Patent Publication Number: US-11050548-B2

Title: Image transmission system, imaging terminal, display terminal, adjustment method, adjustment assistance method, and non-transitory computer-readable recording medium storing program

Description:
This application is a continuation application based on a PCT International Application No. PCT/JP2017/023172, filed on Jun. 23, 2017. The content of the PCT International Application is incorporated herein by reference. 
    
    
     BACKGROUND 
     Field of the Technology 
     The present invention relates to an image transmission system, an imaging terminal, a display terminal, an adjustment method, an adjustment assistance method, a non-transitory computer-readable recording medium storing an adjustment support program, and a non-transitory computer-readable recording medium storing an adjustment assistance program. 
     Conventionally, an image transfer system wirelessly transferring captured image data between an imaging terminal and a display terminal using a wireless communication standard such as that of Institute Of Electrical and Electronics Engineers (IEEE) 802.11, that is, a so-called high-speed wireless communication technique represented by WiFi (registered trademark) has been put to practical use. In an image transfer system, an imaging terminal transmits captured image data captured by an imaging unit included in the imaging terminal to a display terminal. Further, in the image transfer system, the display terminal displays an image corresponding to the captured image data transmitted from the imaging terminal to a display unit included in the display terminal. 
     In such an image transfer system, a synchronization signal such as a vertical synchronization signal or a horizontal synchronization signal based on a reference clock signal generated by, for example, a crystal oscillation IC or the like is generated in each of the imaging terminal and the display terminal. In addition, each of the imaging terminal and the display terminal is operated in accordance with a timing of the generated synchronization signal. That is, in the imaging terminal, an image is captured by the imaging unit in accordance with a timing of the synchronization signal generated in the imaging terminal, and the captured image data is transmitted to the display terminal. Further, in the display terminal, the image corresponding to the captured image data transmitted from the imaging terminal is displayed on the display unit in accordance with a timing of the synchronization signal generated in the display terminal. 
     Incidentally, in an image transfer system, even when a crystal oscillation IC mounted on each of an imaging terminal and a display terminal is a crystal oscillation IC that generates clock signals having the same phase or period, a shift may occur between a synchronization signal generated by the imaging terminal and a synchronization signal generated by the display terminal. That is, a shift of a phase or a period may occur between the clock signal generated by the crystal oscillation IC mounted on the imaging terminal and the clock signal generated by the crystal oscillation IC mounted on the display terminal. This is caused by an error (for example, an error in units of 100 ppm due to temperature characteristics) of the phase or the period of an output signal of a crystal oscillator included in each of the crystal oscillation ICs, variations in a timing when power is supplied to each of the imaging terminal and the display terminal, or the like. In addition, a shift amount of a synchronization signal generated by each of the imaging terminal and the display terminal increases in proportion to the elapse of time. For this reason, even when a process of matching a timing when the imaging terminal generates a synchronization signal to a timing when the display terminal generates a synchronization signal is performed a finite number of times (for example, once, twice, or the like) after power is supplied, the synchronization signal generated by the imaging terminal and the synchronization signal generated by the display terminal may shift from each other with the elapse of time. 
     In addition, when the synchronization signals are shifted in the imaging terminal and the display terminal, a validity period of an image in captured image data transmitted from the imaging terminal may not fall within that of the synchronization signal generated by the display terminal. That is, a start timing or a termination timing of the validity period of the image in the captured image data transmitted from the imaging terminal may deviate from a start timing or a termination timing of a validity period of an image based on the synchronization signal generated by the display terminal. In addition, when the validity period of the image in the captured image data transmitted from the imaging terminal deviates from the validity period of the image based on the synchronization signal generated by the display terminal, the display terminal cannot correctly display an image corresponding to the captured image data transmitted from the imaging terminal. 
     Consequently, for example, as disclosed in Japanese Unexamined Patent Application, First Publication No. 2016-005204, a technique of a communication device that synchronizes communication timings of two communication means with each other has been proposed. In a technique disclosed in Patent Literature 1, at least any one communication means out of two communication means is controlled so that beacons transmitted by the two communication means independently performing communication with different frequencies are synchronized with each other. In this case, in the technique disclosed in Patent Literature 1, each of the communication means transmits a beacon at predetermined time intervals and then issues a predetermined signal after a predetermined period of time elapses. Further, in the technique disclosed in Patent Literature 1, communication timings of two communication means are synchronized with each other on the basis of a time difference between timings when signals issued from the respective communication means in response to beacon signals being transmitted by the respective communication means are received. 
     SUMMARY 
     According to a first aspect of the present invention, An image transmission system has an imaging terminal configured to transmit image data at a cycle of imaging timing; and a display terminal configured to receive the image data and display the image data at a cycle of display timing. The image transmission system further has a communication-delay-time calculation circuit, an accuracy estimation circuit, an adjustment-determination circuit, and a cycle adjustment circuit provided in either of the imaging terminal or the display terminal, and a communication-delay-time-calculation-assistance circuit provided in the other terminal of the imaging terminal and the display terminal. The communication-delay-time calculation circuit is configured to generate a first measurement signal and transmit the first measurement signal to the other terminal at a timing synchronized to the display timing, receive a second measurement signal transmitted from the other terminal in accordance with the first measurement signal, and calculate communication delay time according to the transmission timing of the first measurement signal, the reception timing of the second measurement signal, and lapse time data included in the second measurement signal, wherein the lapse time data indicates lapse time from reception timing of the first measurement signal by the other terminal until transmission timing of the second measurement signal. The communication-delay-time-calculation-assistance circuit is configured to receive the first measurement signal, generate the second measurement signal including the lapse time data, and transmit the second measurement signal to either of the imaging terminal or the display terminal having the communication-delay-time calculation circuit. The accuracy estimation circuit is configured to define either of the imaging timing and the display timing as an adjustment target timing and calculate an accuracy estimation value to estimate an accuracy after the adjustment of the cycle of the adjustment target timing, wherein the accuracy estimation value is calculated in accordance with a plurality of the communication delay time calculated in a predetermined period and a plurality of the transmission timing of the first measurement signal corresponding to the plurality of the communication delay time. The adjustment-determination circuit is configured to determine whether the accuracy estimation value is improved compared to the accuracy at the current adjustment target timing in accordance with the accuracy estimation value and the current accuracy at the adjustment target timing, and the adjustment-determination circuit is configured to determine whether to perform a cycle adjustment of the adjustment target timing in accordance with the determination result. The cycle adjustment circuit is configured to adjust the cycle of the adjustment target timing when it is determined to perform the cycle adjustment of the adjustment target timing. 
     According to a second aspect of the present invention, in the image transmission system according to the first aspect, the cycle adjustment circuit may be configured to calculate an adjustment amount for adjusting the cycle of the adjustment target timing and perform the cycle adjustment of the adjustment target timing according to the calculated adjustment amount, when it is determined to perform the cycle adjustment of the adjustment target timing, and the adjustment amount may be calculated in accordance with the accuracy estimation value, the transmission timing of the first measurement signal, and the transmission timing of the second signal. 
     According to a third aspect of the present invention, in the image transmission system according to the second aspect, the accuracy estimation circuit may be configured to define the plurality of the communication delay time calculated during the predetermined period as a population, extract a minimum communication delay time per each population, and calculate the accuracy estimation value in accordance with a difference between two minimum communication delay time among the plurality of the communication delay time extracted from the plurality of populations and a difference between two transmission timing of the two first measurement signals which are transmitted to calculate the two extracted minimum communication delay time. The adjustment-determination circuit may be configured to determine to perform the cycle adjustment of the adjustment target timing when the accuracy estimation value is improved compared to the accuracy at the current adjustment target timing, and the adjustment-determination circuit may be configured to not to perform the cycle adjustment of the adjustment target timing when the accuracy estimation value is not improved compared to the accuracy at the current adjustment target timing. The cycle adjustment circuit may be configured to calculate the adjustment amount according to a difference between the transmission timing of the two first measurement signals transmitted to calculate the two minimum communication delay time extracted to calculate the accuracy estimation value and a difference between the transmission timing of the tow second measurement signals corresponding to the two first measurement signals. 
     According to a fourth aspect of the invention, in the image transmission system according to the third aspect, the cycle adjustment circuit may be configured to calculate the adjustment amount by multiplying the accuracy estimation value by a ratio of the difference between the transmission timing of the two first measurement signals to the difference between the transmission timing of the two second measurement signals. 
     According to a fifth aspect of the invention, in the image transmission system according to the third aspect, the accuracy estimation circuit may be configured to calculate the accuracy estimation value by increasing a number of the communication delay time included in the population when it is determined to not perform the cycle adjustment of the adjustment target timing. 
     According to a sixth aspect of the present invention, in the image transmission system according to the fifth aspect, the accuracy estimation circuit may be configured to include the communication delay time included in the population before the number of the communication delay time is increased into the population in which the number of the communication delay time is increased. 
     According to a seventh aspect of the present invention, the image transmission system according to the third aspect may further have a phase-adjustment-determination circuit configured to determine whether to perform an adjustment of a phase shift between the imaging timing and the display timing according to the communication delay time. 
     According to an eighth aspect of the present invention, the image transmission system according to the third aspect may further have a phase adjustment circuit configured to calculate a phase-adjustment amount for adjusting the phase shift between the imaging timing and the display timing according to the communication delay time and adjust the phase shift between the imaging timing and the display timing according to the calculated phase-adjustment amount. 
     According to a ninth aspect of the present invention, an imaging terminal included in an image transmission system, wherein the image transmission system has the imaging terminal configured to transmit image data at a cycle of imaging timing; and a display terminal configured to receive the image data and display the image data at a cycle of display timing, has a communication-delay-time calculation circuit configured to generate a first measurement signal and transmit the first measurement signal to the other terminal at a timing synchronized to the display timing, receive a second measurement signal transmitted from the other terminal in accordance with the first measurement signal, and calculate communication delay time according to the transmission timing of the first measurement signal, the reception timing of the second measurement signal, and lapse time data included in the second measurement signal, wherein the lapse time data indicates lapse time from reception timing of the first measurement signal by the other terminal until transmission timing of the second measurement signal; an accuracy estimation circuit configured to calculate an accuracy estimation value to estimate an accuracy after adjusting the cycle of the imaging timing in accordance with a plurality of the communication delay time calculated in a predetermined period and a plurality of the transmission timing of the first measurement signal corresponding to the plurality of the communication delay time; an adjustment-determination circuit configured to determine whether the accuracy estimation value is improved compared to the accuracy at the current imaging timing in accordance with the accuracy estimation value and the current accuracy at the imaging timing, and the adjustment-determination circuit is configured to determine whether to perform a cycle adjustment of the imaging timing in accordance with the determination result, and a cycle adjustment circuit configured to adjust the cycle of the imaging timing when it is determined to perform the cycle adjustment of the imaging timing. 
     According to a tenth aspect of the present invention, an adjustment method for adjusting an imaging timing and a displaying timing in an image transmission system so as to match the imaging timing with the display imaging, wherein the image transmission system has an imaging terminal configured to transmit image data at a cycle of the imaging timing; and a display terminal configured to receive the image data and display the image data at a cycle of the display timing, has a process of generating a first measurement signal by either of the imaging terminal or the display terminal and transmitting the first measurement signal to the other terminal at a timing synchronized to the display timing; a process of receiving the first measurement signal in the other terminal; a process of generating a second measurement signal in the other terminal in accordance with the first measurement signal and transmitting the second measurement signal to either of the imaging terminal and the display terminal, wherein the second measurement signal includes lapse time data indicating lapse time from reception timing of the first measurement signal by the other terminal until transmission timing of the second measurement signal; a process of receiving the second measurement signal to calculate communication delay time according to the transmission timing of the first measurement signal, the reception timing of the second measurement signal, and lapse time data included in the second measurement signal by either of the imaging terminal and the display terminal; a process of defining either of the imaging timing and the display timing as an adjustment target timing and calculating an accuracy estimation value to estimate an accuracy after the adjustment of the cycle of the adjustment target timing by either of the imaging terminal and the display terminal, wherein the accuracy estimation value is calculated in accordance with a plurality of the communication delay time calculated in a predetermined period and a plurality of the transmission timing of the first measurement signal corresponding to the plurality of the communication delay time; a process of determining whether the accuracy estimation value is improved compared to the accuracy at the current adjustment target timing in accordance with the accuracy estimation value and the current accuracy at the adjustment target timing, and determining whether to perform a cycle adjustment of the adjustment target timing in accordance with the determination result by either of the imaging terminal and the display terminal; and a process of adjusting the cycle of the adjustment target timing by either of the imaging terminal and the display terminal, when it is determined to perform the cycle adjustment of the adjustment target timing. 
     According to another aspect of the present invention, an adjustment method for adjusting an imaging timing and a displaying timing in an image transmission system so as to match the imaging timing with the display imaging, wherein the image transmission system has an imaging terminal configured to transmit image data at a cycle of the imaging timing; and a display terminal configured to receive the image data and display the image data at a cycle of the display timing, has a process of generating a first measurement signal by the imaging terminal and transmitting the first measurement signal to the display terminal at a timing synchronized to the display timing; a process of receiving a second measurement signal transmitted from the display terminal in accordance with the first measurement signal, wherein the second measurement signal includes lapse time data indicating lapse time from reception timing of the first measurement signal until transmission timing of the second measurement signal; a process of calculating communication delay time according to the transmission timing of the first measurement signal, the reception timing of the second measurement signal, and the lapse time data; a process of calculating an accuracy estimation value to estimate an accuracy after the adjustment of the cycle of the imaging timing, wherein the accuracy estimation value is calculated in accordance with a plurality of the communication delay time calculated in a predetermined period and a plurality of the transmission timing of the first measurement signal corresponding to the plurality of the communication delay time; a process of determining whether the accuracy estimation value is improved compared to the accuracy at the current imaging timing in accordance with the accuracy estimation value and the current accuracy at the imaging timing, and determining whether to perform a cycle adjustment of the imaging timing in accordance with the determination result; and a process of adjusting the cycle of the imaging timing, when it is determined to perform the cycle adjustment of the imaging timing. 
     According to further another aspect of the present invention, a non-transitory computer-readable recording medium storing program for causing a computer to execute an adjustment method for adjusting an imaging timing and a displaying timing in an image transmission system so as to match the imaging timing with the display imaging, wherein the image transmission system has an imaging terminal configured to transmit image data at a cycle of the imaging timing; and a display terminal configured to receive the image data and display the image data at a cycle of the display timing, wherein the adjustment method has a process of generating a first measurement signal by the imaging terminal and transmitting the first measurement signal to the display terminal at a timing synchronized to the display timing; a process of receiving a second measurement signal transmitted from the display terminal in accordance with the first measurement signal, wherein the second measurement signal includes lapse time data indicating lapse time from reception timing of the first measurement signal until transmission timing of the second measurement signal; a process of calculating communication delay time according to the transmission timing of the first measurement signal, the reception timing of the second measurement signal, and the lapse time data; a process of calculating an accuracy estimation value to estimate an accuracy after the adjustment of the cycle of the imaging timing, wherein the accuracy estimation value is calculated in accordance with a plurality of the communication delay time calculated in a predetermined period and a plurality of the transmission timing of the first measurement signal corresponding to the plurality of the communication delay time; a process of determining whether the accuracy estimation value is improved compared to the accuracy at the current imaging timing in accordance with the accuracy estimation value and the current accuracy at the imaging timing, and determining whether to perform a cycle adjustment of the imaging timing in accordance with the determination result; and a process of adjusting the cycle of the imaging timing, when it is determined to perform the cycle adjustment of the imaging timing. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a flow chart showing schematic processing procedures in an image transmission system according to an embodiment of the present invention. 
         FIG. 2  is a block diagram showing a schematic configuration of an image transmission system according to a first embodiment of the present invention. 
         FIG. 3  is a flow chart showing processing procedures in the image transmission system according to the first embodiment of the present invention. 
         FIG. 4  is a flow chart showing processing procedures of transmission and reception processing of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal in an imaging terminal configuring the image transmission system according to the first embodiment of the present invention. 
         FIG. 5  is a flow chart showing processing procedures of transmission and reception processing of the round-trip-propagation-time-measurement outgoing signal and the round-trip-propagation-time-measurement returning signal in a display terminal configuring the image transmission system according to the first embodiment of the present invention. 
         FIG. 6  is a view showing an example of the timing when the transmission and reception of the round-trip-propagation-time-measurement outgoing signal and the round-trip-propagation-time-measurement returning signal are performed. 
         FIG. 7  is a view showing another example of the timing when the transmission and reception of the round-trip-propagation-time-measurement outgoing signal and the round-trip-propagation-time-measurement returning signal are performed. 
         FIG. 8  is a view showing an example of a calculation method for calculating round-trip-propagation time of wireless transmission by the imaging terminal configuring the image transmission system according to the first embodiment of the present invention. 
         FIG. 9  is a flow chart showing processing procedures of updating a determination value by the imaging terminal configuring the image transmission system according to the first embodiment of the present invention. 
         FIG. 10  is a view showing a processing example to calculate an after-cycle-adjustment accuracy value in the image transmission system according to the first embodiment of the present invention. 
         FIG. 11  is a view showing another processing example to calculate the after-cycle-adjustment accuracy value in the image transmission system according to the first embodiment of the present invention. 
         FIG. 12  is a view showing further another processing example to calculate the after-cycle-adjustment accuracy value in the image transmission system according to the first embodiment of the present invention. 
         FIG. 13  is a flow chart showing processing procedures of adjustment processing for a shooting synchronous signal by the imaging terminal configuring the image transmission system according to the first embodiment of the present invention. 
         FIG. 14  is a timing chart showing an example of transmission and reception of shooting image data via wireless transmission in the image transmission system according to the first embodiment of the present invention. 
         FIG. 15  is a view showing relationship between the synchronous signal and time during wireless transmission of the shooting image data in the image transmission system according to the first embodiment of the present invention. 
         FIG. 16  is a block diagram showing a schematic configuration of an image transmission system according to a second embodiment of the present invention. 
         FIG. 17  is a flow chart showing processing procedures of the image transmission system according to the second embodiment of the present invention. 
         FIG. 18  is a block diagram showing a schematic configuration of an image transmission system according to a third embodiment of the present invention. 
         FIG. 19  is a flow chart showing processing procedures of the image transmission system according to the third embodiment of the present invention. 
         FIG. 20  is a block diagram showing a schematic configuration of an image transmission system according to a fourth embodiment of the present invention. 
         FIG. 21  is a flow chart showing processing procedures of the image transmission system according to the fourth embodiment of the present invention. 
         FIG. 22  is a block diagram showing a schematic configuration of an image transmission system according to a fifth embodiment of the present invention. 
         FIG. 23  is a flow chart showing processing procedures of the image transmission system according to the fifth embodiment of the present invention. 
         FIG. 24  is a block diagram showing a schematic configuration of an image transmission system according to a sixth embodiment of the present invention. 
         FIG. 25  is a flow chart showing processing procedures of the image transmission system according to the sixth embodiment of the present invention. 
         FIG. 26  is a block diagram showing a schematic configuration of an image transmission system according to a seventh embodiment of the present invention. 
         FIG. 27  is a flow chart showing processing procedures of the image transmission system according to the seventh embodiment of the present invention. 
         FIG. 28  is a block diagram showing a schematic configuration of an image transmission system according to an eighth embodiment of the present invention. 
         FIG. 29  is a flow chart showing processing procedures of the image transmission system according to the eighth embodiment of the present invention. 
         FIG. 30  is a block diagram showing a schematic configuration of an image transmission system according to a ninth embodiment of the present invention. 
         FIG. 31  is a flow chart showing processing procedures of the image transmission system according to the ninth embodiment of the present invention. 
         FIG. 32  is a block diagram showing a schematic configuration of an image transmission system according to a tenth embodiment of the present invention. 
         FIG. 33  is a flow chart showing processing procedures of the image transmission system according to the tenth embodiment of the present invention. 
         FIG. 34  is a block diagram showing a schematic configuration of an image transmission system according to an eleventh embodiment of the present invention. 
         FIG. 35  is a flow chart showing processing procedures of the image transmission system according to the eleventh embodiment of the present invention. 
         FIG. 36  is a block diagram showing a schematic configuration of an image transmission system according to a twelfth embodiment of the present invention. 
         FIG. 37  is a flow chart showing processing procedures of the image transmission system according to the twelfth embodiment of the present invention. 
         FIG. 38  is a block diagram showing a schematic configuration of an image transmission system according to a thirteenth embodiment of the present invention. 
         FIG. 39  is a flow chart showing processing procedures of the image transmission system according to the thirteenth embodiment of the present invention. 
         FIG. 40  is a block diagram showing a schematic configuration of an image transmission system according to a fourteenth embodiment of the present invention. 
         FIG. 41  is a flow chart showing processing procedures of the image transmission system according to the fourteenth embodiment of the present invention. 
         FIG. 42  is a block diagram showing a schematic configuration of an image transmission system according to a fifteenth embodiment of the present invention. 
         FIG. 43  is a flow chart showing processing procedures of the image transmission system according to the fifteenth embodiment of the present invention. 
         FIG. 44  is a block diagram showing a schematic configuration of an image transmission system according to a sixteenth embodiment of the present invention. 
         FIG. 45  is a flow chart showing processing procedures of the image transmission system according to the sixteenth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Each of image transfer systems according to the embodiments of the present invention is an image display system in which an imaging terminal wirelessly transfers (transmits) captured image data of an image captured by an imaging unit to a display terminal using a wireless communication technique, and the display terminal displays an image (display image) corresponding to the captured image data wirelessly transferred (transmitted) from the imaging terminal on a display unit. First, an outline of the overall operation in the image transfer system will be described. 
     In the image transfer system, when the imaging terminal and the display terminal are started up, each of the imaging terminal and the display terminal generates a reference clock signal and starts to operate. More specifically, the imaging terminal generates a reference clock signal within the imaging terminal (hereinafter, referred to as an “imaging reference clock signal”) and starts to operate in accordance with a timing of the generated imaging reference clock signal. In addition, the display terminal generates a reference clock signal within the display terminal (hereinafter, referred to as a “display reference clock signal”) and starts to operate in accordance with a timing of the generated display reference clock signal. Further, in the image transfer system, an operation of establishing wireless connection between the imaging terminal and the display terminal is performed. 
     For example, the imaging terminal transmits a connection request to the display terminal until wireless connection between the imaging terminal and the display terminal is established. When the connection request is transmitted from the imaging terminal until the wireless connection between the imaging terminal and the display terminal is established, the display terminal transmits a response signal corresponding to the connection request to the imaging terminal. The wireless connection between the imaging terminal and the display terminal is established through the transmission of the connection request by the imaging terminal and the transmission of the response signal corresponding to the connection request by the display terminal. 
     Meanwhile, a process performed to establish wireless connection between the imaging terminal and the display terminal can be easily conceived on the basis of existing wireless communication techniques. Therefore, a detailed description related to a process performed to establish wireless connection between the imaging terminal and the display terminal will be omitted. In addition, each of the imaging terminal and the display terminal monitors the quality of wireless communication by monitoring interference due to another wireless communication device or the like on a wireless communication channel currently being used after wireless connection is established therebetween. In addition, each of the imaging terminal and the display terminal is operated to select and switch a wireless communication channel at all times so that wireless transfer can be performed using a channel with high communication quality. Monitoring of the quality of wireless communication and a wireless transfer method using a channel with an excellent communication quality can also be easily conceived on the basis of existing wireless communication techniques. Therefore, a detailed description related to a wireless transfer method in the imaging terminal and the display terminal is also omitted. 
     Thereafter, in the image transfer system, after wireless connection is established between the imaging terminal and the display terminal, the imaging terminal transmits captured image data of an image captured by the imaging unit to the display terminal in accordance with a timing of a synchronization signal (hereinafter, referred to as an “imaging synchronization signal”) such as a vertical synchronization signal or a horizontal synchronization signal generated on the basis of an imaging reference clock signal. On the other hand, the display terminal displays an image corresponding to the received captured image data from the imaging terminal on the display unit in accordance with a timing of a synchronization signal (hereinafter, referred to as “display synchronization signal”) such as a vertical synchronization signal or a horizontal synchronization signal generated on the basis of a display reference clock signal. 
     Further, in the image transfer system, when wireless connection is established between the imaging terminal and the display terminal, a cycle adjustment process is performed between the imaging terminal and the display terminal. Here, a schematic processing procedure of the cycle adjustment process performed after wireless connection between the imaging terminal and the display terminal is established will be described.  FIG. 1  is a flowchart showing a schematic processing procedure of processing in an image transfer system according to an embodiment of the present invention. 
     In the image transfer system, when a cycle adjustment process is started, first, transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between an imaging terminal and a display terminal (step S 302 ). More specifically, in the process of step S 302 , any one terminal out of the imaging terminal and the display terminal transmits a round-trip-propagation-time-measurement outgoing signal for calculating a round trip propagation-time required for transmission and reception at the time of wireless transfer to the other terminal. In addition, one terminal having received the round-trip-propagation-time-measurement outgoing signal transmits a round-trip-propagation-time-measurement returning signal corresponding to the received round-trip-propagation-time-measurement outgoing signal to the other terminal having transmitted the round-trip-propagation-time-measurement outgoing signal. 
     Subsequently, in the image transfer system, one terminal having received the round-trip-propagation-time-measurement returning signal calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal and the display terminal (step S 303 ). More specifically, in the process of step S 303 , any one terminal out of the imaging terminal and the display terminal calculates a round trip propagation-time of a signal making a round trip through wireless transfer on the basis of a transmission time of the round-trip-propagation-time-measurement outgoing signal, a reception time of the round-trip-propagation-time-measurement returning signal transmitted from the other terminal, and information included in the round-trip-propagation-time-measurement returning signal. Here, the round trip propagation-time is a time obtained by adding up a time required for wireless communication (delay time) in wireless transfer of the round-trip-propagation-time-measurement returning signal to be transmitted from one terminal to the other terminal and a time required for wireless communication (delay time) in wireless transfer of the round-trip-propagation-time-measurement outgoing signal to be transmitted from the other terminal to one terminal. 
     Subsequently, in the image transfer system, one terminal having received the round-trip-propagation-time-measurement returning signal adjusts the phase of a timing signal used when the terminal operates on the basis of the calculated round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal and the display terminal (step S 304 ). More specifically, in a case where one terminal is the imaging terminal, the imaging terminal adjusts the phase of an imaging synchronization signal by regenerating the imaging synchronization signal which is a timing signal when imaging is performed by the imaging unit or when captured image data of a captured image is transmitted to the display terminal on the basis of an imaging reference clock signal. Further, in a case where one terminal is the display terminal, the display terminal adjusts the phase of a display synchronization signal by regenerating the display synchronization signal which is a timing signal when an image corresponding to the received captured image data from the imaging terminal is displayed on the display unit on the basis of a display reference clock signal. Thereby, in the image transfer system, phases of the imaging synchronization signal and the display synchronization signal are adjusted so as not to be shifted between the imaging terminal and the display terminal. 
     Subsequently, in the image transfer system, subsequently to the phase adjustment process for a timing signal which is performed in step S 304 , the period of a timing signal is adjusted on the basis of the information used in the phase adjustment process (step S 306 ). More specifically, in a case where one terminal is the imaging terminal, the imaging terminal adjusts the period of the imaging synchronization signal on the basis of the information used in the phase adjustment process for the imaging synchronization signal. Further, in a case where one terminal is the display terminal, the display terminal adjusts the period of the display synchronization signal on the basis of the information used in the phase adjustment process for the display synchronization signal. Thereby, in the image transfer system, periods of the imaging synchronization signal and the display synchronization signal are adjusted so as not to be shifted between the imaging terminal and the display terminal. 
     The processing having been performed so far is a cycle adjustment process for a timing signal which is performed between the imaging terminal and the display terminal in the image transfer system. In the image transfer system, after the cycle adjustment process for a timing signal in step S 306  is terminated, the processing returns to step S 302  to repeat the cycle adjustment process in steps S 302  to S 306 . Through such a cycle adjustment process, in the image transfer system, phases or periods of the imaging synchronization signal and the display synchronization signal are adjusted so as not to be shifted with the elapse of time between the imaging terminal and the display terminal. 
     Here, a more detailed operation of a cycle adjustment process for a timing signal which is performed in the image transfer system will be described with a case where the imaging terminal constituting the image transfer system is set to be one terminal and the display terminal is set to be the other terminal as an example. Meanwhile, an operation in a case where the display terminal constituting the image transfer system is set to be one terminal and the imaging terminal is set to be the other terminal can be easily understood by understanding the imaging terminal and the display terminal to be interchanged in the following description. 
     When wireless connection between the imaging terminal and the display terminal is established, the imaging terminal generates the time of the imaging terminal itself on the basis of an imaging reference clock signal. For example, the imaging terminal sets the time when the wireless connection between the imaging terminal and the display terminal is established to be a reference time (for example, time 0) and starts to generate a time indicating an elapsed time from the reference time (hereinafter, referred to as an “imaging terminal time”) on the basis of an imaging reference clock signal. Further, in step S 302 , the imaging terminal determines a time when a round-trip-propagation-time-measurement outgoing signal is scheduled to be transmitted to the display terminal (imaging terminal time) and generates the round-trip-propagation-time-measurement outgoing signal by the determined scheduled transmission time. In addition, the imaging terminal transmits the round-trip-propagation-time-measurement outgoing signal including information of a packet identification number to the display terminal. Thereafter, the imaging terminal waits for a round-trip-propagation-time-measurement returning signal corresponding to the transmitted round-trip-propagation-time-measurement outgoing signal to be transmitted from the display terminal. 
     On the other hand, when wireless connection between the display terminal and the imaging terminal is established, the display terminal generates the time of the display terminal itself on the basis of a display reference clock signal. For example, the display terminal sets the time when the wireless connection between the display terminal and the imaging terminal is established to be a reference time (for example, time 0) and starts to generate a time indicating an elapsed time from the reference time (hereinafter, referred to as a “display terminal time”). In addition, the display terminal waits for a round-trip-propagation-time-measurement outgoing signal to be transmitted from the imaging terminal. Thereafter, when the display terminal receives the round-trip-propagation-time-measurement outgoing signal transmitted from the imaging terminal, the display terminal extracts information of a packet identification number included in the received round-trip-propagation-time-measurement outgoing signal in step S 302 . In addition, the display terminal measures a display terminal time indicating the reception time of the round-trip-propagation-time-measurement outgoing signal. In addition, the display terminal determines a time when a round-trip-propagation-time-measurement returning signal corresponding to the received round-trip-propagation-time-measurement outgoing signal is scheduled to be transmitted to the imaging terminal (display terminal time) and generates the round-trip-propagation-time-measurement returning signal by the determined scheduled transmission time. In this case, the display terminal calculates a difference between the reception time of the round-trip-propagation-time-measurement outgoing signal and the determined scheduled transmission time of the round-trip-propagation-time-measurement returning signal as a receiver elapsed time. In addition, the display terminal transmits the round-trip-propagation-time-measurement returning signal including information of the calculated receiver elapsed time, information of the scheduled transmission time of the round-trip-propagation-time-measurement returning signal, and information of the packet identification number extracted from the round-trip-propagation-time-measurement outgoing signal to the imaging terminal. 
     Thereafter, when the imaging terminal receives the round-trip-propagation-time-measurement returning signal transmitted from the display terminal, the imaging terminal measures an imaging terminal time indicating the reception time of the round-trip-propagation-time-measurement returning signal. In addition, the imaging terminal extracts the information of the receiver elapsed time, the information of the scheduled transmission time of the round-trip-propagation-time-measurement returning signal, and the information of the packet identification number which are included in the received round-trip-propagation-time-measurement returning signal. Further, in step S 303 , the imaging terminal calculates a difference between the scheduled transmission time of the round-trip-propagation-time-measurement outgoing signal which is determined in step S 302  and the reception time of the round-trip-propagation-time-measurement returning signal. In addition, the imaging terminal calculates a round trip propagation-time required for transmission and reception at the time of wireless transfer performed between the imaging terminal and the display terminal by subtracting the extracted receiver elapsed time from the calculated difference time. That is, the imaging terminal calculates a delay time of a signal making a round trip in only wireless transfer between the imaging terminal and the display terminal. 
     In this manner, in the image transfer system, a delay time of a signal making a round trip in only wireless transfer is calculated as a round trip propagation-time on the basis of information of the time when any one terminal out of the imaging terminal and the display terminal transmits a round-trip-propagation-time-measurement outgoing signal, information of the time when a round-trip-propagation-time-measurement returning signal transmitted from the other terminal is received, and information of a time between when the other terminal receives a round-trip-propagation-time-measurement outgoing signal and when the other terminal transmits the round-trip-propagation-time-measurement returning signal. 
     Thereafter, in the image transfer system, the imaging terminal performs a phase adjustment process for a timing signal which is performed in step S 304 . In the phase adjustment process for a timing signal which is performed by the imaging terminal, a phase is adjusted without changing the period of an imaging synchronization signal by regenerating the imaging synchronization signal (timing signal) used at the time of imaging using the imaging unit in accordance with a round trip propagation-time calculated on the basis of information of a round-trip-propagation-time-measurement returning signal corresponding to a transmitted round-trip-propagation-time-measurement outgoing signal. Here, the imaging terminal adjusts the phase of the imaging synchronization signal by regenerating the imaging synchronization signal (timing signal) when the round trip propagation-time calculated in step S 303  is equal to or less than a determination value (hereinafter, referred to as a “round trip propagation-time determination value”) which is a threshold value for determining a round trip propagation-time which is determined in advance. On the other hand, when the round trip propagation-time calculated in step S 303  is greater than a round trip propagation-time determination value determined in advance, the imaging terminal terminates the phase adjustment process for a timing signal which is performed in step S 304  by maintaining the current generation timing without regenerating an imaging synchronization signal (timing signal), that is, without adjusting the phase of the imaging synchronization signal. 
     Thereafter, in the image transfer system, the imaging terminal performs the cycle adjustment process for a timing signal which is performed in step S 306 . In the image transfer system, the imaging terminal performs the cycle adjustment process for a timing signal whenever a phase adjustment process for a timing signal (the process of step S 304 ) is performed a number of times determined in advance, that is, whenever a period of time determined in advance elapses. In the phase adjustment process for a timing signal which is performed by the imaging terminal, the accuracy of an imaging synchronization signal in a case where a period is temporarily adjusted is estimated on the basis of a plurality of round trip propagation-time determination values equivalent to a predetermined number of times of the phase adjustment process for an imaging synchronization signal (timing signal) which is performed in step S 304 . Further, in a case where the imaging terminal determines that the estimated accuracy of the imaging synchronization signal has been improved, the imaging terminal adjusts the period of the imaging synchronization signal (timing signal) with a calculated cycle adjustment amount. On the other hand, in a case where the imaging terminal determines that the estimated accuracy of the imaging synchronization signal has not been improved, the imaging terminal terminates the cycle adjustment process for a timing signal which is performed in step S 306  while maintaining the current period without calculating a cycle adjustment amount, that is, without adjusting the period of the imaging synchronization signal. 
     Hereinafter, in the image transfer system, the imaging terminal repeats the cycle adjustment process for a timing signal which is performed in steps S 302  to S 306 . 
     As described above, in the image transfer system, after wireless connection between the imaging terminal and the display terminal is established, any one terminal out of the imaging terminal and the display terminal transmits a round-trip-propagation-time-measurement outgoing signal to the other terminal to start a cycle adjustment process for a timing signal. Further, in the image transfer system, phase adjustment for a timing signal is performed on the basis of a round trip propagation-time for only transmission and reception of wireless transfer between the imaging terminal and the display terminal which is calculated on the basis of information regarding a round-trip-propagation-time-measurement outgoing signal transmitted by any one terminal and information included in a round-trip-propagation-time-measurement returning signal corresponding to the transmitted round-trip-propagation-time-measurement outgoing signal. Further, in the image transfer system, cycle adjustment for a timing signal is performed on the basis of the accuracy of the timing signal which is estimated from a plurality of round trip propagation-time determination values when the phase adjustment process for the timing signal is perform a number of times determined in advance. 
     First Embodiment 
     Next, a specific configuration and operation for performing phase adjustment for a timing signal in an image transfer system will be described.  FIG. 2  is a block diagram showing a schematic configuration of an image transfer system in a first embodiment of the present invention. An image transfer system  1  includes an imaging terminal  100  and a display terminal  200 . Meanwhile, the imaging terminal  100  starts to operate in accordance with a timing of an imaging reference clock signal generated within the imaging terminal  100 . In addition, the imaging terminal  100  transmits captured image data of an image captured by an imaging unit to the display terminal  200  in accordance with a timing of an imaging synchronization signal which is a timing signal generated on the basis of the imaging reference clock signal. In addition, the display terminal  200  starts to operate in accordance with a timing of a display reference clock signal generated within the display terminal  200 . In addition, the display terminal  200  displays an image corresponding to the captured image data received from the imaging terminal  100  in accordance with a timing of a display synchronization signal which is a timing signal generated on the basis of the display reference clock signal to the display unit. 
     In addition, the image transfer system  1  is an image transfer system configured such that the imaging terminal  100  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  200 , and cycle adjustment for a timing signal generated by the imaging terminal  100  (imaging synchronization signal) is performed on the basis of information regarding the transmitted round-trip-propagation-time-measurement outgoing signal and information included in a round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200  in response to the transmitted round-trip-propagation-time-measurement outgoing signal. 
     The imaging terminal  100  includes a synchronization signal generation unit  102 , a cycle adjustment unit  101 , a source oscillation clock generation unit  103 , a wireless communication unit  108 , a cycle adjustment determination unit  104 , an after-cycle-adjustment accuracy estimation unit  105 , a round-trip-propagation-time measurement unit  106 , a clocking unit  107 , and an antenna  120 . In addition, the display terminal  200  includes a synchronous signal generation unit  204 , a source oscillation clock generation unit  205 , a wireless communication unit  201 , a round-trip-propagation-time-measurement assistance unit  202 , a clocking unit  203 , and an antenna  220 . 
     First, each of the components included in the imaging terminal  100  will be described. 
     The source oscillation clock generation unit  103  generates a source oscillation clock signal which is the source of an imaging reference clock signal for operating the imaging terminal  100 . The source oscillation clock generation unit  103  is a so-called clock generator which is configured to include, for example, a crystal oscillation IC or the like. The source oscillation clock generation unit  103  generates a source oscillation clock signal when the imaging terminal  100  is started up. The imaging reference clock signal is generated on the basis of the source oscillation clock signal generated by the source oscillation clock generation unit  103 . Meanwhile, in the following description, for ease of description, the description will be provided on the assumption that the source oscillation clock generation unit  103  generates an imaging reference clock signal. 
     The clocking unit  107  measures a time in the imaging terminal  100  (imaging terminal time) on the basis of the imaging reference clock signal generated by the source oscillation clock generation unit  103 . The clocking unit  107  outputs information of the clocked imaging terminal time to the round-trip-propagation-time measurement unit  106 . Meanwhile, the clocking unit  107  may also output information of the clocked imaging terminal time to the wireless communication unit  108 . 
     The synchronization signal generation unit  102  generates a synchronization signal (imaging synchronization signal) such as a vertical synchronization signal or a horizontal synchronization signal indicating the start or termination of a validity period of captured image data which is captured and output by an imaging unit not shown in the drawing and included in the imaging terminal  100  on the basis of the imaging reference clock signal generated by the source oscillation clock generation unit  103 . The synchronization signal generation unit  102  outputs the generated imaging synchronization signal to each of the imaging unit not shown in the drawing and the round-trip-propagation-time measurement unit  106 . 
     In a case where an instruction for adjusting the period of an imaging synchronization signal is input from the cycle adjustment unit  101 , the synchronization signal generation unit  102  regenerates the imaging synchronization signal (that is, adjusts the period of an imaging synchronization signal to be generated) in response to the input cycle adjustment instruction. Further, in a case where an instruction for adjusting the phase of an imaging synchronization signal is input from a phase adjustment unit not shown in the drawing, the synchronization signal generation unit  102  regenerates the imaging synchronization signal (that is, adjusts the phase of an imaging synchronization signal to be generated) in response to the input phase adjustment instruction. Meanwhile, in the imaging terminal  100 , it is assumed that a phase adjustment unit, not shown in the drawing, which gives an instruction for performing phase adjustment for an imaging synchronization signal is included in the round-trip-propagation-time measurement unit  106 . Therefore, the synchronization signal generation unit  102  adjusts the phase of the imaging synchronization signal in accordance with the phase adjustment instruction which is output from the phase adjustment unit, not shown in the drawing, provided in the round-trip-propagation-time measurement unit  106 . In addition, the synchronization signal generation unit  102  outputs a regenerated imaging synchronization signal to each of the imaging unit not shown in the drawing and the round-trip-propagation-time measurement unit  106 . 
     The round-trip-propagation-time measurement unit  106  measures (calculates) a round trip propagation-time required for transmission and reception when wireless transfer is performed between the imaging terminal  100  and the display terminal  200 , on the basis of information of the imaging terminal time which is output from the clocking unit  107 . When the round-trip-propagation-time measurement unit  106  determines a round trip propagation-time in wireless transfer, first, the round-trip-propagation-time measurement unit  106  measures a scheduled transmission time which is an imaging terminal time when a round-trip-propagation-time-measurement outgoing signal for calculating a round trip propagation-time in wireless transfer is scheduled to be transmitted to the display terminal  200 . In addition, the round-trip-propagation-time measurement unit  106  generates a round-trip-propagation-time-measurement outgoing signal by the determined scheduled transmission time, outputs the generated round-trip-propagation-time-measurement outgoing signal to the wireless communication unit  108 , and transmits the round-trip-propagation-time-measurement outgoing signal to the display terminal  200 . 
     Meanwhile, the round-trip-propagation-time measurement unit  106  temporarily stores information of the determined scheduled transmission time. Here, the information of the scheduled transmission time which is temporarily stored by the round-trip-propagation-time measurement unit  106  may be a scheduled transmission time determined by the round-trip-propagation-time measurement unit  106 . In addition, the information of the scheduled transmission time which is temporarily stored by the round-trip-propagation-time measurement unit  106  may be information of an actual transmission time when the wireless communication unit  108  transmits the round-trip-propagation-time-measurement outgoing signal to the display terminal  200 . In the following description, it is assumed that the scheduled transmission time determined by the round-trip-propagation-time measurement unit  106  and the actual transmission time when the wireless communication unit  108  transmits the round-trip-propagation-time-measurement outgoing signal to the display terminal  200  are the same time, and a description will be provided with reference to the scheduled transmission time and the actual transmission time as a “scheduled transmission time”. 
     Thereafter, when a round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200  is output from the wireless communication unit  108 , the round-trip-propagation-time measurement unit  106  calculates a round trip propagation-time of a signal making a round trip through wireless transfer between the round-trip-propagation-time measurement unit  106  and the display terminal  200  on the basis of the temporarily stored information of the scheduled transmission time of the round-trip-propagation-time-measurement outgoing signal and the information of the round-trip-propagation-time-measurement returning signal which is output from the wireless communication unit  108 . Here, the information of the round-trip-propagation-time-measurement returning signal includes information of a reception time when the round-trip-propagation-time-measurement outgoing signal is received, in addition to information of a receiver elapsed time included in the round-trip-propagation-time-measurement returning signal, information of a scheduled transmission time of the round-trip-propagation-time-measurement returning signal, and information of a packet identification number. 
     Meanwhile, the round-trip-propagation-time measurement unit  106  temporarily stores the information of the round-trip-propagation-time-measurement returning signal, inclusive of information of a reception time when the round-trip-propagation-time-measurement returning signal is received. Here, the information of the reception time which is temporarily stored by the round-trip-propagation-time measurement unit  106  may be an input time when the round-trip-propagation-time-measurement returning signal is input to the round-trip-propagation-time measurement unit  106  from the wireless communication unit  108 . In addition, the information of the reception time which is temporarily stored by the round-trip-propagation-time measurement unit  106  may be information of a reception time when the wireless communication unit  108  actually receives the round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200 . In the following description, it is assumed that an input time when the round-trip-propagation-time-measurement returning signal is input from the wireless communication unit  108  and a reception time when the wireless communication unit  108  actually receives the round-trip-propagation-time-measurement returning signal are the same time, and a description will be provided with reference to the input time and the reception time to as a “reception time”. 
     The round-trip-propagation-time measurement unit  106  outputs information of a calculated round trip propagation-time to the phase adjustment unit, not shown in the drawing, which is included in the round-trip-propagation-time measurement unit  106 . The phase adjustment unit, not shown in the drawing, determines whether or not phase adjustment for an imaging synchronization signal is to be performed on the basis of the information of the round trip propagation-time which is output from the round-trip-propagation-time measurement unit  106 . 
     More specifically, the phase adjustment unit, not shown in the drawing, compares the round trip propagation-time output from the round-trip-propagation-time measurement unit  106  with a round trip propagation-time determination value stored in advance. That is, the phase adjustment unit, not shown in the drawing, compares a delay time of a signal delayed due to only wireless transmission and reception between the imaging terminal  100  and the display terminal  200  with a delay time indicated by a round trip propagation-time determination value determined in advance. Further, in a case where the round trip propagation-time output from the round-trip-propagation-time measurement unit  106  is larger than the round trip propagation-time determination value determined in advance, the phase adjustment unit, not shown in the drawing, determines that phase adjustment for an imaging synchronization signal is not performed. On the other hand, in a case where the round trip propagation-time output from the round-trip-propagation-time measurement unit  106  is equal to or less than the round trip propagation-time determination value determined in advance, the phase adjustment unit, not shown in the drawing, determines that phase adjustment for an imaging synchronization signal is performed. Further, in a case where it is determined that phase adjustment for an imaging synchronization signal is performed, the phase adjustment unit, not shown in the drawing, outputs an instruction for adjusting the phase of an imaging synchronization signal to the synchronization signal generation unit  102  on the basis of the information of the round trip propagation-time which is output from the round-trip-propagation-time measurement unit  106 . Here, the phase adjustment instruction which is output by the phase adjustment unit, not shown in the drawing, is an instruction for adjusting the phase of an imaging synchronization signal by causing the synchronization signal generation unit  102  to regenerate the imaging synchronization signal. More specifically, the phase adjustment unit, not shown in the drawing, adjusts the phase of an imaging synchronization signal by instructing the synchronization signal generation unit  102  to temporarily stop generating an imaging synchronization signal and to restart generating an imaging synchronization signal after waiting for a period of time shown in the information of the round trip propagation-time. In the following description, a series of instructions for causing the phase adjustment unit, not shown in the drawing, to adjust the phase of an imaging synchronization signal to be output to the synchronization signal generation unit  102  will be referred to as a “phase adjustment instruction”. 
     Meanwhile, in a case where it is determined that phase adjustment for an imaging synchronization signal is not performed, the phase adjustment unit, not shown in the drawing, does not output a phase adjustment instruction to the synchronization signal generation unit  102 . That is, in a case where phase adjustment for an imaging synchronization signal is not performed, the phase adjustment unit, not shown in the drawing, continues causing the synchronization signal generation unit  102  to generate an imaging synchronization signal without outputting a phase adjustment instruction for an imaging synchronization signal to the synchronization signal generation unit  102 . 
     The after-cycle-adjustment accuracy estimation unit  105  estimates (calculates) an accuracy in a case where cycle adjustment for an imaging synchronization signal is temporarily executed whenever a period of time determined in advance elapses. When the accuracy of an imaging synchronization signal is estimated, first, the after-cycle-adjustment accuracy estimation unit  105  acquires information used in order for the phase adjustment unit, not shown in the drawing, included in the round-trip-propagation-time measurement unit  106  to adjust the phase of an imaging synchronization signal in a period determined in advance. More specifically, the after-cycle-adjustment accuracy estimation unit  105  acquires a plurality of round trip propagation-time determination values used to determine whether or not phase adjustment for an imaging synchronization signal is to be performed during a period determined in advance from the phase adjustment unit not shown in the drawing. Thereafter, the after-cycle-adjustment accuracy estimation unit  105  estimates (calculates) the accuracy of an imaging synchronization signal in a case where cycle adjustment is temporarily executed, on the basis of information of the plurality of round trip propagation-time determination values acquired. The after-cycle-adjustment accuracy estimation unit  105  outputs an estimation value indicating the estimated accuracy of the imaging synchronization signal (hereinafter, referred to as a “period-adjusted accuracy estimation value”) to the cycle adjustment determination unit  104 . 
     The cycle adjustment determination unit  104  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value which is output from the after-cycle-adjustment accuracy estimation unit  105 . More specifically, the cycle adjustment determination unit  104  compares the current accuracy of the imaging synchronization signal with the accuracy of an imaging synchronization signal indicated by the period-adjusted accuracy estimation value which is output from the after-cycle-adjustment accuracy estimation unit  105 . That is, the cycle adjustment determination unit  104  compares an accuracy before the cycle adjustment for an imaging synchronization signal is performed with an accuracy after the cycle adjustment for an imaging synchronization signal is performed. Further, in a case where the estimated accuracy of the imaging synchronization signal is equal to the current accuracy of the imaging synchronization signal or has not been improved, the cycle adjustment determination unit  104  determines that cycle adjustment for an imaging synchronization signal is not performed. On the other hand, in a case where the estimated accuracy of the imaging synchronization signal is improved compared with the current accuracy of the imaging synchronization signal, the cycle adjustment determination unit  104  determines that cycle adjustment for an imaging synchronization signal is performed. In addition, the cycle adjustment determination unit  104  outputs information indicating a result obtained by determining whether or not cycle adjustment for an imaging synchronization signal is performed (hereinafter, referred to as a “cycle adjustment execution determination result”) to the cycle adjustment unit  101 . 
     In a case where the cycle adjustment execution determination result output from the cycle adjustment determination unit  104  indicates that cycle adjustment for an imaging synchronization signal is performed, the cycle adjustment unit  101  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal. In addition, the cycle adjustment unit  101  outputs the calculated cycle adjustment amount and an instruction for adjusting the period of an imaging synchronization signal (hereinafter, referred to as a “cycle adjustment instruction”) to the synchronization signal generation unit  102 . 
     The wireless communication unit  108  is a communication unit that transmits and receives signals and data through wireless transfer based on wireless connection established between the wireless communication unit and the display terminal  200 . The wireless communication unit  108  transmits captured image data which is captured by the imaging unit not shown in the drawing and included in the imaging terminal  100  and is output to the display terminal  200  through the antenna  120  for wireless communication. In addition, the wireless communication unit  108  transmits a round-trip-propagation-time-measurement outgoing signal which is output from the round-trip-propagation-time measurement unit  106  to the display terminal  200  through the antenna  120 . 
     In addition, the wireless communication unit  108  receives a round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200  in response to the transmitted round-trip-propagation-time-measurement outgoing signal through the antenna  120 . The wireless communication unit  108  outputs the received round-trip-propagation-time-measurement returning signal to the round-trip-propagation-time measurement unit  106 . 
     The antenna  120  is an antenna for wireless communication for performing wireless communication between the imaging terminal  100  and the display terminal  200 . The antenna  120  transmits a wireless signal corresponding to the captured image data or the round-trip-propagation-time-measurement outgoing signal which is output from the wireless communication unit  108  to the display terminal  200 . In addition, the antenna  120  receives a wireless signal corresponding to the round-trip-propagation-time-measurement returning signal which is transmitted from the display terminal  200 . In addition, the antenna  120  outputs the received round-trip-propagation-time-measurement returning signal to the wireless communication unit  108 . 
     With such a configuration, the imaging terminal  100  adjusts the phase or period of an imaging synchronization signal (timing signal) which is used when an image is captured by the imaging unit not shown in the drawing or captured image data of the image captured by the imaging unit not shown in the drawing is transmitted to the display terminal  200 . That is, the imaging terminal  100  adjusts the phase or period of an imaging synchronization signal, which is generated by the imaging terminal itself such that phases or periods of an imaging synchronization signal and a display synchronization signal are not shifted between the imaging terminal and the display terminal  200  which together constitute the image transfer system  1 , so as to be matched to the phase or period of a display synchronization signal generated by the display terminal  200 . 
     Meanwhile, a configuration may be adopted in which some or all of functions of the respective components for adjusting the phase or period of an imaging synchronization signal (for example, functions of the cycle adjustment unit  101 , the cycle adjustment determination unit  104 , the after-cycle-adjustment accuracy estimation unit  105 , and the round-trip-propagation-time measurement unit  106  (the phase adjustment unit not shown in the drawing is included)), which are included in the imaging terminal  100 , are realized as processors. In this case, a configuration in which all of the above-described functions in the imaging terminal  100  are realized by one processor may be adopted. In addition, a configuration in which the functions are realized by individual processors corresponding to the above-described respective functions in the imaging terminal  100 , that is, a plurality of processors may be adopted. Meanwhile, the above-described processors can be realized by, for example, programs recorded on a general-purpose central processing unit (CPU) and a memory. In addition, a configuration in which some or all of the above-described respective functions in the imaging terminal  100  are realized by an integrated circuit such as a dedicated large scale integration (LSI), that is, a so-called application specific integrated circuit (ASIC) may be adopted. 
     Subsequently, the respective components included in the display terminal  200  will be described. 
     The source oscillation clock generation unit  205  generates a source oscillation clock signal which is the source of a display reference clock signal for operating the display terminal  200 . The source oscillation clock generation unit  205  is a so-called clock generator which is configured to include, for example, a crystal oscillation IC or the like. The source oscillation clock generation unit  205  generates a source oscillation clock signal when the display terminal  200  is started up. The display reference clock signal is generated on the basis of the source oscillation clock signal generated by the source oscillation clock generation unit  205 . Meanwhile, in the following description, for ease of description, a description will be provided on the assumption that the source oscillation clock generation unit  205  generates a display reference clock signal. 
     The clocking unit  203  measures a time in the display terminal  200  (display terminal time) on the basis of the display reference clock signal generated by the source oscillation clock generation unit  205 . The clocking unit  203  outputs information of the clocked display terminal time to the round-trip-propagation-time-measurement assistance unit  202 . Meanwhile, the clocking unit  203  may also output information of the clocked display terminal time to the wireless communication unit  201 . 
     The synchronous signal generation unit  204  generates a synchronization signal (display synchronization signal) such as a vertical synchronization signal or a horizontal synchronization signal indicating the start or termination of a validity period of an image corresponding to captured image data transmitted from the imaging terminal  100  and displayed on a display unit not shown in the drawing and included in the display terminal  200  on the basis of the display reference clock signal generated by the source oscillation clock generation unit  205 . The synchronous signal generation unit  204  outputs the generated display synchronization signal to a display image processing unit, not shown in the drawing, which performs a process of generating an image corresponding to captured image data transmitted from the imaging terminal  100  and displaying the generated image on a display unit, not shown in the drawing, which is configured to include a display device such as a liquid crystal display (LCD), the display unit not shown in the drawing, or the like. 
     The round-trip-propagation-time-measurement assistance unit  202  generates a round-trip-propagation-time-measurement returning signal corresponding to a round-trip-propagation-time-measurement outgoing signal transmitted from the imaging terminal  100 , on the basis of the information of the display terminal time which is output from the clocking unit  203 . In addition, the round-trip-propagation-time-measurement assistance unit  202  outputs the generated round-trip-propagation-time-measurement returning signal to the wireless communication unit  201  and transmits the round-trip-propagation-time-measurement returning signal to the imaging terminal  100 . Thereby, the round-trip-propagation-time-measurement assistance unit  202  assists measurement (calculation) of a round trip propagation-time, performed in the imaging terminal  100 , which is required for transmission and reception when wireless transfer is performed between the imaging terminal  100  and the display terminal  200 . 
     When the round-trip-propagation-time-measurement assistance unit  202  generates a round-trip-propagation-time-measurement returning signal, first, the round-trip-propagation-time-measurement assistance unit  202  determines a scheduled transmission time which is a display terminal time when the round-trip-propagation-time-measurement returning signal corresponding to a received round-trip-propagation-time-measurement outgoing signal is scheduled to be transmitted to the display terminal  200 . In addition, the round-trip-propagation-time-measurement assistance unit  202  calculates a difference between the determined scheduled transmission time of the round-trip-propagation-time-measurement returning signal and a reception time of the round-trip-propagation-time-measurement outgoing signal as a receiver elapsed time. 
     Meanwhile, the round-trip-propagation-time-measurement assistance unit  202  temporarily stores information of the reception time of the round-trip-propagation-time-measurement outgoing signal used when the receiver elapsed time is calculated. Here, the information of the reception time which is temporarily stored by the round-trip-propagation-time-measurement assistance unit  202  may be an input time when the round-trip-propagation-time-measurement outgoing signal is input to the round-trip-propagation-time-measurement assistance unit  202  from the wireless communication unit  201 . In addition, the information of the reception time which is temporarily stored by the round-trip-propagation-time-measurement assistance unit  202  may be information of a reception time when the wireless communication unit  201  actually receives the round-trip-propagation-time-measurement outgoing signal transmitted from the imaging terminal  100  and output from the wireless communication unit  201 . In the following description, it is assumed that the input time when the round-trip-propagation-time-measurement outgoing signal is input from the wireless communication unit  201  and the reception time when the wireless communication unit  201  actually receives the round-trip-propagation-time-measurement outgoing signal are the same, and a description will be provided with reference to the input time and the reception time as an “input time” in order to make a distinction from the reception time used in the round-trip-propagation-time measurement unit  106 . 
     In addition, the round-trip-propagation-time-measurement assistance unit  202  generates a round-trip-propagation-time-measurement returning signal including the calculated receiver elapsed time, information of the determined scheduled transmission time of the round-trip-propagation-time-measurement returning signal, and information of a packet identification number extracted from the round-trip-propagation-time-measurement outgoing signal by the determined scheduled transmission time, outputs the generated round-trip-propagation-time-measurement returning signal to the wireless communication unit  201 , and transmits the round-trip-propagation-time-measurement returning signal to the imaging terminal  100 . 
     The wireless communication unit  201  is a communication unit that transmits and receives signals and data through wireless transfer based on wireless connection established between the wireless communication unit and the imaging terminal  100 . The wireless communication unit  201  receives captured image data or a round-trip-propagation-time-measurement outgoing signal which is transmitted from the imaging terminal  100  through the antenna  220  for wireless communication. The wireless communication unit  201  outputs the received captured image data to the display image processing unit not shown in the drawing. In addition, the wireless communication unit  201  outputs the received round-trip-propagation-time-measurement outgoing signal to the round-trip-propagation-time-measurement assistance unit  202 . 
     In addition, the wireless communication unit  201  transmits a round-trip-propagation-time-measurement returning signal which is output from the round-trip-propagation-time-measurement assistance unit  202  in response to a round-trip-propagation-time-measurement outgoing signal transmitted from the imaging terminal  100  to the imaging terminal  100  through the antenna  220 . 
     The antenna  220  is an antenna for wireless communication for performing wireless communication between the display terminal  200  and the imaging terminal  100 . The antenna  220  receives a wireless signal corresponding to captured image data or a round-trip-propagation-time-measurement outgoing signal which is transmitted from the imaging terminal  100 . In addition, the antenna  220  outputs the received captured image data or round-trip-propagation-time-measurement outgoing signal to the wireless communication unit  201 . In addition, the antenna  220  transmits a wireless signal corresponding to a round-trip-propagation-time-measurement returning signal which is output from the wireless communication unit  201  to the imaging terminal  100 . 
     With such a configuration, the display terminal  200  assists adjustment of the phase or period of an imaging synchronization signal (timing signal) which is used when an image is captured by the imaging unit not shown in the drawing and included in the imaging terminal  100  or captured image data of the image captured by the imaging unit not shown in the drawing is transmitted to the display terminal  200 . That is, the display terminal  200  assists a process in which the imaging terminal  100  constituting the image transfer system  1  together with the display terminal adjusts the phase or period of an imaging synchronization signal, which is generated such that the phases or periods of an imaging synchronization signal and a display synchronization signal are not shifted between the imaging terminal and the display terminal  200 , so as to match the phase or period of a display synchronization signal generated by the display terminal  200  itself. 
     Meanwhile, a configuration may be adopted in which some or all of functions of the respective components for assisting adjustment of the phase or period of an imaging synchronization signal performed by the imaging terminal  100  (for example, the function of the round-trip-propagation-time-measurement assistance unit  202  and the function of the display image processing unit, not shown in the drawing, which performs a process of generating an image corresponding to captured image data transmitted from the imaging terminal  100  and displaying the image on the display unit not shown in the drawing), which are included in the display terminal  200 , are realized as processors. In this case, a configuration in which all of the above-described functions in the display terminal  200  are realized by one processor may be adopted. In addition, a configuration in which the functions are realized by individual processors corresponding to the above-described respective functions in the display terminal  200 , that is, a plurality of processors may be adopted. Meanwhile, the above-described processors can be realized by, for example, programs recorded on a general-purpose CPU and a memory. In addition, a configuration in which some or all of the above-described respective functions in the display terminal  200  are realized by an integrated circuit such as a dedicated LSI (a so-called ASIC) may be adopted. 
     With such a configuration, in the image transfer system  1 , the imaging terminal  100  adjusts the phase or period of an imaging synchronization signal generated on the basis of an imaging reference clock signal generated by the source oscillation clock generation unit  103  included in the imaging terminal  100 , on the basis of a round-trip-propagation-time-measurement outgoing signal transmitted to the display terminal  200  and a round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200 . That is, in the image transfer system  1 , a timing when the imaging terminal  100  wirelessly transfers captured image data of an image captured by the imaging unit not shown in the drawing to the display terminal  200  is matched to a timing when the display terminal  200  displays an image corresponding to the captured image data on the display unit not shown in the drawing. Thereby, in the image transfer system  1 , the display terminal  200  can stably display an image corresponding to captured image data wirelessly transferred from the imaging terminal  100  on the display unit not shown in the drawing. 
     Next, a more specific operation of performing phase adjustment for a timing signal in the image transfer system  1  will be described.  FIG. 3  is a flowchart showing a processing procedure of the image transfer system  1  in the first embodiment of the present invention. 
     In the image transfer system  1 , when a cycle adjustment process is started, the imaging terminal  100  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  200 , and the display terminal  200  transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal to the imaging terminal  100  in step S 302 . More specifically, in the imaging terminal  100 , the round-trip-propagation-time measurement unit  106  transmits the round-trip-propagation-time-measurement outgoing signal to the display terminal  200  through the wireless communication unit  108 , and the antenna  120  and receives the round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200  in the processes of steps S 302 -A and S 302 -B included in step S 302 . On the other hand, in the display terminal  200 , when a cycle adjustment process is started, the round-trip-propagation-time-measurement assistance unit  202  receives a round-trip-propagation-time-measurement outgoing signal transmitted from the imaging terminal  100  through the antenna  220  and the wireless communication unit  201  in the processes of steps S 302 -C and S 302 -D included in step S 302  and transmits a round-trip-propagation-time-measurement returning signal corresponding to the received round-trip-propagation-time-measurement outgoing signal to the imaging terminal  100 . 
     Thereafter, in the image transfer system  1 , in step S 303 , the imaging terminal  100  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  100  and the display terminal  200 . More specifically, the round-trip-propagation-time measurement unit  106  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  100  and the display terminal  200  on the basis of information regarding the transmitted round-trip-propagation-time-measurement outgoing signal and information included in the round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200 . 
     Thereafter, in the image transfer system  1 , in step S 304 , the imaging terminal  100  adjusts the phase of an imaging synchronization signal on the basis of the round trip propagation-time, required for transmission and reception in wireless transfer between the imaging terminal  100  and the display terminal  200 , which is calculated in step S 303 . 
     More specifically, in step S 304 , the phase adjustment unit, not shown in the drawing, which is included in the round-trip-propagation-time measurement unit  106  determines whether or not the round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  is equal to or less than a round trip propagation-time determination value determined in advance (step S 1709 ). In a result of the determination in step S 1709 , in a case where the calculated round trip propagation-time is equal to or less than the round trip propagation-time determination value determined in advance (“YES” in step S 1709 ), the phase adjustment unit not shown in the drawing determines that phase adjustment is performed. In this case, the phase adjustment unit not shown in the drawing updates the round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106 , that is, a round trip propagation-time which is a short period of time (small value) determined to be equal to or less than the round trip propagation-time determination value determined in advance, as a round trip propagation-time determination value when a round trip propagation-time calculated next by the round-trip-propagation-time measurement unit  106  is determined (step S 1916 ). Thereafter, the phase adjustment unit not shown in the drawing outputs a phase adjustment instruction for adjusting the phase of an imaging synchronization signal to the synchronization signal generation unit  102  on the basis of the round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  (step S 1915 ). Thereby, the synchronization signal generation unit  102  performs phase adjustment for an imaging synchronization signal to be generated, in response to the phase adjustment instruction which is output from the phase adjustment unit not shown in the drawing. 
     On the other hand, in a result of the determination in step S 1709 , in a case where the calculated round trip propagation-time is not equal to or less than the round trip propagation-time determination value determined in advance, that is, the calculated round trip propagation-time is greater than the round trip propagation-time determination value determined in advance (“NO” in step S 1709 ), the phase adjustment unit not shown in the drawing determines that phase adjustment is not performed. In this case, the phase adjustment unit not shown in the drawing terminates the process of step S 304 . 
     Meanwhile, the flowchart of the processing procedure of the image transfer system  1  which is shown in  FIG. 3  shows a case where the phase adjustment unit not shown in the drawing performs a process of updating a round trip propagation-time determination value at the time of determining a round trip propagation-time calculated next by the round-trip-propagation-time measurement unit  106  (step S 1916 ) in step S 304  even when it is determined that the round trip propagation-time calculated in step S 303  is not equal to or less than the round trip propagation-time determination value determined in advance (“NO” in step S 1709 ). 
     More specifically, the phase adjustment unit not shown in the drawing measures (accumulates) a period of time for which it is determined that phase adjustment for an imaging synchronization signal is not performed in step S 1709  (hereinafter, referred to as a “phase-unadjusted time”) and calculates a shift amount between phases of an imaging synchronization signal and a display synchronization signal (hereinafter, referred to as a “phase shift cumulative value”) on the basis of the accumulated phase-unadjusted time (step S 1703 ). In addition, the phase adjustment unit not shown in the drawing determines the calculated phase shift cumulative value on the basis of the round trip propagation-time determination value determined in advance (step S 2103 ). In a result of the determination in step S 2103 , in a case where the calculated phase shift cumulative value is greater than a predetermined rate of the round trip propagation-time determination value determined in advance, the phase adjustment unit not shown in the drawing estimates that the shift amount between the phases of the imaging synchronization signal and the display synchronization signal is large. In this case, the phase adjustment unit not shown in the drawing updates the roundtrip propagation-time determination value to a short period of time (small value) in step S 1916 . 
     Meanwhile, in a case where the phase adjustment unit not shown in the drawing estimates that the shift amount between the phases of the imaging synchronization signal and the display synchronization signal is large, the phase adjustment unit may be configured to output a phase adjustment instruction based on the round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  to the synchronization signal generation unit  102  in step S 1915 . Thereby, even when the phase adjustment unit not shown in the drawing estimates that the shift amount between the phases of the imaging synchronization signal and the display synchronization signal is large, the synchronization signal generation unit  102  performs phase adjustment for an imaging synchronization signal to be generated. Meanwhile, in a result of the determination in step S 1709 , in a case where the calculated round trip propagation-time is not equal to or less than the round trip propagation-time determination value determined in advance (“NO” in step S 1709 ), the phase adjustment unit not shown in the drawing may terminate the process of step S 304  without performing the process of step S 1915 . That is, in a case where it is determined that phase adjustment is not performed, the phase adjustment unit not shown in the drawing may not output a phase adjustment instruction. 
     On the other hand, in a result of determination in step S 2103 , in a case where the calculated phase shift cumulative value is equal to or less than the predetermined rate of the round trip propagation-time determination value determined in advance, the phase adjustment unit not shown in the drawing estimates that the shift amount between the phases of the imaging synchronization signal and the display synchronization signal is small. In this case, the phase adjustment unit not shown in the drawing continues accumulating a phase-unadjusted time to calculate a phase shift cumulative value. Meanwhile, in a case where the phase adjustment unit not shown in the drawing determines that phase adjustment is performed (“YES” in step S 1709 ), the phase adjustment unit initializes the phase shift cumulative value, that is, sets the phase-unadjusted time to be 0. Thereby, the phase adjustment unit not shown in the drawing measures (accumulates) again a phase-unadjusted time for which a state where phase adjustment for an imaging synchronization signal is not performed from a time when the last phase adjustment for an imaging synchronization signal was performed. 
     In this manner, in the image transfer system  1 , the imaging terminal  100  adjusts the phases of the imaging synchronization signal and the display synchronization signal so as not to be shifted with the elapse of time in the process of step S 304 . 
     Meanwhile, in the image transfer system  1 , when the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  100  can acquire a plurality of round trip propagation-time determination values in a period determined in advance which are used by the phase adjustment unit not shown in the drawing in order to determine whether or not phase adjustment for an imaging synchronization signal is to be performed, it is possible to estimate (calculate) an accuracy in a case where cycle adjustment for an imaging synchronization signal is temporarily executed. More specifically, in the image transfer system  1 , in step S 304  in the flowchart of the processing procedure of the image transfer system  1  shown in  FIG. 3 , when at least a process of updating the round trip propagation-time determination value in step S 1916  is performed, the after-cycle-adjustment accuracy estimation unit  105  can estimate the accuracy of an imaging synchronization signal. In this case, the phase adjustment unit not shown in the drawing updates the round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  as a round trip propagation-time determination value. In other words, in the image transfer system  1 , in step S 304 , the phase of an imaging synchronization signal does not necessarily need to be adjusted. Therefore, in the image transfer system  1 , in step S 304  in the flowchart of the processing procedure of the image transfer system  1  shown in  FIG. 3 , the processes of steps S 1703 , S 2103 , and S 1916  may be performed without performing the processes of steps S 1709  and S 1915 . Further, in the image transfer system  1 , in step S 304  in the flowchart of the processing procedure of the image transfer system  1  shown in  FIG. 3 , the processes of steps S 1916  and S 1915  may be performed without performing the processes of steps S 1709 , S 1703 , and S 2103 . Further, in the image transfer system  1 , in step S 304  in the flowchart of the processing procedure of the image transfer system  1  shown in  FIG. 3 , the processes of steps S 1703 , S 2103 , and S 1916  may be performed after the process of step S 1709  is performed. Further, in the image transfer system  1 , in step S 304  in the flowchart of the processing procedure of the image transfer system  1  shown in  FIG. 3 , the processes of steps S 1916  and S 1915  may be performed after the process of step S 1709  is performed. 
     Thereafter, in the image transfer system  1 , in step S 306 , the imaging terminal  100  adjusts the period of an imaging synchronization signal on the basis of the plurality of round trip propagation-time determination values updated in the process of step S 1916  included in step S 304 . 
     More specifically, in step S 306 , the after-cycle-adjustment accuracy estimation unit  105  determines whether or not a predetermined period of time determined in advance has elapsed after the period of a previous imaging synchronization signal is adjusted (step S 305 ). In a result of the determination in step S 305 , in a case where the predetermined period of time determined in advance has not elapsed after the period of the previous imaging synchronization signal is adjusted (“NO” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  105  determines that cycle adjustment is not performed. In this case, the after-cycle-adjustment accuracy estimation unit  105  terminates the process of step S 305 . 
     On the other hand, in a result of the determination in step S 305 , in a case where the predetermined period of time determined in advance has elapsed after the period of the previous imaging synchronization signal is adjusted (“YES” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  105  determines that cycle adjustment is performed. In this case, the after-cycle-adjustment accuracy estimation unit  105  acquires a combination of a plurality of round trip propagation-time determination values equivalent to a predetermined number of times per unit time determined in advance, scheduled transmission times of corresponding outward path signals for round trip propagation-time measurement, and scheduled transmission times of corresponding return path signals for round trip propagation-time measurement from the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  (step S 802 ). 
     Meanwhile, in the flowchart of the processing procedure of the image transfer system  1  which is shown in  FIG. 3 , in step S 304 , the phase adjustment unit not shown in the drawing performs a phase adjustment process for an imaging synchronization signal. For this reason, in step S 802 , the after-cycle-adjustment accuracy estimation unit  105  finally performs phase adjustment for an imaging synchronization signal and acquires a combination of the plurality of round trip propagation-time determination values equivalent to the predetermined number of times per unit time determined in advance which are used to determine a round trip propagation-time in a period in which is determined that phase adjustment is not performed, the scheduled transmission times of the corresponding outward path signals for round trip propagation-time measurement transmitted to calculate the respective round trip propagation-times, and the scheduled transmission times of the corresponding return path signals for round trip propagation-time measurement from the phase adjustment unit, not shown in the drawing. 
     Subsequently, the after-cycle-adjustment accuracy estimation unit  105  extracts a minimum round trip propagation-time determination value for each unit time from the plurality of roundtrip propagation-time determination values acquired (step S 803 ). That is, the after-cycle-adjustment accuracy estimation unit  105  extracts a minimum round trip propagation-time determination value of a predetermined number of times (hereinafter, referred to as a “minimum determination value”). 
     Thereafter, the after-cycle-adjustment accuracy estimation unit  105  calculates an estimation value of the accuracy of an imaging synchronization signal (period-adjusted accuracy estimation value) in a case where cycle adjustment is performed by temporarily waiting for the generation of an imaging synchronization signal for a period of time represented by a round trip propagation-time determination value of a minimum determination value, on the basis of information of the extracted minimum determination value of the predetermined number of times (step S 806 ). In addition, the after-cycle-adjustment accuracy estimation unit  105  outputs the calculated period-adjusted accuracy estimation value and information of a combination of a round trip propagation-time determination value, a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, and a scheduled transmission time of a round-trip-propagation-time-measurement returning signal which are acquired this time from the phase adjustment unit not shown in the drawing to the cycle adjustment determination unit  104 . 
     Subsequently, the cycle adjustment determination unit  104  determines whether or not the period-adjusted accuracy estimation value output from the after-cycle-adjustment accuracy estimation unit  105  is a value indicating that the accuracy of the period of an imaging synchronization signal has been improved (step S 1004 ). In the result of the determination in step S 1004 , in a case where cycle adjustment determination unit  104  determines that the accuracy of the period of an imaging synchronization signal has not been improved (“NO” in step S 1004 ), the process of step S 305  is terminated. That is, the imaging terminal  100  does not perform cycle adjustment to the period of an imaging synchronization signal which is temporarily estimated by the after-cycle-adjustment accuracy estimation unit  105  on the basis of a combination of a round trip propagation-time determination value, a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, and a scheduled transmission time of a round-trip-propagation-time-measurement returning signal which are acquired this time from the phase adjustment unit not shown in the drawing. 
     On the other hand, in a result of the determination in step S 1004 , in a case where the cycle adjustment determination unit  104  determines that the accuracy of the period of an imaging synchronization signal has been improved (“YES” in step S 1004 ), the cycle adjustment determination unit  104  outputs information of the cycle adjustment execution determination result indicating that the accuracy has been improved to the cycle adjustment unit  101 . Here, the information of the cycle adjustment execution determination result which is output to the cycle adjustment unit  101  by the cycle adjustment determination unit  104  includes a period-adjusted accuracy estimation value which is output from the after-cycle-adjustment accuracy estimation unit  105 , and information of a combination of a round trip propagation-time determination value, a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, and a scheduled transmission time of a round-trip-propagation-time-measurement returning signal which are acquired this time from the phase adjustment unit not shown in the drawing. 
     Thereby, the cycle adjustment unit  101  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal on the basis of a period-adjusted accuracy estimation value which is output from the cycle adjustment determination unit  104 , and a combination of a round trip propagation-time determination value, a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, and a scheduled transmission time of a round-trip-propagation-time-measurement returning signal (step S 1206 ). In addition, the cycle adjustment unit  101  outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, the synchronization signal generation unit  102  performs cycle adjustment for adjusting the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  101 . 
     In this manner, in the image transfer system  1 , the imaging terminal  100  performs adjustment so that the periods of the imaging synchronization signal and the display synchronization signal are not shifted with the elapse of time in the process of step S 306 . 
     Meanwhile, in step S 306  in the flowchart of the processing procedure of the image transfer system  1  which is shown in  FIG. 3 , a process in which the after-cycle-adjustment accuracy estimation unit  105  calculates a period-adjusted accuracy estimation value in steps S 802  to S 806  is collectively shown as step S 2106 . Further, in step S 306  in the flowchart of the processing procedure of the image transfer system  1  which is shown in  FIG. 3 , a process in which the cycle adjustment determination unit  104  determines whether or not the accuracy of the period of an imaging synchronization signal has been improved in step S 1004  and a process in which the cycle adjustment unit  101  calculates a cycle adjustment amount in step S 1206  are collectively shown as step S 2107 . 
     Hereinafter, each of processes in the image transfer system  1  will be described in more detail. First, a process of transmitting and receiving a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal in the image transfer system  1  (step S 302 ) will be described in more detail.  FIG. 4  is a flowchart showing a procedure of a process of transmitting and receiving a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal by the imaging terminal  100  constituting the image transfer system  1  in the first embodiment of the present invention. In addition,  FIG. 5  is a flowchart showing a procedure of a process of transmitting and receiving a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal by the display terminal  200  constituting the image transfer system  1  in the first embodiment of the present invention. 
     First, a procedure of a process of transmitting and receiving a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal by the imaging terminal  100  will be described with reference to  FIG. 4 . When a cycle adjustment process is started in the imaging terminal  100 , the round-trip-propagation-time measurement unit  106  determines a scheduled transmission time when a round-trip-propagation-time-measurement outgoing signal is scheduled to be transmitted to the display terminal  200  (step S 105 ). In addition, the round-trip-propagation-time measurement unit  106  generates a round-trip-propagation-time-measurement outgoing signal by the determined scheduled transmission time, outputs the generated round-trip-propagation-time-measurement outgoing signal to the wireless communication unit  108 , and transmits the round-trip-propagation-time-measurement outgoing signal to the display terminal  200  (step S 508 ). 
     Thereafter, the round-trip-propagation-time measurement unit  106  confirms whether or not the wireless communication unit  108  has received a round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200  in response to the transmitted round-trip-propagation-time-measurement outgoing signal (step S 501 ). Meanwhile, the process of step S 501  in which the round-trip-propagation-time measurement unit  106  confirms whether or not the wireless communication unit  108  has received a round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200  can be performed according to whether or not the round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200  has been output from the wireless communication unit  108 . 
     As a result of the confirmation in step S 501 , in a case where the wireless communication unit  108  has not received the round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200  (“NO” in step S 501 ), the round-trip-propagation-time measurement unit  106  repeats the process of step S 501  to wait for the round-trip-propagation-time-measurement returning signal to be transmitted from the display terminal  200 . On the other hand, as a result of the confirmation in step S 501 , in a case where the wireless communication unit  108  has received the round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200  (“YES” in step S 501 ), the round-trip-propagation-time measurement unit  106  measures a reception time of the round-trip-propagation-time-measurement returning signal on the basis of the round-trip-propagation-time-measurement returning signal which is output from the wireless communication unit  108  (step S 104 ). 
     Subsequently, a procedure of a process of transmitting and receiving a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal by the display terminal  200  will be described with reference to  FIG. 5 . When a cycle adjustment process is started in the display terminal  200 , the round-trip-propagation-time-measurement assistance unit  202  confirms whether or not the wireless communication unit  201  has received a round-trip-propagation-time-measurement outgoing signal which is transmitted from the imaging terminal  100  (step S 602 ). Meanwhile, the process of step S 602  in which the round-trip-propagation-time-measurement assistance unit  202  confirms whether or not the wireless communication unit  201  has received a round-trip-propagation-time-measurement outgoing signal transmitted from the imaging terminal  100  can be performed according to whether or not the round-trip-propagation-time-measurement outgoing signal transmitted from the imaging terminal  100  has been output from the wireless communication unit  201 . 
     As a result of the confirmation in step S 602 , in a case where the wireless communication unit  201  has not received the round-trip-propagation-time-measurement outgoing signal transmitted from the imaging terminal  100  (“NO” in step S 602 ), the round-trip-propagation-time-measurement assistance unit  202  repeats the process of step S 602  to wait for the round-trip-propagation-time-measurement outgoing signal to be transmitted from the imaging terminal  100 . On the other hand, as a result of the confirmation in step S 602 , in a case where the wireless communication unit  201  has received the round-trip-propagation-time-measurement outgoing signal transmitted from the imaging terminal  100  (“YES” in step S 602 ), the round-trip-propagation-time-measurement assistance unit  202  measures an input time which is a reception time of the round-trip-propagation-time-measurement outgoing signal on the basis of the round-trip-propagation-time-measurement outgoing signal which is output from the wireless communication unit  201  (step S 1104 ). 
     Thereafter, the round-trip-propagation-time-measurement assistance unit  202  determines a scheduled transmission time when a round-trip-propagation-time-measurement returning signal corresponding to a round-trip-propagation-time-measurement outgoing signal transmitted from the imaging terminal  100  is scheduled to be transmitted to the imaging terminal  100  (step S 1106 ). In addition, the round-trip-propagation-time-measurement assistance unit  202  generates a round-trip-propagation-time-measurement returning signal by the determined scheduled transmission time, outputs the generated round-trip-propagation-time-measurement returning signal to the wireless communication unit  201 , and transmits the round-trip-propagation-time-measurement returning signal to the imaging terminal  100  (step S 605 ). 
     In this manner, in the imaging terminal  100  of the image transfer system  1 , the round-trip-propagation-time measurement unit  106  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  200 , and measures a reception time of a round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200  in response to the transmitted round-trip-propagation-time-measurement outgoing signal. Further, in the display terminal  200  of the image transfer system  1 , the round-trip-propagation-time-measurement assistance unit  202  measures an input time of a round-trip-propagation-time-measurement outgoing signal transmitted from the imaging terminal  100  and transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal to the imaging terminal  100 . That is, in the image transfer system  1 , the display terminal  200  returns the round-trip-propagation-time-measurement returning signal in response to the round-trip-propagation-time-measurement outgoing signal transmitted from the imaging terminal  100 . 
     Meanwhile, in the image transfer system  1 , captured image data is wirelessly transferred from the imaging terminal  100  to the display terminal  200 . For this reason, transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal between the round-trip-propagation-time measurement unit  106  and the round-trip-propagation-time-measurement assistance unit  202  are performed in a period in which the captured image data is not wirelessly transferred. That is, in the image transfer system  1 , each of the round-trip-propagation-time measurement unit  106  and the round-trip-propagation-time-measurement assistance unit  202  performs transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal at a timing when wireless transfer of captured image data from the imaging terminal  100  to the display terminal  200  is not disturbed. Here, an example of a timing when transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between the round-trip-propagation-time measurement unit  106  and the round-trip-propagation-time-measurement assistance unit  202  will be described. 
       FIG. 6  is a diagram showing an example of a timing when transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed in the image transfer system  1  according to the first embodiment of the present invention.  FIG. 6  shows an example of a case where transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between the round-trip-propagation-time measurement unit  106  and the round-trip-propagation-time-measurement assistance unit  202  at a timing determined in advance in each of frames of an image in which captured image data is wirelessly transferred from the imaging terminal  100  to the display terminal  200 . The round-trip-propagation-time measurement unit  106  and the round-trip-propagation-time-measurement assistance unit  202  transmit and receive a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal in a period in which a packet of captured image data in a validity period in an image of one frame (hereinafter, referred to as a “valid packet”) is not wirelessly transferred. 
     An example shown in (a) of  FIG. 6  shows a case where a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are transmitted and received in a period until a valid packet is wirelessly transferred, in response to a timing of an imaging synchronization signal indicating the start of a validity period of an image in each of frames wirelessly transferred from the imaging terminal  100  to the display terminal  200 . More specifically, in the example shown in (a) of  FIG. 6 , the round-trip-propagation-time measurement unit  106  transmits a round-trip-propagation-time-measurement outgoing signal to the round-trip-propagation-time-measurement assistance unit  202 , and the round-trip-propagation-time-measurement assistance unit  202  transmits (returns) a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal to the round-trip-propagation-time measurement unit  106  in a period until the period of a valid packet is started (here, referred to as a “blanking period”) in wireless transfer of captured image data of each of the frames. 
     Meanwhile, in the example shown in (a) of  FIG. 6 , a timing when the round-trip-propagation-time measurement unit  106  and the round-trip-propagation-time-measurement assistance unit  202  transmit and receive a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal may be a timing when a header packet, including information such as the size of an image represented by captured image data, which is considered to be transferred during a blanking period is wirelessly transferred. 
     Further, an example shown in (b) of  FIG. 6  shows a case where a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are transmitted and received in a period until wireless transfer of the next frame is started, in response to a timing of an imaging synchronization signal indicating the termination of a validity period of an image in each of frames wirelessly transferred from the imaging terminal  100  to the display terminal  200 . More specifically, in the example shown in (b) of  FIG. 6 , the round-trip-propagation-time measurement unit  106  transmits a round-trip-propagation-time-measurement outgoing signal to the round-trip-propagation-time-measurement assistance unit  202 , and the round-trip-propagation-time-measurement assistance unit  202  transmits (returns) a round-trip-propagation-time-measurement returning signal to the round-trip-propagation-time measurement unit  106  in a period from a time when the period of a valid packet is terminated in wireless transfer of captured image data of each of the frames to a time when the period of the valid packet is started (here, also referred to as a “blanking period”) in wireless transfer of the next frame. 
     Meanwhile, in the example shown in (b) of  FIG. 6 , a timing when the round-trip-propagation-time measurement unit  106  and the round-trip-propagation-time-measurement assistance unit  202  transmit and receive a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal may be a timing when a header packet related to captured image data which is transferred during a blanking period is transmitted and received. 
     An example shown in (c) of  FIG. 6  shows a case where a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are transmitted and received during a period in which a valid packet is wirelessly transferred, in response to a timing of an imaging synchronization signal indicating one point in time determined in advance from a timing of an imaging synchronization signal indicating the start of a validity period of an image in each of the frames wirelessly transferred from the imaging terminal  100  to the display terminal  200 . More specifically, in the example shown in (c) of  FIG. 6 , the round-trip-propagation-time measurement unit  106  transmits a round-trip-propagation-time-measurement outgoing signal to the round-trip-propagation-time-measurement assistance unit  202 , and the round-trip-propagation-time-measurement assistance unit  202  transmits (returns) a round-trip-propagation-time-measurement returning signal to the round-trip-propagation-time measurement unit  106  at a timing of an imaging synchronization signal indicating that a delay period D determined in advance has elapsed after the period of a valid packet is started in wireless transfer of captured image data of each of the frames. 
     Meanwhile, also in the example shown in (c) of  FIG. 6 , the fact that the round-trip-propagation-time measurement unit  106  and the round-trip-propagation-time-measurement assistance unit  202  transmit and receive a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal so that wireless transfer of valid captured image data in an image of one frame is not disturbed is not changed. Therefore, also in the example shown in (c) of  FIG. 6 , the round-trip-propagation-time measurement unit  106  and the round-trip-propagation-time-measurement assistance unit  202  transmit and receive a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal in a period between any two valid packets in which captured image data is divided into a plurality of parts and wirelessly transferred during a period of a valid packet. For this reason, an imaging synchronization signal indicating one point in time determined in advance in the example shown in (c) of  FIG. 6  is an imaging synchronization signal indicating one point in time between any two valid packets after a delay period D has elapsed from the start of a period of a valid packet. Meanwhile, the imaging synchronization signal indicating one point in time determined in advance may be a synchronized packet indicating one point in time between any two valid packets after a delay period D has elapsed from the start of a period of a valid packet. 
     Meanwhile, a timing when the round-trip-propagation-time measurement unit  106  and the round-trip-propagation-time-measurement assistance unit  202  transmit and receive a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal is not limited to the timings shown in (a) to (c) of  FIG. 6 . That is, the round-trip-propagation-time measurement unit  106  and the round-trip-propagation-time-measurement assistance unit  202  may transmit and receive a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal at any timing as long as the timing is a timing when wireless transfer of valid captured image data in an image of one frame is not disturbed. 
     Here, an example of another timing when a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are transmitted and received between the round-trip-propagation-time measurement unit  106  and the round-trip-propagation-time-measurement assistance unit  202  will be described.  FIG. 7  is a diagram showing an example of another timing when a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are transmitted and received in the image transfer system  1  according to the first embodiment of the present invention.  FIG. 7  shows an example of a case where a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal which are included in a packet of an acknowledge (ACK) signal or a negative acknowledge (NAK) signal indicating the state of wireless transfer of a valid packet are transmitted and received when captured image data of an image of one frame is divided into a plurality of parts and wirelessly transferred from the imaging terminal  100  to the display terminal  200 . The round-trip-propagation-time measurement unit  106  and the round-trip-propagation-time-measurement assistance unit  202  transmit and receive a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal using a packet of an acknowledge (ACK) signal or a negative acknowledge (NAK) signal (hereinafter, referred to as an “ACK/NAK packet”) in any valid packet. 
     The example shown in  FIG. 7  shows a case where a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal which are included in an ACK/NAK packet corresponding to a first valid packet wirelessly transferred after an imaging synchronization signal indicating the start of a validity period of an image, among validity periods equivalent to one frame wirelessly transferred from the imaging terminal  100  to the display terminal  200 , are transmitted and received. More specifically, in the example shown in  FIG. 7 , the round-trip-propagation-time measurement unit  106  transmits a round-trip-propagation-time-measurement outgoing signal to the round-trip-propagation-time-measurement assistance unit  202  by including the round-trip-propagation-time-measurement outgoing signal in an ACK/NAK packet indicating a state where a first valid packet has been wirelessly transferred, and the round-trip-propagation-time-measurement assistance unit  202  transmits (returns) a round-trip-propagation-time-measurement returning signal to the round-trip-propagation-time measurement unit  106  after a period until the period of a valid packet is started (here, also referred to as a “blanking period”) in wireless transfer of captured image data of one frame. 
     Meanwhile, the example shown in  FIG. 7  shows a case where a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal which are included in an ACK/NAK packet corresponding to a first valid packet are transmitted and received, but the ACK/NAK packet for transmitting and receiving a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal may be an ACK/NAK packet corresponding to a valid packet with a specific order (for example, a fifth valid packet or the like) which is counted after a blanking period. Further, the example shown in  FIG. 7  shows a case where outward path signals for round trip propagation-time measurement and return path signals for round trip propagation-time measurement which are included in ACK/NAK packets corresponding to the same valid packet are transmitted and received, but an ACK/NAK packet for transmitting a round-trip-propagation-time-measurement outgoing signal and an ACK/NAK packet for transmitting a round-trip-propagation-time-measurement returning signal may be ACK/NAK packets corresponding to specific valid packet with different orders. 
     In this manner, in the image transfer system  1 , each of the round-trip-propagation-time measurement unit  106  and the round-trip-propagation-time-measurement assistance unit  202  transmits and receives a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal at a timing when wireless transfer of captured image data from the imaging terminal  100  to the display terminal  200  is not disturbed. Meanwhile, in the examples of a timing when a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are transmitted and received shown in  FIGS. 6 and 7 , a case where a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal which are included in any one packet at the timing of wirelessly transferring captured image data are transmitted and received has been described. However, the present invention is not limited to a configuration in which a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal which are included in a packet related to wireless transfer of captured image data are transmitted and received. For example, a configuration in which a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal which are included in another packet wirelessly transferred between the imaging terminal  100  and the display terminal  200  are transmitted and received may be adopted. In addition, for example, a configuration in which a dedicated packet indicating each of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal is wirelessly transferred between the imaging terminal  100  and the display terminal  200  may be adopted. 
     Thereafter, in the image transfer system  1 , times when each of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal is transmitted and received are clocked. Further, in the image transfer system  1 , a round trip propagation-time required for transmission and reception at the time of performing wireless transfer between the imaging terminal  100  and the display terminal  200  is calculated on the basis of information of the clocked times. 
     In the image transfer system  1 , the imaging terminal  100  calculates a round trip propagation-time required for transmission and reception at the time of performing wireless transfer between the imaging terminal  100  and the display terminal  200  through the process of step S 303 . In the process of calculating a round trip propagation-time in step S 303 , first, the round-trip-propagation-time measurement unit  106  calculates a difference between the scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal which is determined in step S 105  and the reception time of a round-trip-propagation-time-measurement returning signal which is clocked in step S 104 . In addition, the round-trip-propagation-time measurement unit  106  calculates a round trip propagation-time of a signal making a round trip in only wireless transfer between the imaging terminal  100  and the display terminal  200  by subtracting a receiver elapsed time extracted from the received round-trip-propagation-time-measurement returning signal from the time of the calculated difference. That is, the round-trip-propagation-time measurement unit  106  calculates a period of time required for the transmission of a round-trip-propagation-time-measurement outgoing signal and the reception of a round-trip-propagation-time-measurement returning signal in the process of calculating a round trip propagation-time in step S 303 , as a round trip propagation-time in wireless transfer. 
     Here, an example of a method of calculating a round trip propagation-time which is performed by the round-trip-propagation-time measurement unit  106  in step  3303  will be described.  FIG. 8  is a diagram showing an example of a method in which the imaging terminal  100  constituting the image transfer system  1  according to the first embodiment of the present invention calculates a round trip propagation-time of wireless transfer.  FIG. 8  shows a temporal relationship between a round-trip-propagation-time-measurement outgoing signal to be transmitted from the imaging terminal  100  to the display terminal  200  and a round-trip-propagation-time-measurement returning signal to be transmitted from the display terminal  200  to the imaging terminal  100 . 
     When the imaging terminal  100  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  200  in a case of an imaging terminal time=time Tsa (scheduled transmission time Tsa), the display terminal  200  can receive the round-trip-propagation-time-measurement outgoing signal when a display terminal time is time Tra 0  (input time Tra 0 ) in an ideal state with less delay time in wireless transfer. However, the round-trip-propagation-time-measurement outgoing signal transmitted by the imaging terminal  100  at the scheduled transmission time Tsa is received by the display terminal  200  when the display terminal time=time Tra (input time Tra) due to a delay of wireless transfer. On the other hand, when the display terminal  200  transmits a round-trip-propagation-time-measurement returning signal corresponding to a round-trip-propagation-time-measurement outgoing signal to the imaging terminal  100  in a case of a display terminal time=time Trb (scheduled transmission time Trb), the imaging terminal  100  can receive the round-trip-propagation-time-measurement returning signal when an imaging terminal time=time Tsb 0  (reception time Tsb 0 ) in an ideal state with less delay time in wireless transfer. However, the round-trip-propagation-time-measurement returning signal transmitted by the display terminal  200  at the scheduled transmission time Trb is received by the imaging terminal  100  when an imaging terminal time=time Tsb (reception time Tsb) due to a delay of wireless transfer. In addition, the round-trip-propagation-time measurement unit  106  calculates a round trip propagation-time in only wireless transfer between the imaging terminal  100  and the display terminal  200  in consideration of the state of wireless transfer shown in  FIG. 8 . 
     Here, in  FIG. 8 , a period of time between the scheduled transmission time Tsa and the input time Tra 0  is set to be a transmission time Td_a_ideal, and a period of time between the scheduled transmission time Trb and the reception time Tsb 0  is set to be a reception time Td_b_ideal. The transmission time Td_a_ideal and the reception time Td_b_ideal are stored in the round-trip-propagation-time measurement unit  106  in advance as components of an antenna passing time Td_ideal when a round trip propagation-time=0. In addition, in  FIG. 8 , a period of time between the scheduled transmission time Tsa and the input time Tra is set to be a transmission time Td_a_all, and a period of time between the scheduled transmission time Trb and the reception time Tsb is set to be a reception time Td_b_all. The transmission time Td_a_all and the reception time Td_b_all are calculated by the round-trip-propagation-time measurement unit  106  as components of an antenna passing time Td_all when a round trip propagation-time&gt;0. 
     In addition, in  FIG. 8 , a period of time between the scheduled transmission time Tsa when the round-trip-propagation-time-measurement outgoing signal is transmitted and the reception time Tsb when the imaging terminal  100  has received the round-trip-propagation-time-measurement returning signal, which can be clocked by the imaging terminal  100  on the basis of an imaging terminal time Ts, is set to be a total required time=send. In addition, in  FIG. 8 , a period of time between the input time Tra when the round-trip-propagation-time-measurement outgoing signal is received and the scheduled transmission time Trb when the display terminal  200  transmits the round-trip-propagation-time-measurement returning signal, that is, a receiver elapsed time which can be clocked by the display terminal  200  on the basis of a display terminal time Tr is set to be a receiver elapsed time ΔTrev. The receiver elapsed time=rev is calculated by the round-trip-propagation-time-measurement assistance unit  202 . More specifically, the round-trip-propagation-time-measurement assistance unit  202  calculates the receiver elapsed time ΔTrev by the following Math (1) in the state of wireless transfer shown in  FIG. 8 .
 
[Math 1]
 
Δ Trev=Trb−Tra   (1)
 
     In addition, the round-trip-propagation-time measurement unit  106  calculates a round trip propagation-time Td in only wireless transfer between the imaging terminal  100  and the display terminal  200  on the basis of the following Maths (2) to (6) which are established according to the state of wireless transfer shown in  FIG. 8  and the above-described definition.
 
[Math 2]
 
Δ T send= Tsb−Tsa   (2)
 
[Math 3]
 
 Td _all=Δ T send−Δ Trev   (3)
 
[Math 4]
 
 Td _all= Td _ a _all+ Td _ b _all  (4)
 
[Math 5]
 
 Td _ideal= Td _ a _ideal+ Td _ b _ideal  (5)
 
[Math 6]
 
 Td=Td _all− Td _ideal  (6)
 
     Meanwhile, the round-trip-propagation-time measurement unit  106  may calculate the round trip propagation-time Td without using the antenna passing time Td_ideal when a round trip propagation-time=0 and components thereof (a transmission time Td_a_ideal, a reception time Td_b_ideal). That is, the round-trip-propagation-time measurement unit  106  may calculates the round trip propagation-time Td by setting each of the antenna passing time Td_ideal, the transmission time Td_a_ideal, and the reception time Td_b_ideal to be “0”. The round trip propagation-time Td in this case can be calculated on the basis of the above-described Math (3). That is, the round trip propagation-time is set to be the same time (Td=Td_all) as the antenna passing time Td_all when a round trip propagation-time&gt;0. 
     The round-trip-propagation-time measurement unit  106  outputs information of the calculated round trip propagation-time Td to the phase adjustment unit not shown in the drawing. Thereby, in the image transfer system  1 , the phase of an imaging synchronization signal generated by the synchronization signal generation unit  102  is adjusted on the basis of a determination result for the round trip propagation-time Td calculated by the round-trip-propagation-time measurement unit  106  through the process of step S 304 . 
     In the process of step S 304 , in step S 1709 , in a case where the phase adjustment unit not shown in the drawing determines that the round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  is equal to or less than a round trip propagation-time determination value determined in advance (“YES” in step S 1709 ), the phase adjustment unit determines that phase adjustment for an imaging synchronization signal generated by the synchronization signal generation unit  102  is performed. For example, in a case where the round trip propagation-time determination value determined in advance is set to be a round trip propagation-time determination value Td_th, the phase adjustment unit not shown in the drawing compares the round trip propagation-time determination value with the round trip propagation-time Td calculated by the round-trip-propagation-time measurement unit  106  in step S 303 . Further, in a case where the compared round trip propagation-time Td is equal to or less than the round trip propagation-time determination value Td_th, the phase adjustment unit not shown in the drawing determines that phase adjustment for an imaging synchronization signal generated by the synchronization signal generation unit  102  is performed. In this case, the phase adjustment unit not shown in the drawing updates the round trip propagation-time Td which is equal to or less than the round trip propagation-time determination value Td_th, that is, a round trip propagation-time Td which is a shorter period of time (smaller value) than the round trip propagation-time determination value Td_th as a new round trip propagation-time determination value Td_th in step S 1916 . 
     Meanwhile, the new round trip propagation-time determination value Td_th updated in step S 1916  may be, for example, an average value between a current round trip propagation-time Td used for determination and a current round trip propagation-time determination value Td_th. In addition, the new round trip propagation-time determination value Td_th updated in step S 1916  may be, for example, a value obtained by statistical computation, such as a most frequent value of the current round trip propagation-time Td used for determination and a plurality of round trip propagation-times Td determined in the past. In addition, the new round trip propagation-time determination value Td_th updated in step S 1916  may be a fixed value determined in advance instead of a value obtained using a round trip propagation-time Td. 
     In a case where the round trip propagation-time determination value Td_th is updated in step S 1916 , the phase adjustment unit not shown in the drawing combines a previous round trip propagation-time determination value Td_th with a scheduled transmission time of the corresponding round-trip-propagation-time-measurement outgoing signal and a scheduled transmission time of the corresponding round-trip-propagation-time-measurement returning signal and stores the combination for a period determined in advance. Here, information of the combination of the round trip propagation-time determination value Td_th, the scheduled transmission time of the round-trip-propagation-time-measurement outgoing signal, and the scheduled transmission time of the round-trip-propagation-time-measurement returning signal, which is stored in the phase adjustment unit not shown in the drawing, is used when the after-cycle-adjustment accuracy estimation unit  105  calculates a period-adjusted accuracy estimation value later. 
     Further, in step S 1915 , the phase adjustment unit not shown in the drawing outputs a phase adjustment instruction for adjusting the phase of an imaging synchronization signal to the synchronization signal generation unit  102 . More specifically, the phase adjustment unit not shown in the drawing outputs a phase adjustment instruction which is regenerated after waiting for an imaging synchronization signal to be generated for the round trip propagation-time Td calculated by the round-trip-propagation-time measurement unit  106  in step S 303  to the synchronization signal generation unit  102 . Thereby, the synchronization signal generation unit  102  performs phase adjustment for an imaging synchronization signal to be generated, in response to the phase adjustment instruction which is output from the phase adjustment unit not shown in the drawing. 
     Meanwhile, the flowchart of the processing procedure of the image transfer system  1  which is shown in  FIG. 3  shows a case where the phase adjustment unit not shown in the drawing performs a process of updating a round trip propagation-time determination value (step S 1916 ) even when it is determined in step S 1709  that phase adjustment for an imaging synchronization signal to be generated by the synchronization signal generation unit  102  is not performed (“NO” in step S 1709 ) because the round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  is not equal to or less than the round trip propagation-time determination value determined in advance. In this case, the phase adjustment unit not shown in the drawing estimates a shift amount between phases of an imaging synchronization signal and a display synchronization signal by calculating a phase shift cumulative value. In addition, the phase adjustment unit not shown in the drawing updates a roundtrip propagation-time determination value determined in advance which is used to determine whether or not phase adjustment for an imaging synchronization signal is to be performed, on the basis of a result of the estimation of the shift amount between the phases of the imaging synchronization signal and the display synchronization signal. Here, a process in which the phase adjustment unit not shown in the drawing estimates a shift amount between phases of an imaging synchronization signal and a display synchronization signal to update a round trip propagation-time determination value (steps S 1703  to S 1916 ) will be described in more detail. 
       FIG. 9  is a flowchart showing a processing procedure of a process of updating a determination value of a round trip propagation-time (round trip propagation-time determination value) by the imaging terminal  100  constituting the image transfer system  1  according to the first embodiment of the present invention. 
     When the phase adjustment unit not shown in the drawing starts a process of updating a round trip propagation-time determination value, first, information of a cycle adjustment amount when the period of an imaging synchronization signal is adjusted in response to the cycle adjustment instruction output from the cycle adjustment unit  101  is acquired from the synchronization signal generation unit  102 . In addition, the phase adjustment unit not shown in the drawing determines whether or not the acquired cycle adjustment amount is a value other than “0” (the cycle adjustment amount≠0) (step S 2019 ). That is, the phase adjustment unit not shown in the drawing determines whether or not the synchronization signal generation unit  102  has performed cycle adjustment for an imaging synchronization signal in step S 2019 . 
     In a result of the determination in step S 2019 , in a case where the cycle adjustment amount is not a value other than “0”, that is, the cycle adjustment amount is “0” (“NO” in step S 2019 ), the phase adjustment unit not shown in the drawing causes the processing to proceed to step S 1703 . On the other hand, in a result of the determination in step S 2019 , in a case where the cycle adjustment amount is a value other than “0” (“YES” in step S 2019 ), the phase adjustment unit not shown in the drawing replaces the cycle adjustment amount with a periodic error (step S 2012 ). Here, the periodic error is defined as a predetermined value based on an output error in a source oscillation clock signal which is generated in the source oscillation clock generation unit  103  generating the source oscillation clock signal. Meanwhile, even when the source oscillation clock generation unit  103  and the source oscillation clock generation unit  205  are clock generators having the same specifications, the periodic error is an error which occurs in units of 100 ppm due to, for example, temperature characteristics according to the accuracy of the clock generators. For this reason, the periodic error may be set on the basis of information of specifications representing the accuracy of the clock generator, instead of replacing the cycle adjustment amount. 
     Thereafter, the phase adjustment unit not shown in the drawing measures an elapsed time in a state where phase adjustment for an imaging synchronization signal is not performed, that is, a period of time for which it is continuously determined that phase adjustment for an imaging synchronization signal is not performed (phase-unadjusted time) from a time when the last phase adjustment for an imaging synchronization signal was performed (step S 2018 ). 
     In addition, the phase adjustment unit not shown in the drawing calculates a phase shift cumulative value on the basis of the periodic error and the clocked phase-unadjusted time (step S 1703 ). For example, in a case where the cycle adjustment amount acquired from the synchronization signal generation unit  102  in step S 2019  is set to be a cycle adjustment amount E and the cycle adjustment amount E is a value other than “0”, it is assumed that the cycle adjustment amount is replaced with a periodic error E′ in step S 2012 . In addition, a case where the phase adjustment unit not shown in the drawing measures a phase-unadjusted time T′ in step S 2018  will be considered. In this case, the phase adjustment unit not shown in the drawing calculates a phase shift cumulative value V by multiplying the phase-unadjusted time T′ by the periodic error E′ as in the following Math (7).
 
[Math 7]
 
 V=T′×E′   (7)
 
     Meanwhile, in a result of the determination in step S 2019 , in a case where the cycle adjustment amount E is not a value other than “0”, that is, the cycle adjustment amount E=0 (“NO” in step S 2019 ), the phase adjustment unit not shown in the drawing considers the cycle adjustment amount E=periodic error E′=0 and calculates the phase shift cumulative value Vas the phase shift cumulative value V=0. 
     Subsequently, the phase adjustment unit not shown in the drawing determines whether or not the calculated phase shift cumulative value is equal to or less than a predetermined rate of the roundtrip propagation-time determination value determined in advance (step S 2103 ). In a result of the determination in step S 2103 , in a case where the calculated phase shift cumulative value is equal to or less than a predetermined rate of the round trip propagation-time determination value determined in advance (“YES” in step S 2103 ), the phase adjustment unit not shown in the drawing determines that the round trip propagation-time determination value is not updated and terminates the process of updating the round trip propagation-time determination value. 
     On the other hand, in a result of the determination in step S 2103 , in a case where the calculated phase shift cumulative value is not equal to or less than a predetermined rate of the round trip propagation-time determination value determined in advance, that is, the calculated phase shift cumulative value is greater than the predetermined rate of the round trip propagation-time determination value determined in advance (“NO” in step S 2103 ), the phase adjustment unit not shown in the drawing determines that the round trip propagation-time determination value is updated. In addition, the phase adjustment unit not shown in the drawing updates the round trip propagation-time determination value to a short period of time (small value) (step S 1916 ). For example, the phase adjustment unit not shown in the drawing updates a round trip propagation-time determination value Td_newth set to be a short period of time (small value) by multiplying the current round trip propagation-time determination value Td_th by a constant ε determined in advance (ε is a real number satisfying 1&lt;ε) as a new determination value Td_th as in the following Math (8).
 
[Math 8]
 
 Td _newth= Td _ th×ε   (8)
 
     Meanwhile, the new round trip propagation-time determination value updated by the phase adjustment unit not shown in the drawing in step S 1916  may be a value obtained by the same approach as that at the time of updating the round trip propagation-time determination value in step S 1916  in a case where it is determined that phase adjustment for an imaging synchronization signal is performed. That is, also in a case where the round trip propagation-time determination value is updated on the basis of a result of the estimation of a shift amount between phases of an imaging synchronization signal and a display synchronization signal, the phase adjustment unit not shown in the drawing may update a value on which statistical computation has been performed, such as an average value between the current round trip propagation-time Td used for the determination in step S 1709  and the current round trip propagation-time determination value Td_th or a most frequent value of a plurality of round trip propagation-times Td, a fixed value determined in advance, or the like as the new round trip propagation-time determination value Td_th in step S 1916 . 
     Further, in the image transfer system  1 , subsequently to phase adjustment for an imaging synchronization signal, cycle adjustment for an imaging synchronization signal is performed in step S 306  whenever a period of time determined in advance elapses. In the image transfer system  1 , first, when the after-cycle-adjustment accuracy estimation unit  105  determines that cycle adjustment for an imaging synchronization signal is performed due to the elapse of a predetermined period of time determined in advance (for example, one minute, five minutes, or several tens of minutes) in step S 305  included in step S 306  (YES in step S 305 ), the period-adjusted accuracy estimation unit calculates a period-adjusted accuracy estimation value in step S 2106  included in step S 306 . 
     More specifically, in step S 802  included in step S 2106 , the after-cycle-adjustment accuracy estimation unit  105  acquires a combination of a plurality of round trip propagation-time determination values equivalent to a predetermined number of times per unit time determined in advance (for example, 10 seconds), scheduled transmission times of corresponding outward path signals for round trip propagation-time measurement, and scheduled transmission times of corresponding return path signals for round trip propagation-time measurement from the phase adjustment unit not shown in the drawing. 
     Meanwhile, in step S 802 , the after-cycle-adjustment accuracy estimation unit  105  may acquire, for example, a combination of a plurality of round trip propagation-time determination values equivalent to a predetermined number of frames per unit time determined in advance, scheduled transmission times of corresponding outward path signals for round trip propagation-time measurement, and scheduled transmission times of corresponding return path signals for round trip propagation-time measurement from the phase adjustment unit not shown in the drawing. 
     Further, in step S 803  included in step S 2106 , the after-cycle-adjustment accuracy estimation unit  105  extracts a minimum round trip propagation-time determination value (minimum determination value) for each unit time from the plurality of round trip propagation-time determination values acquired. For example, in a case where a unit time B is 10 seconds and a predetermined number of times is six, the after-cycle-adjustment accuracy estimation unit  105  acquires a combination of roundtrip propagation-time determination values equivalent to 60 seconds (one minute), scheduled transmission times of outward path signals for round trip propagation-time measurement, and scheduled transmission times of return path signals for round trip propagation-time measurement from the phase adjustment unit not shown in the drawing in step S 802 . In this case, the after-cycle-adjustment accuracy estimation unit  105  extracts six minimum determination values. 
     Meanwhile, in step S 803 , the after-cycle-adjustment accuracy estimation unit  105  may extract, for example, an average value of a plurality of round trip propagation-time determination values acquired for each unit time. Further, in step S 803 , the after-cycle-adjustment accuracy estimation unit  105  may extract, for example, a maximum round trip propagation-time determination value for each unit time from the plurality of round trip propagation-time determination values acquired. 
     Further, in a case where the after-cycle-adjustment accuracy estimation unit  105  acquires each of pieces of information equivalent to a predetermined number of frames per unit time determined in advance in step S 802 , for example, an average value of the round trip propagation-time determination values equivalent to a predetermined number of times per unit time determined in advance which are obtained for each of the frames may be extracted in step S 803 . In this case, the after-cycle-adjustment accuracy estimation unit  105  also sets each of the scheduled transmission times of the corresponding outward path signals for round trip propagation-time measurement and the scheduled transmission time of the corresponding round-trip-propagation-time-measurement returning signal to be an average value, similar to the obtained average value of the round trip propagation-time determination values. Further, in a case where the after-cycle-adjustment accuracy estimation unit  105  acquires each of the pieces of information equivalent to the predetermined number of frames per unit time determined in advance in step S 802 , the period-adjusted accuracy estimation unit may extract, for example, a minimum value of the round trip propagation-time determination values equivalent to the predetermined number of times per unit time determined in advance which are obtained for each of the frames in step S 803 . In this case, the after-cycle-adjustment accuracy estimation unit  105  also sets each of the scheduled transmission times of the corresponding outward path signals for round trip propagation-time measurement and the scheduled transmission time of the corresponding round-trip-propagation-time-measurement returning signal to be a minimum value, similar to the obtained minimum value of the round trip propagation-time determination values. Further, in a case where the after-cycle-adjustment accuracy estimation unit  105  acquires each of the pieces of information equivalent to the predetermined number of frames per unit time determined in advance in step S 802 , the period-adjusted accuracy estimation unit may extract, for example, a maximum value of the round trip propagation-time determination values equivalent to the predetermined number of times per unit time determined in advance which are obtained for each of the frames in step S 803 . In this case, the after-cycle-adjustment accuracy estimation unit  105  also sets each of the scheduled transmission times of the corresponding outward path signals for round trip propagation-time measurement and the scheduled transmission times of the corresponding return path signals for round trip propagation-time measurement to be a maximum value, similar to the obtained maximum value of the round trip propagation-time determination values. 
     Thereafter, in step S 806  included in step S 2106 , the after-cycle-adjustment accuracy estimation unit  105  calculates a period-adjusted accuracy estimation value in a case where the period of an imaging synchronization signal is temporarily adjusted, on the basis of the extracted minimum determination value equivalent to a predetermined number of times. 
     Here, an example of the process of step S 2106  in which the after-cycle-adjustment accuracy estimation unit  105  calculates a period-adjusted accuracy estimation value will be described. 
       FIG. 10  is a diagram showing an example of a process of calculating a period-adjusted accuracy estimation value in the image transfer system  1  according to the first embodiment of the present invention.  FIG. 10  shows a case where the after-cycle-adjustment accuracy estimation unit  105  acquires a plurality of round trip propagation-time determination values for each unit time and extracts a minimum determination value from the plurality of round trip propagation-time determination values acquired for each unit time. More specifically, the drawing shows a case where a minimum determination value Tdmin_y_1 to a minimum determination value Tdmin_y_2 m equivalent to 2m times by setting a plurality of round trip propagation-time determination values Td_th included in a unit time B to be a population, acquiring the population 2m times (m is a natural number of 2 or greater, a positive integer) to acquire a plurality of round trip propagation-time determination values Td_th, and extract a minimum determination value for each of the populations (unit time B). 
     The after-cycle-adjustment accuracy estimation unit  105  calculates an accuracy error of the period of an imaging synchronization signal in a case where a generation timing is waited for a period of time represented in a minimum determination value so as to be delayed as a period-adjusted accuracy estimation value, on the basis of information regarding a minimum determination value extracted first and information regarding a minimum determination value extracted last, among the extracted minimum determination values equivalent to a predetermined number of times. In other words, the after-cycle-adjustment accuracy estimation unit  105  calculates a period-adjusted accuracy estimation value on the basis of information regarding a minimum determination value separated by a period equivalent to a unit time B×(2m−2) times. More specifically, the after-cycle-adjustment accuracy estimation unit  105  performs phase adjustment for an imaging synchronization signal lastly and then calculates a period-adjusted accuracy estimation value on the basis of a difference between a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal transmitted to calculate a round trip propagation-time for which it is first determined that phase adjustment is not performed and a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal transmitted to calculate a round trip propagation-time for which it is lastly determined that phase adjustment is not performed. 
     Here, a method of estimating an accuracy error of the period of an imaging synchronization signal (period-adjusted accuracy estimation value) will be described in more detail with reference to  FIG. 10 . As shown in  FIG. 10 , it is assumed that the after-cycle-adjustment accuracy estimation unit  105  acquires round trip propagation-time determination values equivalent to a predetermined number of times=2m times for each unit time B [seconds (sec)], and minimum determination values extracted in the respective unit times B are a minimum determination value Tdmin_y_1 [seconds (sec)] to a minimum determination value Tdmin_y_2m [seconds (sec)]. 
     The after-cycle-adjustment accuracy estimation unit  105  calculates (estimates) a period-adjusted accuracy estimation value Er [ppm] on the basis of the following Math (9) because a ratio of a value, obtained by subtracting a difference between a scheduled transmission time and a minimum determination value in a first unit time B from a difference between a scheduled transmission time and a minimum determination value in a last unit time B, to a difference between the scheduled transmission time in the last unit time B and the scheduled transmission time in the first unit time B is an accuracy error occurring in cycle adjustment for an imaging synchronization signal. 
     
       
         
           
             
               
                 
                   
                       
                   
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     In the above-described Math (9), TS_y_1 [seconds (sec)] is a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal transmitted to calculate the minimum determination value Tdmin_y_1 [seconds (sec)] extracted in the first unit time B. Further, in the above-described Math (9), TS_y_2m[seconds (sec)] is a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal transmitted to calculate the minimum determination value Tdmin_y_2m [seconds (sec)] extracted in the last unit time B. 
     Meanwhile, the after-cycle-adjustment accuracy estimation unit  105  may acquire a larger number of round trip propagation-time determination values to calculate a period-adjusted accuracy estimation value in a case where the cycle adjustment determination unit  104  determines that the period-adjusted accuracy estimation value calculated on the basis of the above-described Math (9) is a value indicating that the accuracy of the period of an imaging synchronization signal has not been improved (“NO” in step S 1004 ) and the cycle adjustment unit  101  has not output a cycle adjustment instruction to the synchronization signal generation unit  102 . Here, a description will be provided of an example of the process of step S 2106  in a case where the after-cycle-adjustment accuracy estimation unit  105  acquires a larger number of round trip propagation-time determination values to calculate a period-adjusted accuracy estimation value when the cycle adjustment determination unit  104  determines that the accuracy of the period of an imaging synchronization signal has not been improved. 
       FIG. 11  is a diagram showing another example of a process of calculating a period-adjusted accuracy estimation value in the image transfer system  1  according to the first embodiment of the present invention. The example shown in  FIG. 11  shows a case where the after-cycle-adjustment accuracy estimation unit  105  increases a unit time for acquiring a round trip propagation-time determination value to extract a minimum determination value from a plurality of round trip propagation-time determination values acquired in respective unit times. More specifically, the example shown in  FIG. 10  shows a case where the same number of minimum determination values as that in the example shown in  FIG. 10 , that is, a minimum determination value Tdmin_y_1′ to a minimum determination value Tdmin_y_2m′ equivalent to 2m times are extracted by setting a population of round trip propagation-time determination values Td_th acquired in units of unit times B [seconds (sec)] to be twice, that is, by doubling the unit time B (unit time 2B [seconds (sec)]) and acquiring the population m times to extract a minimum determination value. 
     In this case, the after-cycle-adjustment accuracy estimation unit  105  does not acquire around trip propagation-time determination value Td_th, which is already acquired before the cycle adjustment determination unit  104  determines that the accuracy of the period of an imaging synchronization signal has not been improved, again at the time of extracting a minimum determination value for each unit time 2B [seconds (sec)]. In addition, the after-cycle-adjustment accuracy estimation unit  105  compares minimum determination values of two adjacent unit times B, which are already extracted for each unit time B [seconds (sec)], with each other and selects a smaller minimum determination value as shown in an upper stage of  FIG. 11  to extract a minimum determination value Tdmin_y_1′ to a minimum determination value Tdmin_y_m′ equivalent to m times for each unit time 2B as shown in a lower stage of  FIG. 11 . 
     More specifically, in the example shown in  FIG. 11 , a minimum determination value Tdmin_y_1 extracted in a first unit time B [seconds (sec)] is compared with a minimum determination value Tdmin_y_2 extracted in a second unit time B [seconds (sec)], and the minimum determination value Tdmin_y_2 smaller than the minimum determination value Tdmin_y_1 is selected and set to be a minimum determination value Tdmin_y_1′. Further, in the example shown in  FIG. 11 , a minimum determination value Tdmin_y_(2m−1) extracted in a (2m−1)-th unit time B [seconds (sec)] is compared with a minimum determination value Tdmin_y_2m extracted in a 2m-th unit time B [seconds (sec)], and the minimum determination value Tdmin_y_2m smaller than the minimum determination value Tdmin_y_(2m−1) is selected and set to be a minimum determination value Tdmin_y_m′. 
     In the example shown in  FIG. 11 , a case where minimum determination values of two adjacent unit times B, which are already extracted for each unit time B, are compared with each other and a smaller minimum determination value is selected has been described, but a minimum determination value selected by the after-cycle-adjustment accuracy estimation unit  105  may be, for example, a larger minimum determination value when minimum determination values of two adjacent unit times B are compared with each other. In addition, a minimum determination value selected by the after-cycle-adjustment accuracy estimation unit  105  may be, for example, an average value of minimum determination values of two adjacent unit times B. 
     In addition, the after-cycle-adjustment accuracy estimation unit  105  further acquires round trip propagation-time determination values Td_th equivalent to m times for each unit time 2B [seconds (sec)] and extracts a minimum determination value for each unit time 2B. Meanwhile, also in the example shown in  FIG. 11 , a method of extracting a minimum determination value for each unit time 2B is the same as a method of extracting a minimum determination value for each unit time B in the example shown in  FIG. 10  except that the number of round trip propagation-time determination values Td_th included in a unit time 2B (population) is different. The example shown in  FIG. 11  shows a case where a minimum round trip propagation-time determination value Td_th is extracted as a minimum determination value Tdmin_y_2m′ from a plurality of round trip propagation-time determination values Td_th included in a last unit time 2B [seconds (sec)]. 
     Thereafter, similarly to the example shown in  FIG. 10 , the after-cycle-adjustment accuracy estimation unit  105  calculates a period-adjusted accuracy estimation value on the basis of information regarding a minimum determination value separated by a period equivalent to a unit time 2B×(2m−2) times. More specifically, the after-cycle-adjustment accuracy estimation unit  105  applies each of a set including a minimum determination value Tdmin_y_1′ [seconds (sec)] extracted first and a scheduled transmission time TS_y_1′ [seconds (sec)] of a corresponding round-trip-propagation-time-measurement outgoing signal and a set including a minimum determination value Tdmin_y_2m′ [seconds (sec)] extracted last and a scheduled transmission time TS_y_2m′ [seconds (sec)] of a corresponding round-trip-propagation-time-measurement outgoing signal to the above-described Math (9) to calculate (estimate) a period-adjusted accuracy estimation value Er′ [ppm]. 
     Meanwhile, the after-cycle-adjustment accuracy estimation unit  105  may acquire a round trip propagation-time determination value in another period in the same unit time to calculate a period-adjusted accuracy estimation value in a case where the cycle adjustment determination unit  104  determines that the period-adjusted accuracy estimation value calculated on the basis of the above-described Math (9) is a value indicating that the accuracy of the period of an imaging synchronization signal has not been improved (“NO” in step S 1004 ) and the cycle adjustment unit  101  has not output a cycle adjustment instruction to the synchronization signal generation unit  102 . Here, a description will be provided of an example of the process of step S 2106  in a case where the after-cycle-adjustment accuracy estimation unit  105  acquires a round trip propagation-time determination value in another period in the same unit time to calculate a period-adjusted accuracy estimation value when the cycle adjustment determination unit  104  determines that the accuracy of the period of an imaging synchronization signal has not been improved. 
       FIG. 12  is a diagram showing still another example of a process of calculating a period-adjusted accuracy estimation value in the image transfer system  1  according to the first embodiment of the present invention. The example shown in FIG.  12  shows a case where the after-cycle-adjustment accuracy estimation unit  105  discards a minimum determination value extracted in each of the previous unit times and newly acquires a plurality of round trip propagation-time determination values equivalent to a predetermined number of times in the same unit time in a period different from the previous period to extract a minimum determination value for each unit time. More specifically, the drawing shows a case where the same number of minimum determination values as the previous number, that is, a minimum determination value Tdmin_y_1″ to a minimum determination value Tdmin_y_2m″ equivalent to 2m times are extracted by discarding a population of round trip propagation-time determination values Td_th acquired in units of unit times B′ [seconds (sec)] similar to the previous example shown in  FIG. 10  and setting round trip propagation-time determination values Td_th acquired in units of unit times B′ [seconds (sec)] in a period different from the previous period as a new population to extract a minimum determination value. Meanwhile, the unit time B′ [seconds (sec)] in the example shown in  FIG. 12  may be the unit time in the example shown  FIG. 10  or  FIG. 12 , that is, the unit time B [seconds (sec)] or the unit time 2B [seconds (sec)], but may be different from the unit time in the example shown in  FIG. 10  or  FIG. 12 . 
     In the example shown in  FIG. 12 , as shown in an upper stage of  FIG. 12 , the minimum determination value Tdmin_y_1″ to the minimum determination value Tdmin_y_2m″ extracted from the round trip propagation-time determination values Td_th acquired before in each unit time B′ [seconds (sec)] are discarded. In addition, as shown in a lower stage of  FIG. 12 , a plurality of round trip propagation-time determination values Td_th are acquired in a new period, and a new minimum determination value Tdmin_y_1″ to minimum determination value Tdmin_y_2m″ are extracted in each of unit times B′ [seconds (sec)]. Meanwhile, also in the example shown in  FIG. 12 , a method of extracting a minimum determination value for each unit time B′ is the same as the method of extracting a minimum determination value for each unit time in the example shown in  FIG. 10  or  FIG. 12 . However, as described above, the number of round trip propagation-time determination values Td_th included in a unit time B′ (population) may be different from the number of round trip propagation-time determination values Td_th included in the unit time B (population) or the unit time 2B (population) in the example shown in  FIG. 10  or  FIG. 12 . 
     Thereafter, similarly to the example shown in  FIG. 10  or  FIG. 12 , the after-cycle-adjustment accuracy estimation unit  105  calculates a period-adjusted accuracy estimation value on the basis of information regarding a minimum determination value separated by a period equivalent to a unit time B′×(2m−2) times. More specifically, the after-cycle-adjustment accuracy estimation unit  105  applies each of a set including a new minimum determination value Tdmin_y_1″ [seconds (sec)] extracted first and a scheduled transmission time TS_y_1″ [seconds (sec)] of a corresponding round-trip-propagation-time-measurement outgoing signal and a set including a new minimum determination value Tdmin_y_2m″ [seconds (sec)] extracted last and a scheduled transmission time TS_y_2m″ [seconds (sec)] of a corresponding round-trip-propagation-time-measurement outgoing signal to the above-described Math (9) to calculate (estimate) a period-adjusted accuracy estimation value Er″ [ppm]. 
     Meanwhile, the examples shown in  FIGS. 10 to 12  show a case where the after-cycle-adjustment accuracy estimation unit  105  calculates a period-adjusted accuracy estimation value according to the above-described Math (9) on the basis of information regarding a minimum determination value extracted first and information regarding a minimum determination value extracted last. However, the minimum determination values used when the after-cycle-adjustment accuracy estimation unit  105  calculates a period-adjusted accuracy estimation value according to the above-described Math (9) are not limited to minimum determination values extracted first and last. For example, the after-cycle-adjustment accuracy estimation unit  105  may calculate a period-adjusted accuracy estimation value according to the above-described Math (9) by using minimum determination values extracted in any two unit times in specific orders among extracted minimum determination values equivalent to a predetermined number of times. 
     Meanwhile, the after-cycle-adjustment accuracy estimation unit  105  executes the above-described process of calculating a period-adjusted accuracy estimation value (step S 2106 ) in a case where it is determined in step S 305  that cycle adjustment for an imaging synchronization signal is performed with the elapse of a predetermined period of time determined in advance (for example, one minute, five minutes, or several tens of minutes) (“YES” in step S 305 ). However, also in a case where the after-cycle-adjustment accuracy estimation unit  105  determines that cycle adjustment for an imaging synchronization signal is not performed in step S 305  (“NO” in step S 305 ), the period-adjusted accuracy estimation unit may perform a process of calculating a period-adjusted accuracy estimation value. That is, the after-cycle-adjustment accuracy estimation unit  105  may perform a process of calculating a period-adjusted accuracy estimation value in step S 2106  in parallel with a phase adjustment process for an imaging synchronization signal which is performed by the phase adjustment unit not shown in the drawing. In this case, for example, the after-cycle-adjustment accuracy estimation unit  105  may acquire a round trip propagation-time determination value from the phase adjustment unit not shown in the drawing for each predetermined period of time (for example, one minute or five minutes) which is shorter than a predetermined period of time (for example, several tens of minutes) for determining whether or not cycle adjustment for an imaging synchronization signal is performed, and may calculate a period-adjusted accuracy estimation value. In addition, the after-cycle-adjustment accuracy estimation unit  105  may repeat a process of calculating a period-adjusted accuracy estimation value in step S 2106  using the same method as in the examples shown in  FIGS. 10 to 12  on the assumption that the cycle adjustment determination unit  104  determines that the calculated period-adjusted accuracy estimation value is a value indicating that the accuracy of the period of an imaging synchronization signal has not been improved (“NO” in step S 1004 ) and the cycle adjustment unit  101  has not output a cycle adjustment instruction to the synchronization signal generation unit  102 . 
     More specifically, when a predetermined period of time (for example, one minute or five minutes) which is shorter than a predetermined period of time (for example, several tens of minutes) for determining whether or not cycle adjustment for an imaging synchronization signal is performed has elapsed first, the after-cycle-adjustment accuracy estimation unit  105  first extracts a minimum determination value for each unit time B to calculate a period-adjusted accuracy estimation value using the same method as that in the example shown in  FIG. 10 . Thereafter, when a predetermined period of time (for example, one minute or five minutes) which is shorter than a predetermined period of time (for example, several tens of minutes) for determining whether or not cycle adjustment for an imaging synchronization signal is performed has further elapsed, the period-adjusted accuracy estimation unit extracts a minimum determination value for each unit time 2B obtained by lengthening the unit time B to calculate a period-adjusted accuracy estimation value using the same method as that in the example shown in  FIG. 11 . Thereafter, when a predetermined period of time (for example, one minute or five minutes) which is shorter than a predetermined period of time (for example, several tens of minutes) for determining whether or not cycle adjustment for an imaging synchronization signal is performed has further elapsed, the period-adjusted accuracy estimation unit extracts a minimum determination value for each unit time B′ (unit time 2B=unit time B′) which is the same as the unit time 2B in a period different from a period in which a period-adjusted accuracy estimation value is previously calculated to calculate a period-adjusted accuracy estimation value using the same method as that in the example shown in  FIG. 12 . Thereby, the after-cycle-adjustment accuracy estimation unit  105  can also estimate the accuracy of an imaging synchronization signal when cycle adjustment for an imaging synchronization signal is not performed. 
     Meanwhile, in a case where the after-cycle-adjustment accuracy estimation unit  105  calculates a period-adjusted accuracy estimation value using the same method as that in the example shown in  FIG. 12 , the number of times a period-adjusted accuracy estimation value is calculated using the same method as that in the example shown in  FIG. 11  before that may be one or two or more. For example, the after-cycle-adjustment accuracy estimation unit  105  may calculate a period-adjusted accuracy estimation value twice using the same method as that in the example shown in  FIG. 11  and then calculate a period-adjusted accuracy estimation value using the same method as that in the example shown in  FIG. 12  by setting the unit time 2B×2=a unit time 4B to be a unit time B′ (unit time 4B=unit time B′). However, the number of times the after-cycle-adjustment accuracy estimation unit  105  calculates a period-adjusted accuracy estimation value using the same method as that in the example shown in  FIG. 11  before calculating a period-adjusted accuracy estimation value using the same method as that in the example shown in  FIG. 12  may be set to be the number of times falling within a range in which the period of the unit time 2B equivalent to a predetermined number of times=2m times does not exceed a predetermined period of time (for example, several tens of minutes) for determining whether or not cycle adjustment for an imaging synchronization signal is performed, 
     Thereafter, in the image transfer system  1 , when the after-cycle-adjustment accuracy estimation unit  105  calculates a period-adjusted accuracy estimation value in step S 2106 , the cycle adjustment determination unit  104  determines the period-adjusted accuracy estimation value in step S 2107  of step S 306 , and the cycle adjustment unit  101  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal. 
     More specifically, in step S 1004  included in step S 2107 , the cycle adjustment determination unit  104  determines whether or not the period-adjusted accuracy estimation value calculated (estimated) by the after-cycle-adjustment accuracy estimation unit  105  is a value indicating that the accuracy of the period of an imaging synchronization signal has been improved. 
     Here, an example of the process of step S 1004  in which the cycle adjustment determination unit  104  determines a period-adjusted accuracy estimation value will be described. In the determination of a period-adjusted accuracy estimation value by the cycle adjustment determination unit  104 , first, a range in which a period-adjusted accuracy estimation value can be taken is obtained, and an absolute maximum value in the obtained range in which a period-adjusted accuracy estimation value can be taken is obtained. In addition, the cycle adjustment determination unit  104  determines whether or not a period-adjusted accuracy estimation value has been improved by comparing a past absolute maximum value with a current absolute maximum value. 
     For example, when a minimum value of a period-adjusted accuracy estimation value Er calculated (estimated) by the after-cycle-adjustment accuracy estimation unit  105  is set to be a minimum value Er_min and a maximum value of the period-adjusted accuracy estimation value Er is set to be a maximum value Er_max, a case where it is assumed that the current period-adjusted accuracy estimation value Er is in a relationship of Er_min≤Er≤Er_max is considered. In this case, the cycle adjustment determination unit  104  determines that the current period-adjusted accuracy estimation value Er has been improved when an absolute value |Er_min| of a minimum value Er_min at the present point in time is smaller than an absolute value |Er_min| at a past point in time, and an absolute value |Er_max| of a maximum value Er_max at the present point in time is smaller than an absolute value |Er_max| at a past point in time. 
     Here, assuming that the cycle adjustment unit  101  has already performed cycle adjustment that has been performed once or more on the basis of a maximum value Er_max until the present time, an accuracy error of the period of an imaging synchronization signal (period-adjusted accuracy estimation value Er) is increased to become a maximum value Er_max when the minimum determination value Tdmin_y_1&gt;0 [seconds (sec)] and the minimum determination value Tdmin_y_2m=0 [seconds (sec)] in the above-described Math (9). For example, in a case where the after-cycle-adjustment accuracy estimation unit  105  extracts a minimum determination value in each unit time B to calculate (estimate) a period-adjusted accuracy estimation value Er as in the example shown in  FIG. 10 , the period-adjusted accuracy estimation value Er is set to be a maximum value Er_max when a minimum determination value Tdmin_y_1 extracted in a first unit time B is larger than 0 seconds and a minimum determination value Tdmin_y_2m extracted in a 2m-th unit time B is 0 seconds. From this, the cycle adjustment determination unit  104  calculates a maximum value Er_max[ppm] of a period-adjusted accuracy estimation value Er on the basis of the following Math (10) in which each of a set including a minimum determination value Tdmin_y_1&gt;0 [seconds (sec)] and a corresponding scheduled transmission time TS_y_1 [seconds (sec)] and a set including a minimum determination value Tdmin_y_2m=0 [seconds (sec)] and a corresponding scheduled transmission time TS_y_2m [seconds (sec)] is applied to the above-described Math (9). 
     
       
         
           
             
               
                 
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                     + 
                     Er_max 
                   
                   = 
                   
                     
                       
                         TS_y 
                         ⁢ 
                         _ 
                         ⁢ 
                         2 
                         ⁢ 
                         m 
                       
                       - 
                       
                         ( 
                         
                           
                             TS_y 
                             ⁢ 
                             _ 
                             ⁢ 
                             1 
                           
                           - 
                           
                             Td 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             min_y 
                             ⁢ 
                             _ 
                             ⁢ 
                             1 
                           
                         
                         ) 
                       
                     
                     
                       
                         TS_y 
                         ⁢ 
                         _ 
                         ⁢ 
                         2 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         m 
                       
                       - 
                       
                         TS_y 
                         ⁢ 
                         _ 
                         ⁢ 
                         1 
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
             
               
                 
                   
                     ∴ 
                     Er_max 
                   
                   = 
                   
                     
                       Td 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       min_y 
                       ⁢ 
                       _ 
                       ⁢ 
                       1 
                     
                     
                       
                         TS_y 
                         ⁢ 
                         _ 
                         ⁢ 
                         2 
                         ⁢ 
                         m 
                       
                       - 
                       
                         TS_y 
                         ⁢ 
                         _ 
                         ⁢ 
                         1 
                       
                     
                   
                 
               
               
                 
                     
                 
               
             
           
         
       
     
     On the other hand, assuming that the cycle adjustment unit  101  has already performed cycle adjustment that has been performed once or more on the basis of a minimum value Er_min until the present time, a period-adjusted accuracy estimation value Er is decreased to become a minimum value Er_min when the minimum determination value Tdmin_y_1=0 [seconds (sec)] and the minimum determination value Tdmin_y_2m&gt;0 [seconds (sec)] in the above-described Math (9). For example, in a case where the after-cycle-adjustment accuracy estimation unit  105  extracts a minimum determination value in each unit time B to calculate (estimate) a period-adjusted accuracy estimation value Er as in the example shown in  FIG. 10 , the period-adjusted accuracy estimation value Er is set to be a minimum value Er_min when a minimum determination value Tdmin_y_1 extracted in a first unit time B is 0 seconds and a minimum determination value Tdmin_y_2m extracted in a 2m-th unit time B is larger than 0 seconds. From this, the cycle adjustment determination unit  104  calculates a minimum value Er_min[ppm] of a period-adjusted accuracy estimation value Er on the basis of the following Math (11) in which each of a set including a minimum determination value Tdmin_y_1=0 [seconds (sec)] and a corresponding scheduled transmission time TS_y_1 [seconds (sec)] and a set including a minimum determination value Tdmin_y_2m&gt;0 [seconds (sec)] and a corresponding scheduled transmission time TS_y_2m [seconds (sec)] is applied to the above-described Math (9). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Math 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     11 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     1 
                     + 
                     Er_min 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             TS_y 
                             ⁢ 
                             _ 
                             ⁢ 
                             2 
                             ⁢ 
                             m 
                           
                           - 
                           
                             Td 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             min_y 
                             ⁢ 
                             _ 
                             ⁢ 
                             2 
                             ⁢ 
                             m 
                           
                         
                         ) 
                       
                       - 
                       
                         TS_y 
                         ⁢ 
                         _ 
                         ⁢ 
                         1 
                       
                     
                     
                       
                         TS_y 
                         ⁢ 
                         _ 
                         ⁢ 
                         2 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         m 
                       
                       - 
                       
                         TS_y 
                         ⁢ 
                         _ 
                         ⁢ 
                         1 
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
             
               
                 
                   
                     ∴ 
                     Er_min 
                   
                   = 
                   
                     
                       
                         - 
                         Td 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       min_y 
                       ⁢ 
                       _ 
                       ⁢ 
                       2 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       m 
                     
                     
                       
                         TS_y 
                         ⁢ 
                         _ 
                         ⁢ 
                         2 
                         ⁢ 
                         m 
                       
                       - 
                       
                         TS_y 
                         ⁢ 
                         _ 
                         ⁢ 
                         1 
                       
                     
                   
                 
               
               
                 
                     
                 
               
             
           
         
       
     
     In addition, the cycle adjustment determination unit  104  determines whether or not a current period-adjusted accuracy estimation value Er is a value indicating that the accuracy of the period of an imaging synchronization signal has been improved according to whether or not a relationship of Er_min≤Er≤Er_max is smaller than that in a past point in time, on the basis of the maximum value Er_max calculated on the basis of the above-described Math (10) and the minimum value Er_min calculated on the basis of the above-described Math (11). 
     Meanwhile, for example, similarly to a case where the after-cycle-adjustment accuracy estimation unit  105  extracts a minimum determination value in each unit time 2B to calculate (estimate) a period-adjusted accuracy estimation value Er′ as in the example shown in  FIG. 11 , the cycle adjustment determination unit  104  determines whether or not the period-adjusted accuracy estimation value Er′ is a value indicating that the accuracy of the period of an imaging synchronization signal has been improved. More specifically, the cycle adjustment determination unit  104  applies each of a set including a minimum determination value Tdmin_y_1′ and a corresponding scheduled transmission time TS_y_1′ and a set including a minimum determination value Tdmin_y_2m′ and a corresponding scheduled transmission time TS_y_2m′ to the above-described Math (9) to calculate a maximum value Er_max′ and a minimum value Er_min′ similar to the above-described Math (10) and the above-described Math (11). In addition, the cycle adjustment determination unit  104  determines whether or not a current period-adjusted accuracy estimation value Er′ is a value indicating that the accuracy of the period of an imaging synchronization signal has been improved according to whether or not a relationship of Er_min′≤Er′≤Er_max′ is smaller than that in a past point in time, on the basis of the calculated maximum value Er_max′ and minimum value Er_min′. 
     In addition, for example, similarly to a case where the after-cycle-adjustment accuracy estimation unit  105  extracts a minimum determination value in each unit time B′ to calculate (estimate) a period-adjusted accuracy estimation value Er″ as in the example shown in  FIG. 12 , the cycle adjustment determination unit  104  determines whether or not the period-adjusted accuracy estimation value Er″ is a value indicating that the accuracy of the period of an imaging synchronization signal has been improved. More specifically, the cycle adjustment determination unit  104  applies each of a set including a minimum determination value Tdmin_y_1″ and a corresponding scheduled transmission time TS_y_1″ and a set including a minimum determination value Tdmin_y_2m″ and a corresponding scheduled transmission time TS_y_2m″ to the above-described Math (9) to calculate a maximum value Er_max″ and a minimum value Er_min″ similar to the above-described Math (10) and the above-described Math (11). In addition, the cycle adjustment determination unit  104  determines whether or not a current period-adjusted accuracy estimation value Er″ is a value indicating that the accuracy of the period of an imaging synchronization signal has been improved according to whether or not a relationship of Er_min″≤Er″≤Er_max″ is smaller than that in a past point in time, on the basis of the calculated maximum value Er_max″ and minimum value Er_min″. 
     Meanwhile, as described above, the after-cycle-adjustment accuracy estimation unit  105  may perform a process of calculating a period-adjusted accuracy estimation value even when it is determined that cycle adjustment for an imaging synchronization signal is not performed. For this reason, the cycle adjustment determination unit  104  may determine whether or not a current period-adjusted accuracy estimation value Er has been improved even when the cycle adjustment unit  101  has not performed cycle adjustment until the present time. In this case, for example, when the above-described relationship of Er_min≤Er≤Er_max is established, the cycle adjustment determination unit  104  defines a minimum value Er_min as a minimum value of a periodic error in an imaging reference clock signal generated by the source oscillation clock generation unit  103  and defines a maximum value Er_max as a maximum value of a periodic error in an imaging reference clock signal generated by the source oscillation clock generation unit  103 . In addition, the cycle adjustment determination unit  104  determines whether or not a current period-adjusted accuracy estimation value Er is a value indicating that the accuracy of the period of an imaging synchronization signal has been improved according to whether or not a relationship of Er_min≤Er≤Er_max is smaller than that in a past point in time, on the basis of the defined maximum value Er_max and minimum value Er_min. Meanwhile, a minimum value and a maximum value of a periodic error in an imaging reference clock signal generated by the source oscillation clock generation unit  103  may be defined on the basis of information of specifications representing the accuracy of a clock generator. 
     Further, in the image transfer system  1 , in a case where the cycle adjustment determination unit  104  determines that the accuracy of the period of an imaging synchronization signal has been improved in step S 1004  (“YES” in step S 1004 ), the cycle adjustment unit  101  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal. 
     More specifically, in step S 1206  included in step S 2107 , the cycle adjustment unit  101  calculates a cycle adjustment amount on the basis of a period-adjusted accuracy estimation value output from the cycle adjustment determination unit  104 , a combination of a round trip propagation-time determination value, a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, and a scheduled transmission time of a round-trip-propagation-time-measurement returning signal. 
     Here, an example of the process of step S 1206  in which the cycle adjustment unit  101  calculates a cycle adjustment amount will be described. For example, a case where the after-cycle-adjustment accuracy estimation unit  105  calculate (estimates) a period-adjusted accuracy estimation value Er [ppm] will be considered. In this case, a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal corresponding to a minimum determination value Tdmin_y_1 extracted in a first unit time is set to be a scheduled transmission time TS_y_1 [seconds (sec)], and a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal corresponding to a minimum determination value Tdmin_y_n extracted in an n-th (n is a natural number of 2 or greater, a positive integer) unit time is set to be a scheduled transmission time TS_y_n [seconds (sec)]. In addition, a scheduled transmission time of a round-trip-propagation-time-measurement returning signal corresponding to a minimum determination value Tdmin_y_1 extracted in a first unit time is set to be a scheduled transmission time Tr_y_1 [seconds (sec)], and a scheduled transmission time of a round-trip-propagation-time-measurement returning signal corresponding to a minimum determination value Tdmin_y_n extracted in an n-th unit time is set to be a scheduled transmission time Tr_y_n [seconds (sec)]. In addition, a correction factor for cycle adjustment when the cycle adjustment unit  101  calculates a cycle adjustment amount is set to be a correction factor for cycle adjustment β. 
     The cycle adjustment unit  101  calculates a cycle adjustment amount E indicating a change in a round trip propagation-time with respect to a scheduled transmission time of a round-trip-propagation-time-measurement returning signal transmitted by the display terminal  200  by multiplying a period-adjusted accuracy estimation value Er indicating a round trip propagation-time with respect to a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal transmitted from the imaging terminal  100  by the correction factor for cycle adjustment β on the basis of the following Math (12) and the following Math (13). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Math 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     12 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   β 
                   = 
                   
                     
                       
                         TS_y 
                         ⁢ 
                         _n 
                       
                       - 
                       
                         TS_y 
                         ⁢ 
                         _ 
                         ⁢ 
                         1 
                       
                     
                     
                       
                         TR_y 
                         ⁢ 
                         _n 
                       
                       - 
                       
                         TR_y 
                         ⁢ 
                         _ 
                         ⁢ 
                         1 
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
             
               
                 
                   [ 
                   
                     Math 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     13 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   E 
                   = 
                   
                     Er 
                     × 
                     β 
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
     In addition, the cycle adjustment unit  101  outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, the synchronization signal generation unit  102  performs cycle adjustment for adjusting the period of an imaging synchronization signal to be generated by an amount corresponding to the cycle adjustment amount, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  101 . 
     Here, an example of processing in which the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated will be described.  FIG. 13  is a flowchart showing a processing procedure of a process of adjusting an imaging synchronization signal by the imaging terminal  100  constituting the image transfer system  1  according to the first embodiment of the present invention. 
     The synchronization signal generation unit  102  starts an operation of generating an imaging synchronization signal when the imaging terminal  100  is started up and an imaging reference clock signal is input from the source oscillation clock generation unit  103 . In addition, the synchronization signal generation unit  102  confirms whether or not a cycle adjustment instruction has been output from the cycle adjustment unit  101  (step S 203 ). 
     As a result of the confirmation in step S 203 , in a case where a cycle adjustment instruction has not been output from the cycle adjustment unit  101  (“NO” in step S 203 ), the synchronization signal generation unit  102  causes the processing to proceed to step S 207 . On the other hand, as a result of the confirmation in step S 203 , in a case where a cycle adjustment instruction has been output from the cycle adjustment unit  101  (“YES” in step S 203 ), the synchronization signal generation unit  102  performs cycle adjustment for an imaging synchronization signal in accordance with a cycle adjustment amount which is output from the cycle adjustment unit  101  together with the cycle adjustment instruction (step S 204 ). 
     For example, when the period of an imaging synchronization signal before cycle adjustment is performed is set to be a period A and a cycle adjustment amount output from the cycle adjustment unit  101  is set to be a cycle adjustment amount E, the synchronization signal generation unit  102  calculates a period A′ of an imaging synchronization signal on which cycle adjustment has been performed according to the following Math (14) to perform cycle adjustment in step S 204 .
 
[Math 14]
 
 A′=A ×(1+ E )  (14)
 
     In addition, the synchronization signal generation unit  102  causes the processing to proceed to step S 207 . In addition, the synchronization signal generation unit  102  generates an imaging synchronization signal on which cycle adjustment has been performed, and outputs the generated imaging synchronization signal to each of the components (the imaging unit not shown in the drawing and the round-trip-propagation-time measurement unit  106 ) included in the imaging terminal  100  (step S 207 ). 
     Thereafter, in a case where a cycle adjustment instruction has been output from the cycle adjustment unit  101  (“YES” in step S 203 ) and cycle adjustment for an imaging synchronization signal has been performed in accordance with the cycle adjustment amount output from the cycle adjustment unit  101  together with the cycle adjustment instruction in step S 204 , the synchronization signal generation unit  102  outputs information of a cycle adjustment amount for an imaging synchronization signal which is adjusted in the performed cycle adjustment to the round-trip-propagation-time measurement unit  106  (step S 205 ). That is, in step S 205 , the synchronization signal generation unit  102  outputs the information of the cycle adjustment amount which is output from the cycle adjustment unit  101  together with the cycle adjustment instruction to the phase adjustment unit, not shown in the drawing, which is included in the round-trip-propagation-time measurement unit  106 . 
     Meanwhile, in the image transfer system  1 , as described above, when the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  100  can acquire a plurality of round trip propagation-time determination values in a period determined in advance from the phase adjustment unit not shown in the drawing, a period-adjusted accuracy estimation value can be calculated (estimated). That is, in the image transfer system  1 , the phase of the imaging synchronization signal has not necessarily be adjusted by the synchronization signal generation unit  102 . However, in the image transfer system  1 , an example of processing in which the phase adjustment unit not shown in the drawing performs phase adjustment for an imaging synchronization signal has been described using  FIG. 3  or  FIG. 9 . In the synchronization signal generation unit  102 , phase adjustment for an imaging synchronization signal is performed before an imaging synchronization signal is generated in step S 207  after cycle adjustment for an imaging synchronization signal is performed in step S 204 . 
     In a phase adjustment process for an imaging synchronization signal performed in the synchronization signal generation unit  102 , first, it is confirmed whether or not a phase adjustment instruction has been output from the phase adjustment unit not shown in the drawing (step S 208 ). As a result of the confirmation in step S 208 , in a case where a phase adjustment instruction has not been output from the phase adjustment unit not shown in the drawing (“NO” in step S 208 ), the synchronization signal generation unit  102  causes the processing to proceed to step S 207 . On the other hand, as a result of the confirmation in step S 208 , in a case where a phase adjustment instruction has been output from the phase adjustment unit not shown in the drawing (“YES” in step S 208 ), the synchronization signal generation unit  102  temporarily stops generating an imaging synchronization signal in response to a phase adjustment instruction. In addition, the synchronization signal generation unit  102  waits until a period of time represented in a round trip propagation-time which is output from the phase adjustment unit not shown in the drawing together with the phase adjustment instruction elapses (step S 206 ). Thereafter, the synchronization signal generation unit  102  causes the processing to proceed to step S 207 . 
     According to such a processing procedure, the synchronization signal generation unit  102  generates and outputs an imaging synchronization signal of which the phase or period is adjusted, in response to the phase adjustment instruction which is output by the phase adjustment unit not shown in the drawing or the cycle adjustment instruction which is output by the cycle adjustment unit  101 . Thereby, in the image transfer system  1 , it is possible to perform adjustment so that the phase or period (at least a period) of an imaging synchronization signal generated in the imaging terminal  100  matches the phase or period (at least a period) of a display synchronization signal generated by the display terminal  200 . 
     Here, an example of a case where the phase or period of an imaging synchronization signal is adjusted in the image transfer system  1  will be described.  FIG. 14  is a timing chart showing an example of transmission and reception of captured image data to be wirelessly transferred in the image transfer system  1  according to the first embodiment of the present invention. In addition,  FIG. 15  is a diagram showing an example of a relationship between a synchronization signal (an imaging synchronization signal and a display synchronization signal) and a time (an imaging terminal time and a display terminal time) when captured image data is wirelessly transferred in the image transfer system  1  according to the first embodiment of the present invention. 
       FIG. 14  shows an example of a timing when the synchronization signal generation unit  102  generates an imaging synchronization signal on a time axis of an imaging terminal time in the imaging terminal  100 . In addition,  FIG. 14  shows an example of a timing when the synchronous signal generation unit  204  generates a display synchronization signal on a time axis of a display terminal time in the display terminal  200 . In addition,  FIG. 14  shows an example of timings of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal which are wirelessly transferred between the imaging terminal  100  and the display terminal  200 . In addition,  FIG. 15  shows an example of a relationship between the phase or period of an imaging synchronization signal or period of a display synchronization signal changing with the elapse of time and the phase when an imaging terminal time in the imaging terminal  100  is set to be an X axis and a display terminal time in the display terminal  200  is set to be a Y axis.  FIGS. 14 and 15  schematically show a case where the phase or period of an imaging synchronization signal generated by the synchronization signal generation unit  102  are adjusted so as to match the phase or period of a display synchronization signal generated by the synchronous signal generation unit  204  on the basis of a round trip propagation-time calculated in the imaging terminal  100 . 
     In the image transfer system  1 , the imaging terminal  100  starts generating an imaging synchronization signal by setting a point in time when wireless connection is established to be a reference imaging terminal time=0 as described above, and transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  200  for each scheduled transmission time. Further, in the image transfer system  1 , the display terminal  200  starts generating a display synchronization signal by setting a point in time when wireless connection is established to be a reference display terminal time=0 as described above, and waits for a round-trip-propagation-time-measurement outgoing signal to be transmitted from the imaging terminal  100 . In addition, when the display terminal  200  receives the round-trip-propagation-time-measurement outgoing signal transmitted from the imaging terminal  100 , the display terminal transmits a round-trip-propagation-time-measurement returning signal corresponding to the received round-trip-propagation-time-measurement outgoing signal to the imaging terminal  100  for each scheduled transmission time. Thereby, the imaging terminal  100  adjusts the phase or period of an imaging synchronization signal on the basis of the transmitted round-trip-propagation-time-measurement outgoing signal and the received round-trip-propagation-time-measurement returning signal. 
     The example shown in  FIG. 14  shows a state where the imaging terminal  100  (more specifically, the round-trip-propagation-time measurement unit  106 ) sets respective times of an imaging terminal time=Ts_y1 to Ts_y5 to be scheduled transmission times and transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  200  at each of the scheduled transmission times. In addition, the example shown in  FIG. 14  shows a state where the display terminal  200  (more specifically, the round-trip-propagation-time-measurement assistance unit  202 ) sets respective times of a display terminal time=Tr_y1 to Tr_y5 after a round-trip-propagation-time-measurement outgoing signal transmitted from the imaging terminal  100  is received to be scheduled transmission times, and transmits a round-trip-propagation-time-measurement returning signal corresponding to the received round-trip-propagation-time-measurement outgoing signal to the imaging terminal  100  at each of the scheduled transmission times. Meanwhile, in the example shown in  FIG. 14 , for ease of description, a receiver elapsed time from an input time when the display terminal  200  receives a round-trip-propagation-time-measurement outgoing signal to a scheduled transmission time when the display terminal transmits a round-trip-propagation-time-measurement returning signal is set to be a receiver elapsed time ΔTrev=0. In addition, respective times of the imaging terminal time=Ts_y1 to Ts_y5 and the display terminal time=Tr_y1 to Tr_y5 shown in  FIG. 15  corresponds to the respective times in the example shown in  FIG. 14 . 
     More specifically, in the example shown in  FIG. 14 , the imaging terminal  100  (the round-trip-propagation-time measurement unit  106 ) transmits a first round-trip-propagation-time-measurement outgoing signal to the display terminal  200  when an imaging terminal time=Ts_y1 and receives a first round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200  (the round-trip-propagation-time-measurement assistance unit  202 ) when a display terminal time=Tr_y1. Thereby, the round-trip-propagation-time measurement unit  106  calculates a first round trip propagation-time Td_y1 on the basis of a round-trip-propagation-time-measurement outgoing signal which is transmitted first and a round-trip-propagation-time-measurement returning signal which is received first. In the example shown in  FIG. 14 , the first round trip propagation-time Td_y1 calculated by the round-trip-propagation-time measurement unit  106  is a round trip propagation-time which is smaller than a round trip propagation-time determination value Td_th (Td_y1&lt;Td_th). For this reason, the phase adjustment unit not shown in the drawing determines that phase adjustment for an imaging synchronization signal is performed, and outputs a phase adjustment instruction, indicating that the phase of an imaging synchronization signal is adjusted by waiting for the generation of an imaging synchronization signal until a period of time represented in the first round trip propagation-time Td_y1 elapses, to the synchronization signal generation unit  102 . Thereby, the synchronization signal generation unit  102  regenerates an imaging synchronization signal after waiting for the generation of an imaging synchronization signal for the first round trip propagation-time Td_y1 instructed by the phase adjustment unit not shown in the drawing to adjust the phase of an imaging synchronization signal to be generated. As a result, the phase of an imaging synchronization signal generated by the synchronization signal generation unit  102  approaches the phase of a display synchronization signal generated by the synchronous signal generation unit  204  as in the example shown in  FIG. 15 . 
     Hereinafter, similarly, the imaging terminal  100  (the round-trip-propagation-time measurement unit  106 ) repeats phase adjustment for an imaging synchronization signal. In the example shown in  FIG. 14 , the imaging terminal  100  (the round-trip-propagation-time measurement unit  106 ) transmits a second round-trip-propagation-time-measurement outgoing signal to the display terminal  200  when an imaging terminal time=Ts_y2, and receives a second round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200  (the round-trip-propagation-time-measurement assistance unit  202 ) when a display terminal time=Tr_y2. Thereby, the round-trip-propagation-time measurement unit  106  calculates a second round trip propagation-time Td_y2 on the basis of a round-trip-propagation-time-measurement outgoing signal which is transmitted second and a round-trip-propagation-time-measurement returning signal which is received second. In the example shown in  FIG. 14 , the second round trip propagation-time Td_y2 calculated by the round-trip-propagation-time measurement unit  106  is also a round trip propagation-time which is smaller than a round trip propagation-time determination value Td_th (Td_y2&lt;Td_th). For this reason, the phase adjustment unit not shown in the drawing determines that phase adjustment for an imaging synchronization signal is performed, and outputs a phase adjustment instruction, indicating that the phase of an imaging synchronization signal is adjusted by waiting for the generation of an imaging synchronization signal until a period of time represented in the second round trip propagation-time Td_y2 elapses, to the synchronization signal generation unit  102 . Thereby, the synchronization signal generation unit  102  regenerates an imaging synchronization signal after waiting for the generation of an imaging synchronization signal for a period of time represented in the second round trip propagation-time Td_y2 instructed by the phase adjustment unit not shown in the drawing to adjust the phase of an imaging synchronization signal to be generated. As a result, the phase of an imaging synchronization signal generated by the synchronization signal generation unit  102  approaches the phase of a display synchronization signal generated by the synchronous signal generation unit  204  as in the example shown in  FIG. 15 . 
     Meanwhile, in the example shown in  FIG. 14 , the imaging terminal  100  (the round-trip-propagation-time measurement unit  106 ) transmits a third round-trip-propagation-time-measurement outgoing signal to the display terminal  200  when an imaging terminal time=Ts_y3, and receives a third round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200  (the round-trip-propagation-time-measurement assistance unit  202 ) when a display terminal time=Tr_y3. In this case, a third round trip propagation-time Td_y3 calculated by the round-trip-propagation-time measurement unit  106  is a round trip propagation-time which is larger than a round trip propagation-time determination value Td_th (Td_y3&gt;Td_th). For this reason, the phase adjustment unit not shown in the drawing determines that phase adjustment for an imaging synchronization signal is not performed. In this case, the phase adjustment unit not shown in the drawing does not output a phase adjustment instruction to the synchronization signal generation unit  102 . That is, the phase adjustment unit not shown in the drawing causes the synchronization signal generation unit  102  to continue generating an imaging synchronization signal without performing phase adjustment for an imaging synchronization signal. Thereby, the synchronization signal generation unit  102  continues generating an imaging synchronization signal at a phase adjusted in accordance with the second round trip propagation-time Td_y2, that is, a timing adjusted previously. 
     Further, in the example shown in  FIG. 14 , a period of time determined in advance elapses, the period of time being required for the imaging terminal  100  (the round-trip-propagation-time measurement unit  106 ) to receive the third round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200  (the round-trip-propagation-time-measurement assistance unit  202 ) when a display terminal time=Tr_y3 and then determine whether or not cycle adjustment is performed. For this reason, the after-cycle-adjustment accuracy estimation unit  105  acquires a plurality of round trip propagation-time determination values used to determine whether or not phase adjustment for an imaging synchronization signal is to be performed during a period determined in advance from the phase adjustment unit not shown in the drawing, and calculates (estimates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed, on the basis of information of the plurality of round trip propagation-time determination values acquired. In addition, the cycle adjustment determination unit  104  determines whether or not cycle adjustment for an imaging synchronization signal is performed, on the basis of the period-adjusted accuracy estimation value calculated (estimated) by the after-cycle-adjustment accuracy estimation unit  105 . In the example shown in  FIG. 14 , the cycle adjustment determination unit  104  determines that cycle adjustment for an imaging synchronization signal is performed. For this reason, the cycle adjustment unit  101  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal in accordance with a determination result indicating that cycle adjustment for an imaging synchronization signal is performed which is obtained by the cycle adjustment determination unit  104 , and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. In the example shown in  FIG. 14 , the cycle adjustment unit  101  outputs a cycle adjustment amount E to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated, in accordance with the cycle adjustment amount E which is output from the cycle adjustment unit  101 . The example shown in  FIG. 14  shows a state where a period A of an imaging synchronization signal before cycle adjustment is performed is adjusted to a period A′ which is increased by an adjustment amount AE according to the cycle adjustment amount E. As a result, the period of an imaging synchronization signal generated by the synchronization signal generation unit  102  becomes similar to the period of a display synchronization signal generated by the synchronous signal generation unit  204  as in the example shown in  FIG. 15 . 
     In this manner, in the image transfer system  1 , the imaging terminal  100  (more specifically, the synchronization signal generation unit  102 ) adjusts the phase or period of an imaging synchronization signal to be generated. Thereby, in the image transfer system  1 , adjustment is performed so that the phase or period (at least a period) of an imaging synchronization signal generated by the synchronization signal generation unit  102  matches the phase or period (at least a period) of a display synchronization signal generated by the display terminal  200  (more specifically, the synchronous signal generation unit  204 ). That is, in the image transfer system  1 , even when the phase or period of each of an imaging synchronization signal and a display synchronization signal is shifted with the elapse of time due to an error of the phase or period between an imaging reference clock signal generated by the source oscillation clock generation unit  103  and a display reference clock signal generated by the source oscillation clock generation unit  205 , the phase or period (at least a period) of the imaging synchronization signal is adjusted so as to match the phase or period (at least a period) of the display synchronization signal in the imaging terminal  100 . 
     As described above, in the image transfer system  1  of the first embodiment, after wireless connection between the imaging terminal  100  and the display terminal  200  is established, transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between the round-trip-propagation-time measurement unit  106  included in the imaging terminal  100  and the round-trip-propagation-time-measurement assistance unit  202  included in the display terminal  200 . Further, in the image transfer system  1  of the first embodiment, the round-trip-propagation-time measurement unit  106  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  100  and the display terminal  200 , on the basis of a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal and a reception time of a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the round-trip-propagation-time-measurement assistance unit  202 . Further, in the image transfer system  1  of the first embodiment, the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  100  acquires a plurality of round trip propagation-time determination values based on the round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  to calculate (estimate) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  1  of the first embodiment, the cycle adjustment determination unit  104  included in the imaging terminal  100  determines whether or not cycle adjustment for an imaging synchronization signal is performed, on the basis of a period-adjusted accuracy estimation value calculated (estimated) by the after-cycle-adjustment accuracy estimation unit  105 . Further, in the image transfer system  1  of the first embodiment, in a case where the cycle adjustment determination unit  104  determines that cycle adjustment for an imaging synchronization signal is performed, the cycle adjustment unit  101  included in the imaging terminal  100  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, in the image transfer system  1  of the first embodiment, the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated by an amount corresponding to the cycle adjustment amount, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  101 . That is, in the image transfer system  1  of the first embodiment, the imaging terminal  100  adjusts the period of an imaging synchronization signal to be generated so as to match the period of a display synchronization signal to be generated by the display terminal  200 . 
     Thereby, in the image transfer system  1  of the first embodiment, even when the phase of each of an imaging synchronization signal and a display synchronization signal is shifted with the elapse of time due to an error of the phase or period between an imaging reference clock signal generated by the source oscillation clock generation unit  103  included in the imaging terminal  100  and a display reference clock signal generated by the source oscillation clock generation unit  205  included in the display terminal  200 , the period of the imaging synchronization signal can be matched to the period of the display synchronization signal. Thus, in the image transfer system  1  of the first embodiment, even when wireless transfer between the imaging terminal  100  and the display terminal  200  is in an unstable communication situation in which the quality of communication suddenly deteriorates due to frequent retransmission of packet transmission and reception or a decrease in a communication rate and thus wireless transfer with variations exceeding a predetermined range is delayed, wireless transfer can be performed in a state where a delay of wireless transfer exceeding the predetermined range is excluded. Thus, in the image transfer system  1  of the first embodiment, it is possible to wirelessly transfer captured image data from the imaging terminal  100  to the display terminal  200  without exceeding a validity period of a display image and to stably display a display image corresponding to the captured image data transmitted from the imaging terminal  100  in the display terminal  200 . 
     Further, in the image transfer system  1  of the first embodiment, it is possible to adjust the phase of an imaging synchronization signal in addition to the period of an imaging synchronization signal. More specifically, in the image transfer system  1  of the first embodiment, the phase adjustment unit, not shown in the drawing, which is included in the imaging terminal  100  can determine whether or not the synchronization signal generation unit  102  included in the imaging terminal  100  performs phase adjustment for an imaging synchronization signal to be generated, on the basis of a round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106 . Further, in the image transfer system  1  of the first embodiment, in a case where the phase adjustment unit not shown in the drawing determines that phase adjustment for an imaging synchronization signal is performed, a phase adjustment instruction for adjusting the phase of an imaging synchronization signal can be output to the synchronization signal generation unit  102 . In this case, in the image transfer system  1  of the first embodiment, the synchronization signal generation unit  102  adjusts the phase of an imaging synchronization signal to be generated, in response to a phase adjustment instruction which is output from the phase adjustment unit not shown in the drawing. That is, in the image transfer system  1  of the first embodiment, the imaging terminal  100  adjusts the phase of an imaging synchronization signal to be generated so as to match the phase of a display synchronization signal which is generated by the display terminal  200 . Thereby, in the image transfer system  1  of the first embodiment, a display image corresponding to captured image data transmitted from the imaging terminal  100  can be displayed more stably in the display terminal  200 . 
     Meanwhile, in the image transfer system  1  of the first embodiment, a description has been provided of a configuration in which the round-trip-propagation-time measurement unit  106 , the after-cycle-adjustment accuracy estimation unit  105 , the cycle adjustment determination unit  104 , and the cycle adjustment unit  101  which are components for adjusting the period of an imaging synchronization signal generated by the imaging terminal  100  are included in the imaging terminal  100 , and the round-trip-propagation-time-measurement assistance unit  202  is included in the display terminal  200 . In other words, in the image transfer system  1  of the first embodiment, a description has been provided of a configuration in which a round trip propagation-time is calculated by the imaging terminal  100  transmitting a round-trip-propagation-time-measurement outgoing signal to the display terminal  200  and receiving a round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200 . However, in the image transfer system of the present invention, even in a configuration in which components for adjusting the period of an imaging synchronization signal which is generated by an imaging terminal are included in either one of the imaging terminal or a display terminal, a function of adjusting the period of an imaging synchronization signal which is generated by the imaging terminal can be realized similarly. For example, contrary to the image transfer system  1  of the first embodiment, even in a configuration in which a round trip propagation-time is calculated by a display terminal transmitting a round-trip-propagation-time-measurement outgoing signal to an imaging terminal and receiving a round-trip-propagation-time-measurement returning signal transmitted from the imaging terminal, a function of adjusting the period of an imaging synchronization signal which is generated by the imaging terminal can be realized similarly. 
     Second Embodiment 
     Hereinafter, an image transfer system of a second embodiment of the present invention will be described.  FIG. 16  is a block diagram showing a schematic configuration of the image transfer system in the second embodiment of the present invention. An image transfer system  2  includes an imaging terminal  1100  and a display terminal  1200 . The imaging terminal  1100  includes a synchronization signal generation unit  102 , a cycle adjustment unit  101 , a source oscillation clock generation unit  103 , a wireless communication unit  108 , a cycle adjustment determination unit  104 , an after-cycle-adjustment accuracy estimation unit  105 , a round trip propagation-time measurement assistance unit  1106 , a clocking unit  107 , and an antenna  120 . In addition, the display terminal  1200  includes a synchronous signal generation unit  204 , a source oscillation clock generation unit  205 , a wireless communication unit  201 , a round trip propagation-time measurement unit  1202 , a clocking unit  203 , and an antenna  220 . 
     Meanwhile, in  FIG. 16 , in components of the image transfer system  2 , the same components as the components included in the image transfer system  1  of the first embodiment shown in  FIG. 2  are denoted by the same reference numerals and signs. 
     Similarly to the image transfer system  1  of the first embodiment, the image transfer system  2  is also an image transfer system configured such that transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between the imaging terminal  1100  and the display terminal  1200 , and the imaging terminal  1100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  1200 . 
     However, in the image transfer system  2 , the disposition of some components included in the imaging terminal  100  or the display terminal  200  in the image transfer system  1  of the first embodiment is replaced. More specifically, in the image transfer system  2 , the display terminal  1200  includes a round trip propagation-time measurement unit  1202  that replaces the round-trip-propagation-time measurement unit  106  included in the imaging terminal  100  in the image transfer system  1  of the first embodiment. Further, in the image transfer system  2 , the imaging terminal  1100  includes a round trip propagation-time measurement assistance unit  1106  that replaces the round-trip-propagation-time-measurement assistance unit  202  included in the display terminal  200  in the image transfer system  1  of the first embodiment. 
     For this reason, in the image transfer system  2 , the display terminal  1200  transmits a round-trip-propagation-time-measurement outgoing signal to the imaging terminal  1100 , and the imaging terminal  1100  transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the display terminal  1200  to the display terminal  1200 . Further, in the image transfer system  2 , the display terminal  1200  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  1100  and the display terminal  1200 , on the basis of a transmission time of a round-trip-propagation-time-measurement outgoing signal and a reception time of a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the imaging terminal  1100 . 
     However, in the image transfer system  2 , although there is a process of exchanging signals through wireless transfer in association with a change in the disposition of components, functions and operations of the components are the same as the functions and operations of the corresponding components in the image transfer system  1  of the first embodiment. That is, the functions and operations of the components included in the imaging terminal  1100  and the display terminal  1200  in the image transfer system  2  can be easily understood from the above description of the components included in the imaging terminal  100  and the display terminal  200  in the image transfer system  1  of the first embodiment. Therefore, a detailed description related to the components included in the image transfer system  2  will be omitted. 
     Next, operations of processing in the image transfer system  2  will be described. Meanwhile, in the image transfer system  2 , it is assumed that the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  100  in the image transfer system  1  of the first embodiment is included in the round trip propagation-time measurement unit  1202 .  FIG. 17  is a flowchart showing a processing procedure of the image transfer system  2  in the second embodiment of the present invention. 
     In the image transfer system  2 , a process of transmitting information used to adjust the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  to the imaging terminal  1100  is added, in association with a change to a configuration in which the display terminal  1200  transmits a round-trip-propagation-time-measurement outgoing signal. For this reason, in the image transfer system  2 , information such as a round trip propagation-time determination value which is used to adjust the period of an imaging synchronization signal is transmitted to the imaging terminal  1100 . However, an outline of the overall operation in the image transfer system  2  is the same as that of the image transfer system  1  of the first embodiment. Therefore, in the image transfer system  2 , a description of an outline of the overall operation will be omitted, and a more specific operation of performing cycle adjustment for an imaging synchronization signal in the image transfer system  2  will be described. 
     In the image transfer system  2 , when a cycle adjustment process is started, the display terminal  1200  transmits a round-trip-propagation-time-measurement outgoing signal to the imaging terminal  1100 , and the imaging terminal  1100  transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal to the display terminal  1200  in step S 302 . More specifically, in the display terminal  1200 , the round trip propagation-time measurement unit  1202  transmits a round-trip-propagation-time-measurement outgoing signal to the imaging terminal  1100  through the wireless communication unit  201  and the antenna  220  and receives a round-trip-propagation-time-measurement returning signal transmitted from the imaging terminal  1100  in the processes of steps S 302 -A to S 302 -B included in step S 302 . On the other hand, in the imaging terminal  1100 , when a cycle adjustment process is started, the round trip propagation-time measurement assistance unit  1106  receives a round-trip-propagation-time-measurement outgoing signal transmitted from the display terminal  1200  through the antenna  120  and the wireless communication unit  108  and transmits a round-trip-propagation-time-measurement returning signal corresponding to the received round-trip-propagation-time-measurement outgoing signal to the display terminal  1200  in the processes of steps S 302 -C to S 302 -D included in step S 302 . 
     Meanwhile, in the image transfer system  2 , the process of step S 302  in which a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are transmitted and received between the imaging terminal  1100  and the display terminal  1200  can be easily understood by inversely considering the imaging terminal  100  and the display terminal  200  which perform the process of step S 302  in the image transfer system  1  of the first embodiment. More specifically, the processes of transmitting and receiving a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal by the imaging terminal  100  constituting the image transfer system  1  of the first embodiment (the processes of steps S 302 -A to S 302 -B), shown in  FIG. 4 , can be easily understood by considering that the processes are performed by the display terminal  1200  (more specifically, the round trip propagation-time measurement unit  1202 ). In addition, the processes of transmitting and receiving a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal by the display terminal  200  constituting the image transfer system  1  of the first embodiment (the processes of steps S 302 -C to S 302 -D), shown in  FIG. 5 , can be easily understood by considering that the processes are performed by the imaging terminal  1100  (more specifically, the round trip propagation-time measurement assistance unit  1106 ). Therefore, a detailed description related to the process of step S 302  in the image transfer system  2  will be omitted. 
     Thereafter, in the image transfer system  2 , in step S 303 , the display terminal  1200  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  1100  and the display terminal  1200 . More specifically, the round trip propagation-time measurement unit  1202  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  1100  and the display terminal  1200 , on the basis of information regarding a transmitted round-trip-propagation-time-measurement outgoing signal and information included in a round-trip-propagation-time-measurement returning signal transmitted from the imaging terminal  1100 . 
     Thereafter, in the image transfer system  2 , in step S 304 , the display terminal  1200  adjusts the phase of an imaging synchronization signal, on the basis of the round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  1100  and the display terminal  1200  which is calculated in step S 303 . More specifically, in step S 304 , the phase adjustment unit, not shown in the drawing, which is provided in the round trip propagation-time measurement unit  1202  included in the display terminal  1200  generates a phase adjustment instruction on the basis of the round trip propagation-time calculated by the round trip propagation-time measurement unit  1202  and updates a round trip propagation-time determination value. Meanwhile, the process of adjusting the phase of an imaging synchronization signal in step S 304  is the same as the process of step S 304  in the image transfer system  1  of the first embodiment. Therefore, a detailed description related to the process of step S 304  in the image transfer system  2  will be omitted. 
     Thereafter, in the image transfer system  2 , in step S 307 , the display terminal  1200  generates a measurement notification signal including information of the calculated round trip propagation-time and transmits the generated measurement notification signal to the imaging terminal  1100 . More specifically, the round trip propagation-time measurement unit  1202  generates a measurement notification signal including information of the calculated round trip propagation-time, information of a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, information of a scheduled transmission time included in a received round-trip-propagation-time-measurement returning signal, a phase adjustment instruction generated by the phase adjustment unit not shown in the drawing, and information of a plurality of round trip propagation-time determination values updated by the phase adjustment unit not shown in the drawing. In addition, the round trip propagation-time measurement unit  1202  outputs the generated measurement notification signal to the wireless communication unit  201  and transmits the measurement notification signal to the imaging terminal  1100  through the wireless communication unit  201  and the antenna  220 . Thereby, the imaging terminal  1100  acquires information of the round trip propagation-time determination values from the display terminal  1200 . More specifically, the wireless communication unit  108  receives the measurement notification signal transmitted from the display terminal  1200  through the antenna  120 . In addition, the wireless communication unit  108  outputs each of the information of the round trip propagation-time included in the received measurement notification signal, the information of the scheduled transmission time of the round-trip-propagation-time-measurement outgoing signal, the information of the scheduled transmission time of the round-trip-propagation-time-measurement returning signal, the information of the phase adjustment instruction, and the information of the plurality of round trip propagation-time determination values to the round trip propagation-time measurement assistance unit  1106 . 
     Thereafter, in the image transfer system  2 , in step S 306 , the imaging terminal  1100  adjusts the period of an imaging synchronization signal on the basis of information transmitted from the display terminal  1200 , that is, information of the plurality of round trip propagation-time determination values updated in step S 304  by the display terminal  1200 , similar to the processes in step S 306  in the image transfer system  1  of the first embodiment. In this case, the round trip propagation-time measurement assistance unit  1106  outputs each of the information of the round trip propagation-time included in the measurement notification signal output from the wireless communication unit  108 , the information of the scheduled transmission time of the round-trip-propagation-time-measurement outgoing signal, the information of the scheduled transmission time of the round-trip-propagation-time-measurement returning signal, and the information of the plurality of round trip propagation-time determination values to each of the after-cycle-adjustment accuracy estimation unit  105 , the cycle adjustment determination unit  104 , and the cycle adjustment unit  101 . Thereby, the after-cycle-adjustment accuracy estimation unit  105  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed on the basis of the information of the plurality of round trip propagation-time determination values transmitted from the display terminal  1200  and outputs the estimated period-adjusted accuracy estimation value to the cycle adjustment determination unit  104  (step S 2106 ). In addition, the cycle adjustment determination unit  104  determines whether or not cycle adjustment for an imaging synchronization signal is performed, on the basis of the period-adjusted accuracy estimation value which is output from the after-cycle-adjustment accuracy estimation unit  105  (step S 1004 ). Further, in a case where the cycle adjustment determination unit  104  determines that cycle adjustment for an imaging synchronization signal is performed, the cycle adjustment unit  101  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction (step S 1206 ). Thereby, the synchronization signal generation unit  102  performs cycle adjustment for adjusting the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  101 . 
     Meanwhile, the round trip propagation-time measurement assistance unit  1106  outputs a phase adjustment instruction which is output from the wireless communication unit  108  to the synchronization signal generation unit  102 . Thereby, the synchronization signal generation unit  102  performs phase adjustment for an imaging synchronization signal to be generated, in response to the phase adjustment instruction which is output from the round trip propagation-time measurement assistance unit  1106 , that is, a phase adjustment instruction which is transmitted from the phase adjustment unit, not shown in the drawing, which is provided in the round trip propagation-time measurement unit  1202  included in the display terminal  1200 . 
     In this manner, in the image transfer system  2 , the display terminal  1200  transmits a round-trip-propagation-time-measurement outgoing signal and calculates a round trip propagation-time in wireless transfer between the imaging terminal  1100  and the display terminal  1200 . Further, in the image transfer system  2 , the display terminal  1200  performs a process of updating a round trip propagation-time determination value and transmits information of a plurality of round trip propagation-time determination values updated, information of a round trip propagation-time, information of a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, information of a scheduled transmission time of a round-trip-propagation-time-measurement returning signal, and a phase adjustment instruction to the imaging terminal  1100 . Thereby, in the image transfer system  2 , the imaging terminal  1100  estimates (calculates) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed on the basis of the pieces of information transmitted from the display terminal  1200 , determines whether or not the period of an imaging synchronization signal is adjusted, and adjusts the periods of an imaging synchronization signal and a display synchronization signal so as not to be shifted with the elapse of time. 
     Meanwhile, similarly to the image transfer system  1  of the first embodiment, also in the image transfer system  2 , when at least a process of updating a round trip propagation-time determination value is performed in step S 304 , the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  1100  can estimate the accuracy of an imaging synchronization signal. That is, when the phase adjustment unit  105 , not shown in the drawing, which is provided in the round trip propagation-time measurement unit  1202  included in the display terminal  1200  can acquire a combination of a plurality of round trip propagation-time determination values used to adjust the phase of an imaging synchronization signal, scheduled transmission times of corresponding outward path signals for round trip propagation-time measurement, and scheduled transmission times of corresponding return path signals for round trip propagation-time measurement, the period-adjusted accuracy estimation unit can calculate (estimate) a period-adjusted accuracy estimation value. Therefore, similarly to the image transfer system  1  of the first embodiment, also in the image transfer system  2 , in step S 304 , the phase adjustment unit not shown in the drawing does not necessarily need to adjust the phase of an imaging synchronization signal. In this case, the phase adjustment unit not shown in the drawing updates a round trip propagation-time calculated by the round trip propagation-time measurement unit  1202  as a round trip propagation-time determination value. 
     With such a configuration and processing procedure, in the image transfer system  2 , the display terminal  1200  transmits a round-trip-propagation-time-measurement outgoing signal and measures (calculates) a round trip propagation-time in wireless transfer between the imaging terminal  1100  and the display terminal  1200  to update a round trip propagation-time determination value. Further, in the image transfer system  2 , the imaging terminal  1100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated, on the basis of the round trip propagation-time determination value updated by the display terminal  1200 . Thereby, similarly to the image transfer system  1  of the first embodiment, also in the image transfer system  2 , the phase or period (at least a period) of an imaging synchronization signal generated by the imaging terminal  1100  (more specifically, the synchronization signal generation unit  102 ) is adjusted so as to match the phase or period (at least a period) of a display synchronization signal generated by the display terminal  1200 . That is, similarly to the image transfer system  1  of the first embodiment, also in the image transfer system  2 , a timing when the imaging terminal  1100  wirelessly transfers captured image data of an image captured by an imaging unit not shown in the drawing to the display terminal  1200  is matched to a timing when the display terminal  1200  displays an image corresponding to the captured image data on a display unit not shown in the drawing. Thereby, similarly to the image transfer system  1  of the first embodiment, also in the image transfer system  2 , the display terminal  1200  can stably display an image corresponding to the captured image data wirelessly transferred from the imaging terminal  1100  on the display unit not shown in the drawing. 
     As described above, the image transfer system  2  of the second embodiment performs transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal between the round trip propagation-time measurement unit  1202  included in the display terminal  1200  and the round trip propagation-time measurement assistance unit  1106  included in the imaging terminal  1100  after wireless connection between the imaging terminal  1100  and the display terminal  1200  is established. Further, in the image transfer system  2  of the second embodiment, the round trip propagation-time measurement unit  1202  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  1100  and the display terminal  1200 , on the basis of a transmission time of a round-trip-propagation-time-measurement outgoing signal and a reception time of a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the round trip propagation-time measurement assistance unit  1106 . Further, in the image transfer system  2  of the second embodiment, the round trip propagation-time measurement unit  1202  (more specifically, the phase adjustment unit not shown in the drawing) updates the round trip propagation-time calculated by the round trip propagation-time measurement unit  1202  as a round trip propagation-time determination value. Further, in the image transfer system  2  of the second embodiment, the round trip propagation-time measurement unit  1202  transmits information for adjusting the period of an imaging synchronization signal in the imaging terminal  1100  to the imaging terminal  1100 . Further, in the image transfer system  2  of the second embodiment, the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  1100  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  2  of the second embodiment, the cycle adjustment determination unit  104  included in the imaging terminal  1100  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  1100  is performed, on the basis of the period-adjusted accuracy estimation value. Further, in the image transfer system  2  of the second embodiment, in a case where the cycle adjustment determination unit  104  determines that cycle adjustment for an imaging synchronization signal is performed, the cycle adjustment unit  101  included in the imaging terminal  1100  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, in the image transfer system  2  of the second embodiment, the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  101 . 
     Thereby, similarly to the image transfer system  1  of the first embodiment, also in the image transfer system  2  of the second embodiment, the phase or period (at least a period) of an imaging synchronization signal can be matched to the phase or period (at least a period) of a display synchronization signal. That is, also in the image transfer system  2  of the second embodiment, even when the periods of an imaging synchronization signal and a display synchronization signal are shifted with the elapse of time due to an error of a phase or a period between an imaging reference clock signal and a display reference clock signal, the phase or period (at least a period) of the imaging synchronization signal can be matched to the phase or period (at least a period) of the display synchronization signal. Thus, also in the image transfer system  2  of the second embodiment, the same effects as those in the image transfer system  1  of the first embodiment can be obtained. That is, similarly to the image transfer system  1  of the first embodiment, also in the image transfer system  2  of the second embodiment, it is possible to wirelessly transfer captured image data from the imaging terminal  1100  to the display terminal  1200  without exceeding a validity period of a display image and to stably display a display image corresponding to the captured image data transmitted from the imaging terminal  1100  in the display terminal  1200 . 
     Moreover, in the image transfer system  2  of the second embodiment, the display terminal  1200  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  1100  and the display terminal  1200 . Thereby, in the image transfer system  2  of the second embodiment, it is not necessary to calculate a round trip propagation-time in the imaging terminal  1100 , and thus it is possible to reduce a load of processing performed in the imaging terminal  1100 . 
     Meanwhile, as described above, in the image transfer system of the present invention, even in a configuration in which components for adjusting the period of an imaging synchronization signal which is generated by an imaging terminal are included in either one of the imaging terminal or a display terminal, a function of adjusting the period of an imaging synchronization signal which is generated by the imaging terminal can be realized similarly. 
     Third Embodiment 
     Hereinafter, an image transfer system of a third embodiment of the present invention will be described.  FIG. 18  is a block diagram showing a schematic configuration of the image transfer system in the third embodiment of the present invention. An image transfer system  3  includes an imaging terminal  2100  and a display terminal  2200 . The imaging terminal  2100  includes a synchronization signal generation unit  102 , a cycle adjustment unit  101 , a source oscillation clock generation unit  103 , a wireless communication unit  108 , a cycle adjustment determination unit  104 , a round trip propagation-time measurement assistance unit  1106 , a clocking unit  107 , and an antenna  120 . In addition, the display terminal  2200  includes a synchronous signal generation unit  204 , a source oscillation clock generation unit  205 , a wireless communication unit  201 , a round trip propagation-time measurement unit  1202 , a clocking unit  203 , an after-cycle-adjustment accuracy estimation unit  2105 , and an antenna  220 . 
     Meanwhile, also in  FIG. 18 , in components of the image transfer system  3 , the same components as the components included in the image transfer system  1  of the first embodiment shown in  FIG. 2  and the image transfer system  2  of the second embodiment shown in  FIG. 16  are denoted by the same reference numerals and signs. 
     Similarly to the image transfer system  1  of the first embodiment and the image transfer system  2  of the second embodiment, the image transfer system  3  is also an image transfer system configured such that transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between the imaging terminal  2100  and the display terminal  2200 , and the imaging terminal  2100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  2200 . Meanwhile, similarly to the image transfer system  2  of the second embodiment, the image transfer system  3  is an image transfer system in which the display terminal  2200  transmits a round-trip-propagation-time-measurement outgoing signal to the imaging terminal  2100  to calculate a round trip propagation-time in wireless transfer between the imaging terminal  2100  and the display terminal  2200 , and the imaging terminal  2100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated. 
     However, in the image transfer system  3 , some components included in the imaging terminal  1100  in the image transfer system  2  of the second embodiment are moved to the display terminal  2200 . More specifically, in the image transfer system  3 , the after-cycle-adjustment accuracy estimation unit  2105  that replaces the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  1100  in the image transfer system  2  of the second embodiment is included in the display terminal  2200 . 
     For this reason, in the image transfer system  3 , the display terminal  2200  estimates (calculates) a period-adjusted accuracy estimation value in a case where the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  2100  is temporarily adjusted, and transmits the estimated period-adjusted accuracy estimation value to the imaging terminal  2100 . Further, in the image transfer system  3 , the imaging terminal  2100  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value transmitted from the display terminal  2200 , and adjusts the period of an imaging synchronization signal to be generated. 
     However, also in the image transfer system  3 , although there is a process of exchanging signals through wireless transfer in association with a change in the disposition of components, functions and operations of the components are the same as the functions and operations of the corresponding components in the image transfer system  1  of the first embodiment and the image transfer system  2  of the second embodiment. That is, the functions and operations of the components included in the imaging terminal  2100  and the display terminal  2200  in the image transfer system  3  can be easily understood from the above description of the components included in the imaging terminal and the display terminal in each of the image transfer system  1  of the first embodiment and the image transfer system  2  of the second embodiment. Therefore, a detailed description related to the components included in the image transfer system  3  will be omitted. 
     Next, operations of processing in the image transfer system  3  will be described. Meanwhile, in the image transfer system  3 , it is assumed that a phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  100  in the image transfer system  1  of the first embodiment is included in the round trip propagation-time measurement unit  1202 . That is, in the image transfer system  3 , a configuration in which the phase adjustment unit not shown in the drawing is disposed at the same position as that in the image transfer system  2  of the second embodiment is adopted.  FIG. 19  is a flowchart showing a processing procedure of the image transfer system  3  in the third embodiment of the present invention. 
     In the image transfer system  3 , a process of transmitting information of a period-adjusted accuracy estimation value to the imaging terminal  2100  as information used to adjust the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is added, in association with a change to a configuration in which the after-cycle-adjustment accuracy estimation unit  2105  is included in the display terminal  2200 . However, an outline of the overall operation in the image transfer system  3  is the same as those of the image transfer system  1  of the first embodiment and the image transfer system  2  of the second embodiment. Therefore, also in the image transfer system  3 , a description of an outline of the overall operation will be omitted, and a more specific operation of performing cycle adjustment for an imaging synchronization signal in the image transfer system  3  will be described. 
     Also in the image transfer system  3 , when a cycle adjustment process is started, the display terminal  2200  transmits a round-trip-propagation-time-measurement outgoing signal to the imaging terminal  2100 , and the imaging terminal  2100  transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal to the display terminal  2200  in step S 302 . Meanwhile, the process of step S 302  in the image transfer system  3  is the same as the process of step S 302  in the image transfer system  2  of the second embodiment. 
     Thereafter, also in the image transfer system  3 , in step S 303 , the display terminal  2200  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  2100  and the display terminal  2200 . Meanwhile, the process of step S 303  in the image transfer system  3  is also the same as the process of step S 303  in the image transfer system  2  of the second embodiment. 
     Thereafter, also in the image transfer system  3 , in step S 304 , the display terminal  2200  generates a phase adjustment instruction for the phase adjustment unit, not shown in the drawing, which is provided in the round trip propagation-time measurement unit  1202  to adjust the phase of an imaging synchronization signal, on the basis of the round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  2100  and the display terminal  2200  which is calculated in step S 303 , and updates a round trip propagation-time determination value. Meanwhile, the process of step S 304  in the image transfer system  3  is also the same as the process of step S 304  in the image transfer system  2  of the second embodiment. 
     Thereafter, in the image transfer system  3 , in step S 306 , the period of an imaging synchronization signal is adjusted on the basis of the information of the plurality of round trip propagation-time determination values updated in step S 304 . However, in the image transfer system  3 , the display terminal  2200  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  3 , the imaging terminal  2100  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value, calculates a cycle adjustment amount, and outputs a cycle adjustment instruction to the synchronization signal generation unit  102 . 
     For this reason, in the image transfer system  3 , in step S 305  included in step S 306 , the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  2200  determines whether or not a predetermined period of time determined in advance has elapsed after the period of a previous imaging synchronization signal is adjusted. In a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has not elapsed after the period of the previous imaging synchronization signal is adjusted (“NO” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  2105  determines that cycle adjustment is not performed and terminates the process of step S 305 . 
     On the other hand, in a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has elapsed after the period of the previous imaging synchronization signal is adjusted (“YES” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  2105  determines that cycle adjustment is performed. In this case, similarly to the process of step S 2106  included in step S 306  in the image transfer system  1  of the first embodiment, the after-cycle-adjustment accuracy estimation unit  2105  calculates a period-adjusted accuracy estimation value in step S 2106  included in step S 306 . More specifically, the after-cycle-adjustment accuracy estimation unit  2105  acquires a combination of a plurality of round trip propagation-time determination values equivalent to a predetermined number of times per unit time determined in advance, scheduled transmission times of corresponding outward path signals for round trip propagation-time measurement, and scheduled transmission times of corresponding return path signals for round trip propagation-time measurement from the phase adjustment unit, not shown in the drawing, which is provided in the round trip propagation-time measurement unit  1202  (step S 802 ). In addition, the after-cycle-adjustment accuracy estimation unit  2105  extracts a minimum determination value for each unit time from the plurality of roundtrip propagation-time determination values acquired (step S 803 ). In addition, the after-cycle-adjustment accuracy estimation unit  2105  calculates a period-adjusted accuracy estimation value on the basis of information of the extracted minimum determination values equivalent to a predetermined number of times (step S 806 ). 
     Thereafter, in the image transfer system  3 , in step S 319  included in step S 306 , the display terminal  2200  generates an estimation notification signal including information of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  2105  and transmits the generated estimation notification signal to the imaging terminal  2100 . More specifically, the after-cycle-adjustment accuracy estimation unit  2105  generates an estimation notification signal including information of a calculated period-adjusted accuracy estimation value, information of a round trip propagation-time calculated by the round trip propagation-time measurement unit  1202 , information of a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, information of a scheduled transmission time included in a received round-trip-propagation-time-measurement returning signal, a phase adjustment instruction generated by the phase adjustment unit not shown in the drawing, and information of a plurality of round trip propagation-time determination values updated by the phase adjustment unit not shown in the drawing. In addition, the after-cycle-adjustment accuracy estimation unit  2105  outputs the generated estimation notification signal to the wireless communication unit  201  and transmits the estimation notification signal to the cycle adjustment determination unit  104  provided in the imaging terminal  2100  through the wireless communication unit  201  and the antenna  220 . Thereby, the cycle adjustment determination unit  104  acquires information such as a round trip propagation-time determination value used to adjust the period of an imaging synchronization signal from the after-cycle-adjustment accuracy estimation unit  2105  provided in the display terminal  2200 . More specifically, the wireless communication unit  108  receives an estimation notification signal transmitted from the display terminal  2200  through the antenna  120 . In addition, the wireless communication unit  108  outputs each of information of a period-adjusted accuracy estimation value included in the received estimation notification signal, information of a round trip propagation-time, information of a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, information of a scheduled transmission time of a round-trip-propagation-time-measurement returning signal, information of a phase adjustment instruction, and information of a plurality of round trip propagation-time determination values to the round trip propagation-time measurement assistance unit  1106 . 
     Further, in the image transfer system  3 , in step S 2107  included in step S 306 , the imaging terminal  2100  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of information such as a round trip propagation-time determination value transmitted from the display terminal  2200  similar to the process of step S 2107  included in step S 306  in the image transfer system  1  of the first embodiment, and outputs a calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. In this case, the round trip propagation-time measurement assistance unit  1106  outputs each of information of a period-adjusted accuracy estimation value included in the estimation notification signal output from the wireless communication unit  108 , information of a round trip propagation-time, information of a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, information of a scheduled transmission time of a round-trip-propagation-time-measurement returning signal, information of a phase adjustment instruction, and information of a plurality of round trip propagation-time determination values to each of the cycle adjustment determination unit  104  and the cycle adjustment unit  101 . Thereby, the cycle adjustment determination unit  104  determines whether or not cycle adjustment for an imaging synchronization signal is performed, on the basis of a period-adjusted accuracy estimation value calculated and transmitted by the after-cycle-adjustment accuracy estimation unit  2105  provided in the display terminal  2200  (step S 1004 ). Further, in a case where the cycle adjustment determination unit  104  determines that cycle adjustment for an imaging synchronization signal is performed, the cycle adjustment unit  101  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction (step S 1206 ). Thereby, the synchronization signal generation unit  102  performs cycle adjustment for adjusting the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  101 . 
     Meanwhile, the round trip propagation-time measurement assistance unit  1106  outputs a phase adjustment instruction which is output from the wireless communication unit  108  to the synchronization signal generation unit  102 . Thereby, the synchronization signal generation unit  102  performs phase adjustment for an imaging synchronization signal to be generated, in response to a phase adjustment instruction which is output from the round trip propagation-time measurement assistance unit  1106 , that is, a phase adjustment instruction transmitted from the phase adjustment unit, not shown in the drawing, which is provided in the round trip propagation-time measurement unit  1202  included in the display terminal  2200 . 
     In this manner, in the image transfer system  3 , the display terminal  2200  transmits a round-trip-propagation-time-measurement outgoing signal and calculates a round trip propagation-time in wireless transfer between the imaging terminal  2100  and the display terminal  2200 . Further, in the image transfer system  3 , the display terminal  2200  performs a process of updating a round trip propagation-time determination value and a process of estimating (calculating) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed. Further, in the image transfer system  3 , the display terminal  2200  transmits information of the calculated period-adjusted accuracy estimation value, information of the plurality of round trip propagation-time determination values updated, information of a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, information of a scheduled transmission time of a round-trip-propagation-time-measurement returning signal, and a phase adjustment instruction to the imaging terminal  2100 . Thereby, in the image transfer system  3 , the imaging terminal  2100  determines whether or not the period of an imaging synchronization signal is adjusted, on the basis of the pieces of information transmitted from the display terminal  2200 , and adjusts the periods of an imaging synchronization signal and a display synchronization signal so as not to be shifted with the elapse of time. 
     Meanwhile, similarly to the image transfer system  1  of the first embodiment and the image transfer system  2  of the second embodiment, also in the image transfer system  3 , when at least a process of updating a round trip propagation-time determination value is performed in step S 304 , the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  2200  can estimate the accuracy of an imaging synchronization signal (that is, calculate a period-adjusted accuracy estimation value). Therefore, similarly to the image transfer system  1  of the first embodiment and the image transfer system  2  of the second embodiment, also in the image transfer system  3 , the phase adjustment unit not shown in the drawing does not necessarily need to adjust the phase of an imaging synchronization signal in step S 304  and may be able to update a round trip propagation-time calculated by the round trip propagation-time measurement unit  1202  as a round trip propagation-time determination value. 
     With such a configuration and processing procedure, in the image transfer system  3 , the display terminal  2200  transmits a round-trip-propagation-time-measurement outgoing signal and measures (calculates) a round trip propagation-time in wireless transfer between the imaging terminal  2100  and the display terminal  2200  to update a round trip propagation-time determination value. Further, in the image transfer system  3 , the display terminal  2200  estimates (calculates) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed. Further, in the image transfer system  3 , the imaging terminal  2100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated, on the basis of the round trip propagation-time determination value updated by the display terminal  2200  and the estimated (calculated) period-adjusted accuracy estimation value. Thereby, similarly to the image transfer system  1  of the first embodiment and the image transfer system  2  of the second embodiment, also in the image transfer system  3 , the phase or period (at least a period) of an imaging synchronization signal which is generated by the imaging terminal  2100  (more specifically, the synchronization signal generation unit  102 ) is adjusted so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  2200 . That is, similarly to the image transfer system  1  of the first embodiment and the image transfer system  2  of the second embodiment, also in the image transfer system  3 , a timing when the imaging terminal  2100  wirelessly transfers captured image data of an image captured by the imaging unit not shown in the drawing to the display terminal  2200  is matched to a timing when the display terminal  2200  displays an image corresponding to the captured image data on the display unit not shown in the drawing. Thereby, similarly to the image transfer system  1  of the first embodiment and the image transfer system  2  of the second embodiment, also in the image transfer system  3 , the display terminal  2200  can stably display an image corresponding to captured image data wirelessly transferred from the imaging terminal  2100  on the display unit not shown in the drawing. 
     As described above, the image transfer system  3  of the third embodiment performs transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal between the round trip propagation-time measurement unit  1202  included in the display terminal  2200  and the round trip propagation-time measurement assistance unit  1106  included in the imaging terminal  2100  after wireless connection between the imaging terminal  2100  and the display terminal  2200  is established. Further, in the image transfer system  3  of the third embodiment, the round trip propagation-time measurement unit  1202  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  2100  and the display terminal  2200 , on the basis of a transmission time of a round-trip-propagation-time-measurement outgoing signal and a reception time of a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the round trip propagation-time measurement assistance unit  1106 . Further, in the image transfer system  3  of the third embodiment, the round trip propagation-time measurement unit  1202  (more specifically, the phase adjustment unit not shown in the drawing) updates the round trip propagation-time calculated by the round trip propagation-time measurement unit  1202  as a round trip propagation-time determination value. Further, in the image transfer system  3  of the third embodiment, the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  2200  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  3  of the third embodiment, the after-cycle-adjustment accuracy estimation unit  2105  transmits information for adjusting the period of an imaging synchronization signal in the imaging terminal  2100  to the imaging terminal  2100 , inclusive of a period-adjusted accuracy estimation value. Further, in the image transfer system  3  of the third embodiment, the cycle adjustment determination unit  104  included in the imaging terminal  2100  determines whether or not cycle adjustment for an imaging synchronization signal generated by the synchronization signal generation unit  102  included in the imaging terminal  2100  is performed, on the basis of a period-adjusted accuracy estimation value. Further, in the image transfer system  3  of the third embodiment, in a case where the cycle adjustment determination unit  104  determines that cycle adjustment for an imaging synchronization signal is performed, the cycle adjustment unit  101  included in the imaging terminal  2100  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, in the image transfer system  3  of the third embodiment, the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  101 . 
     Thereby, similarly to the image transfer system  1  of the first embodiment and the image transfer system  2  of the second embodiment, also in the image transfer system  3  of the third embodiment, the phase or period (at least a period) of an imaging synchronization signal can be matched to the phase or period (at least a period) of a display synchronization signal. That is, also in the image transfer system  3  of the third embodiment, even when the periods of an imaging synchronization signal and a display synchronization signal are shifted with the elapse of time due to an error of a phase or a period between an imaging reference clock signal and a display reference clock signal, the phase or period (at least a period) of the imaging synchronization signal can be matched to the phase or period (at least a period) of the display synchronization signal. Thus, also in the image transfer system  3  of the third embodiment, the same effects as those in the image transfer system  1  of the first embodiment and the image transfer system  2  of the second embodiment can be obtained. That is, similarly to the image transfer system  1  of the first embodiment and the image transfer system  2  of the second embodiment, also in the image transfer system  3  of the third embodiment, it is possible to wirelessly transfer captured image data from the imaging terminal  2100  to the display terminal  2200  without exceeding a validity period of a display image and to stably display a display image corresponding to the captured image data transmitted from the imaging terminal  2100  in the display terminal  2200 . 
     Moreover, in the image transfer system  3  of the third embodiment, the display terminal  2200  estimates (calculates) a period-adjusted accuracy estimation value, in addition to calculating a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  2100  and the display terminal  2200 . Thereby, in the image transfer system  3  of the third embodiment, it is not necessary to calculate a round trip propagation-time in the imaging terminal  2100  and estimate (calculate) a period-adjusted accuracy estimation value in the imaging terminal  2100 , and thus it is possible to reduce a load of processing performed in the imaging terminal  2100 . 
     Fourth Embodiment 
     Hereinafter, an image transfer system of a fourth embodiment of the present invention will be described.  FIG. 20  is a block diagram showing a schematic configuration of the image transfer system in the fourth embodiment of the present invention. An image transfer system  4  includes an imaging terminal  3100  and a display terminal  3200 . The imaging terminal  3100  includes a synchronization signal generation unit  102 , a cycle adjustment unit  101 , a source oscillation clock generation unit  103 , a wireless communication unit  108 , a round trip propagation-time measurement assistance unit  1106 , a clocking unit  107 , and an antenna  120 . In addition, the display terminal  3200  includes a synchronous signal generation unit  204 , a source oscillation clock generation unit  205 , a wireless communication unit  201 , a round trip propagation-time measurement unit  1202 , a clocking unit  203 , an after-cycle-adjustment accuracy estimation unit  2105 , a cycle-adjustment determination unit  3104 , and an antenna  220 . 
     Meanwhile, also in  FIG. 20 , in components of the image transfer system  4 , the same components as the components included in the image transfer system  1  of the first embodiment to the image transfer system  3  of the third embodiment are denoted by the same reference numerals and signs. 
     Similarly to the image transfer system  1  of the first embodiment to the image transfer system  3  of the third embodiment, the image transfer system  4  is also an image transfer system configured such that transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between the imaging terminal  3100  and the display terminal  3200 , and the imaging terminal  3100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  3200 . Meanwhile, similarly to the image transfer system  2  of the second embodiment and the image transfer system  3  of the third embodiment, the image transfer system  4  is an image transfer system in which the display terminal  3200  transmits a round-trip-propagation-time-measurement outgoing signal to the imaging terminal  3100  to calculate a round trip propagation-time in wireless transfer between the imaging terminal  3100  and the display terminal  3200 , and the imaging terminal  3100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated. 
     However, in the image transfer system  4 , some components included in the imaging terminal  2100  in the image transfer system  3  of the third embodiment are moved to the display terminal  3200 . More specifically, in the image transfer system  4 , the cycle-adjustment determination unit  3104  that replaces the cycle adjustment determination unit  104  included in the imaging terminal  2100  in the image transfer system  3  of the third embodiment is included in the display terminal  3200 . 
     For this reason, in the image transfer system  4 , the display terminal  3200  estimates (calculates) a period-adjusted accuracy estimation value in a case where the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  3100  is temporarily adjusted, and transmits a cycle adjustment execution determination result indicating a result obtained by determining whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value to the imaging terminal  3100 . Further, in the image transfer system  4 , in a case where the cycle adjustment execution determination result transmitted from the display terminal  3200  indicates that cycle adjustment for an imaging synchronization signal is performed, the imaging terminal  3100  adjusts the period of an imaging synchronization signal to be generated. 
     However, also in the image transfer system  4 , although there is a process of exchanging signals through wireless transfer in association with a change in the disposition of components, functions and operations of the components are the same as the functions and operations of the corresponding components in the image transfer system  1  of the first embodiment to the image transfer system  3  of the third embodiment. That is, the functions and operations of the components included in the imaging terminal  3100  and the display terminal  3200  in the image transfer system  4  can be easily understood from the above description of the components included in the imaging terminal and the display terminal in each of the image transfer system  1  of the first embodiment to the image transfer system  3  of the third embodiment. Therefore, a detailed description related to the components included in the image transfer system  4  will be omitted. 
     Next, operations of processing in the image transfer system  4  will be described. Meanwhile, in the image transfer system  4 , it is assumed that a phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  100  in the image transfer system  1  of the first embodiment is included in the round trip propagation-time measurement unit  1202 . That is, in the image transfer system  4 , a configuration in which the phase adjustment unit not shown in the drawing is disposed at the same position as those in the image transfer system  2  of the second embodiment and the image transfer system  3  of the third embodiment is adopted.  FIG. 21  is a flowchart showing a processing procedure of the image transfer system  4  in the fourth embodiment of the present invention. 
     In the image transfer system  4 , a process of transmitting information of a period-adjusted accuracy estimation value and information of a cycle adjustment execution determination result to the imaging terminal  3100  as information used to adjust the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is added, in association with a change to a configuration in which the after-cycle-adjustment accuracy estimation unit  2105  and the cycle-adjustment determination unit  3104  are included in the display terminal  3200 . However, an outline of the overall operation in the image transfer system  4  is the same as those of the image transfer system  1  of the first embodiment to the image transfer system  3  of the third embodiment. Therefore, also in the image transfer system  4 , a description of an outline of the overall operation will be omitted, and a more specific operation of performing cycle adjustment for an imaging synchronization signal in the image transfer system  4  will be described. 
     Also in the image transfer system  4 , when a cycle adjustment process is started, the display terminal  3200  transmits a round-trip-propagation-time-measurement outgoing signal to the imaging terminal  3100 , and the imaging terminal  3100  transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal to the display terminal  3200  in step S 302 . Meanwhile, the process of step S 302  in the image transfer system  4  is the same as the process of step S 302  in the image transfer system  2  of the second embodiment and the image transfer system  3  of the third embodiment. 
     Thereafter, also in the image transfer system  4 , in step S 303 , the display terminal  3200  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  3100  and the display terminal  3200 . Meanwhile, the process of step S 303  in the image transfer system  4  is also the same as the process of step S 303  in the image transfer system  2  of the second embodiment and the image transfer system  3  of the third embodiment. 
     Thereafter, also in the image transfer system  4 , in step S 304 , the display terminal  3200  generates a phase adjustment instruction for the phase adjustment unit, not shown in the drawing, which is provided in the round trip propagation-time measurement unit  1202  to adjust the phase of an imaging synchronization signal, on the basis of the round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  3100  and the display terminal  3200  which is calculated in step S 303 , and updates a round trip propagation-time determination value. Meanwhile, the process of step S 304  in the image transfer system  4  is also the same as the process of step S 304  in each of the image transfer system  2  of the second embodiment and the image transfer system  3  of the third embodiment. 
     Thereafter, in the image transfer system  4 , in step S 306 , the period of an imaging synchronization signal is adjusted on the basis of the information of the plurality of round trip propagation-time determination values updated in step S 304 . However, in the image transfer system  4 , the display terminal  3200  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed, and determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the estimated (calculated) period-adjusted accuracy estimation value. Further, in the image transfer system  4 , the imaging terminal  3100  calculates a cycle adjustment amount and outputs a cycle adjustment instruction to the synchronization signal generation unit  102 . 
     For this reason, in the image transfer system  4 , in step S 305  included in step S 306 , the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  3200  determines whether or not a predetermined period of time determined in advance has elapsed after the period of a previous imaging synchronization signal is adjusted. In a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has not elapsed after the period of the previous imaging synchronization signal is adjusted (“NO” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  2105  determines that cycle adjustment is not performed and terminates the process of step S 305 . 
     On the other hand, in a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has elapsed after the period of the previous imaging synchronization signal is adjusted (“YES” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  2105  determines that cycle adjustment is performed and calculates a period-adjusted accuracy estimation value in step S 2106  included in step S 306 . Meanwhile, the process of step S 305  and the process of step S 2106  which are included in step S 306  in the image transfer system  4  are the same as the process of step S 305  and the process of step S 2106  which are included in step S 306  in the image transfer system  3  of the third embodiment. 
     Further, in the image transfer system  4 , in step S 1004  included in step S 306 , the display terminal  3200  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the calculated period-adjusted accuracy estimation value, similar to the process of step S 1004  included in step S 306  in the image transfer system  1  of the first embodiment. More specifically, the cycle-adjustment determination unit  3104  included in the display terminal  3200  compares the accuracy of an imaging synchronization signal indicated by the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  2105  with the current accuracy of the imaging synchronization signal, determines that cycle adjustment for an imaging synchronization signal is not performed in a case where the estimated accuracy of the imaging synchronization signal is equal to the current accuracy of the imaging synchronization signal or has not been improved, and terminates the process of step S 1004 . 
     On the other hand, in a case where the estimated accuracy of the imaging synchronization signal has been improved compared with the current accuracy of the imaging synchronization signal, the cycle-adjustment determination unit  3104  determines that cycle adjustment for an imaging synchronization signal is performed, and generates a cycle adjustment execution determination result indicating a determination result. 
     Thereafter, in the image transfer system  4 , in step S 311  in step S 306 , the display terminal  3200  generates a determination notification signal including information of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  2105  and information of the cycle adjustment execution determination result obtained by the cycle-adjustment determination unit  3104 , and transmits the generated determination notification signal to the imaging terminal  3100 . More specifically, the cycle-adjustment determination unit  3104  generates a determination notification signal including the information of the obtained cycle adjustment execution determination result, information of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  2105 , information of a round trip propagation-time calculated by the round trip propagation-time measurement unit  1202 , information of a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, information of a scheduled transmission time included in a received round-trip-propagation-time-measurement returning signal, a phase adjustment instruction generated by the phase adjustment unit not shown in the drawing, and information of a plurality of round trip propagation-time determination values updated by the phase adjustment unit not shown in the drawing. In addition, the cycle-adjustment determination unit  3104  outputs the generated determination notification signal to the wireless communication unit  201  and transmits the determination notification signal to the cycle adjustment unit  101  in the imaging terminal  3100  through the wireless communication unit  201  and the antenna  220 . Thereby, the cycle adjustment unit  101  acquires information such as a round trip propagation-time determination value used to adjust the period of an imaging synchronization signal from the cycle-adjustment determination unit  3104  provided in the display terminal  3200 . More specifically, the wireless communication unit  108  receives the determination notification signal transmitted from the display terminal  3200  through the antenna  120 . In addition, the wireless communication unit  108  outputs each of the information of the cycle adjustment execution determination result, the information of the period-adjusted accuracy estimation value, the information of the round trip propagation-time, the information of the scheduled transmission time of the round-trip-propagation-time-measurement outgoing signal, the information of the scheduled transmission time of the round-trip-propagation-time-measurement returning signal, the information of the phase adjustment instruction, and the information of the plurality of round trip propagation-time determination values which are included in the received determination notification signal to the round trip propagation-time measurement assistance unit  1106 . 
     Further, in the image transfer system  4 , in step S 1206  included in step S 306 , the imaging terminal  3100  calculates a cycle adjustment amount on the basis of information such as a round trip propagation-time determination value transmitted from the display terminal  3200  similar to the process of step S 1206  included in step S 306  in the image transfer system  1  of the first embodiment, and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. In this case, the round trip propagation-time measurement assistance unit  1106  outputs each of the information of the cycle adjustment execution determination result, the information of the period-adjusted accuracy estimation value, the information of the round trip propagation-time, the information of the scheduled transmission time of the round-trip-propagation-time-measurement outgoing signal, the information of the scheduled transmission time of the round-trip-propagation-time-measurement returning signal, the information of the phase adjustment instruction, and the information of the plurality of round trip propagation-time determination values which are included in the determination notification signal output from the wireless communication unit  108  to the cycle adjustment unit  101 . Thereby, the cycle adjustment unit  101  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal on the basis of the period-adjusted accuracy estimation value transmitted by the cycle-adjustment determination unit  3104  provided in the display terminal  3200  and a combination of the round trip propagation-time determination value, the scheduled transmission time of the round-trip-propagation-time-measurement outgoing signal, and the scheduled transmission time of the round-trip-propagation-time-measurement returning signal, and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, the synchronization signal generation unit  102  performs cycle adjustment for adjusting the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  101 . 
     Meanwhile, the round trip propagation-time measurement assistance unit  1106  outputs a phase adjustment instruction which is output from the wireless communication unit  108  to the synchronization signal generation unit  102 . Thereby, the synchronization signal generation unit  102  performs phase adjustment for an imaging synchronization signal to be generated, in response to the phase adjustment instruction which is output from the round trip propagation-time measurement assistance unit  1106 , that is, a phase adjustment instruction which is transmitted from the phase adjustment unit, not shown in the drawing, which is provided in the round trip propagation-time measurement unit  1202  included in the display terminal  3200 . 
     In this manner, in the image transfer system  4 , the display terminal  3200  transmits a round-trip-propagation-time-measurement outgoing signal and calculates a round trip propagation-time in wireless transfer between the imaging terminal  3100  and the display terminal  3200 . Further, in the image transfer system  4 , the display terminal  3200  performs a process of updating a round trip propagation-time determination value, a process of estimating (calculating) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed, and a process of determining whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  3100  is performed. Further, in the image transfer system  4 , the display terminal  3200  transmits the information of the obtained cycle adjustment execution determination result, the information of the calculated period-adjusted accuracy estimation value, the information of the plurality of roundtrip propagation-time determination values updated, the information of the round trip propagation-time, the information of the scheduled transmission time of the round-trip-propagation-time-measurement outgoing signal, the information of the scheduled transmission time of the round-trip-propagation-time-measurement returning signal, and the phase adjustment instruction to the imaging terminal  3100 . Thereby, in the image transfer system  4 , the imaging terminal  3100  calculates a cycle adjustment amount on the basis of the pieces of information transmitted from the display terminal  3200 , and adjusts the periods of an imaging synchronization signal and a display synchronization signal so as not to be shifted with the elapse of time. 
     Meanwhile, similarly to the image transfer system  1  of the first embodiment to the image transfer system  3  of the third embodiment, also in the image transfer system  4 , when at least a process of updating around trip propagation-time determination value is performed in step S 304 , the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  3200  can estimate the accuracy of an imaging synchronization signal (that is, calculate a period-adjusted accuracy estimation value). Therefore, similarly to the image transfer system  1  of the first embodiment to the image transfer system  3  of the third embodiment, also in the image transfer system  4 , the phase adjustment unit not shown in the drawing does not necessarily need to adjust the phase of an imaging synchronization signal in step S 304  and may be able to update a round trip propagation-time calculated by the round trip propagation-time measurement unit  1202  as a round trip propagation-time determination value. 
     With such a configuration and processing procedure, in the image transfer system  4 , the display terminal  3200  transmits a round-trip-propagation-time-measurement outgoing signal and measures (calculates) a round trip propagation-time in wireless transfer between the imaging terminal  3100  and the display terminal  3200  to update a round trip propagation-time determination value. Further, in the image transfer system  4 , the display terminal  3200  further estimates (calculates) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed and determines whether or not cycle adjustment for an imaging synchronization signal is performed in the imaging terminal  3100 . Further, in the image transfer system  4 , the imaging terminal  3100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated, in accordance with a cycle adjustment execution determination result obtained by determining whether or not the display terminal  3200  performs cycle adjustment for an imaging synchronization signal. Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  3  of the third embodiment, also in the image transfer system  4 , the phase or period (at least a period) of an imaging synchronization signal which is generated by the imaging terminal  3100  (more specifically, the synchronization signal generation unit  102 ) is adjusted so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  3200 . That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  3  of the third embodiment, also in the image transfer system  4 , a timing when the imaging terminal  3100  wirelessly transfers captured image data of an image captured by an imaging unit not shown in the drawing to the display terminal  3200  is matched to a timing when the display terminal  3200  displays an image corresponding to the captured image data on a display unit not shown in the drawing. Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  3  of the third embodiment, also in the image transfer system  4 , the display terminal  3200  can stably display an image corresponding to captured image data wirelessly transferred from the imaging terminal  3100  on the display unit not shown in the drawing. 
     As described above, the image transfer system  4  of the fourth embodiment performs transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal between the round trip propagation-time measurement unit  1202  included in the display terminal  3200  and the round trip propagation-time measurement assistance unit  1106  included in the imaging terminal  3100  after wireless connection between the imaging terminal  3100  and the display terminal  3200  is established. Further, in the image transfer system  4  of the fourth embodiment, the round trip propagation-time measurement unit  1202  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  3100  and the display terminal  3200 , on the basis of a transmission time of a round-trip-propagation-time-measurement outgoing signal and a reception time of a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the round trip propagation-time measurement assistance unit  1106 . Further, in the image transfer system  4  of the fourth embodiment, the round trip propagation-time measurement unit  1202  (more specifically, the phase adjustment unit not shown in the drawing) updates the round trip propagation-time calculated by the round trip propagation-time measurement unit  1202  as a round trip propagation-time determination value. Further, in the image transfer system  4  of the fourth embodiment, the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  3200  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  4  of the fourth embodiment, the cycle-adjustment determination unit  3104  included in the display terminal  3200  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  3100  is performed, on the basis of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  2105 . Further, in the image transfer system  4  of the fourth embodiment, the cycle-adjustment determination unit  3104  transmits information for adjusting the period of an imaging synchronization signal in the imaging terminal  3100  to the imaging terminal  3100 , inclusive of a cycle adjustment execution determination result and a period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  2105 . Further, in the image transfer system  4  of the fourth embodiment, the cycle adjustment unit  101  included in the imaging terminal  3100  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal in a case where the cycle adjustment execution determination result indicates that cycle adjustment for an imaging synchronization signal is performed, and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, in the image transfer system  4  of the fourth embodiment, the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  101 . 
     Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  3  of the third embodiment, also in the image transfer system  4  of the fourth embodiment, the phase or period (at least a period) of an imaging synchronization signal can be matched to the phase or period (at least a period) of a display synchronization signal. That is, also in the image transfer system  4  of the fourth embodiment, even when the periods of an imaging synchronization signal and a display synchronization signal are shifted with the elapse of time due to an error of a phase or a period between an imaging reference clock signal and a display reference clock signal, the phase or period (at least a period) of the imaging synchronization signal can be matched to the phase or period (at least a period) of the display synchronization signal. Thus, also in the image transfer system  4  of the fourth embodiment, the same effects as those in the image transfer system  1  of the first embodiment to the image transfer system  3  of the third embodiment can be obtained. That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  3  of the third embodiment, also in the image transfer system  4  of the fourth embodiment, it is possible to wirelessly transfer captured image data from the imaging terminal  3100  to the display terminal  3200  without exceeding a validity period of a display image and to stably display a display image corresponding to the captured image data transmitted from the imaging terminal  3100  in the display terminal  3200 . 
     Moreover, in the image transfer system  4  of the fourth embodiment, the display terminal  3200  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  3100  and the display terminal  3200 , estimates (calculates) a period-adjusted accuracy estimation value, and determines whether or not cycle adjustment for an imaging synchronization signal is performed. Thereby, in the image transfer system  4  of the fourth embodiment, the imaging terminal  3100  may only calculate a cycle adjustment amount. That is, in the image transfer system  4  of the fourth embodiment, it is not necessary to calculate a round trip propagation-time, estimate (calculate) a period-adjusted accuracy estimation value, and determine whether or not cycle adjustment for an imaging synchronization signal is performed in the imaging terminal  3100 , and thus it is possible to reduce a load of processing performed in the imaging terminal  3100 . 
     Fifth Embodiment 
     Hereinafter, an image transfer system of a fifth embodiment of the present invention will be described.  FIG. 22  is a block diagram showing a schematic configuration of the image transfer system in the fifth embodiment of the present invention. An image transfer system  5  includes an imaging terminal  4100  and a display terminal  4200 . The imaging terminal  4100  includes a synchronization signal generation unit  102 , a source oscillation clock generation unit  103 , a wireless communication unit  108 , a round trip propagation-time measurement assistance unit  1106 , a clocking unit  107 , and an antenna  120 . In addition, the display terminal  4200  includes a synchronous signal generation unit  204 , a source oscillation clock generation unit  205 , a wireless communication unit  201 , a round trip propagation-time measurement unit  1202 , a clocking unit  203 , an after-cycle-adjustment accuracy estimation unit  2105 , a cycle-adjustment determination unit  3104 , a cycle adjustment unit  4101 , and an antenna  220 . 
     Meanwhile, also in  FIG. 22 , in components of the image transfer system  5 , the same components as the components included in the image transfer system  1  of the first embodiment to the image transfer system  4  of the fourth embodiment are denoted by the same reference numerals and signs. 
     Similarly to the image transfer system  1  of the first embodiment to the image transfer system  4  of the fourth embodiment, the image transfer system  5  is also an image transfer system configured such that transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between the imaging terminal  4100  and the display terminal  4200 , and the imaging terminal  4100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  4200 . Meanwhile, similarly to the image transfer system  2  of the second embodiment to the image transfer system  4  of the fourth embodiment, the image transfer system  5  is an image transfer system in which the display terminal  4200  transmits a round-trip-propagation-time-measurement outgoing signal to the imaging terminal  4100  to calculate a round trip propagation-time in wireless transfer between the imaging terminal  4100  and the display terminal  4200 , and the imaging terminal  4100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated. 
     However, in the image transfer system  5 , some components included in the imaging terminal  3100  in the image transfer system  4  of the fourth embodiment are moved to the display terminal  4200 . More specifically, in the image transfer system  5 , the cycle adjustment unit  4101  that replaces the cycle adjustment unit  101  included in the imaging terminal  3100  in the image transfer system  4  of the fourth embodiment is included in the display terminal  4200 . 
     For this reason, in the image transfer system  5 , the display terminal  4200  estimates (calculates) a period-adjusted accuracy estimation value in a case where the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  4100  is temporarily adjusted, determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value, calculates a cycle adjustment amount in a case where it is determined that cycle adjustment for an imaging synchronization signal is performed, and transmits the calculated cycle adjustment amount to the imaging terminal  4100  together with a cycle adjustment instruction. Further, in the image transfer system  5 , the imaging terminal  4100  adjusts the period of an imaging synchronization signal to be generated, in response to the cycle adjustment instruction transmitted from the display terminal  4200 . 
     However, also in the image transfer system  5 , although there is a process of exchanging signals through wireless transfer in association with a change in the disposition of components, functions and operations of the components are the same as the functions and operations of the corresponding components in the image transfer system  1  of the first embodiment to the image transfer system  4  of the fourth embodiment. That is, the functions and operations of the components included in the imaging terminal  4100  and the display terminal  4200  in the image transfer system  5  can be easily understood from the above description of the components included in the imaging terminal and the display terminal in each of the image transfer system  1  of the first embodiment to the image transfer system  4  of the fourth embodiment. Therefore, a detailed description related to the components included in the image transfer system  5  will be omitted. 
     Next, operations of processing in the image transfer system  5  will be described. Meanwhile, in the image transfer system  5 , it is assumed that a phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  100  in the image transfer system  1  of the first embodiment is included in the round trip propagation-time measurement unit  1202 . That is, in the image transfer system  5 , a configuration in which the phase adjustment unit not shown in the drawing is disposed at the same position as those in the image transfer system  2  of the second embodiment to the image transfer system  4  of the fourth embodiment is adopted.  FIG. 23  is a flowchart showing a processing procedure of the image transfer system  5  in the fifth embodiment of the present invention. 
     In the image transfer system  5 , a process of transmitting a cycle adjustment instruction including a cycle adjustment amount to the imaging terminal  4100  as information used to adjust the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is added, in association with a change to a configuration in which the after-cycle-adjustment accuracy estimation unit  2105 , the cycle-adjustment determination unit  3104 , and the cycle adjustment unit  4101  are included in the display terminal  4200 . However, an outline of the overall operation in the image transfer system  5  is the same as those of the image transfer system  1  of the first embodiment to the image transfer system  4  of the fourth embodiment. Therefore, also in the image transfer system  5 , a description of an outline of the overall operation will be omitted, and a more specific operation of performing cycle adjustment for an imaging synchronization signal in the image transfer system  5  will be described. 
     Also in the image transfer system  5 , when a cycle adjustment process is started, the display terminal  4200  transmits a round-trip-propagation-time-measurement outgoing signal to the imaging terminal  4100 , and the imaging terminal  4100  transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal to the display terminal  4200  in step S 302 . Meanwhile, the process of step S 302  in the image transfer system  5  is the same as the process of step S 302  in the image transfer system  2  of the second embodiment to the image transfer system  4  of the fourth embodiment. 
     Thereafter, also in the image transfer system  5 , in step S 303 , the display terminal  4200  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  4100  and the display terminal  4200 . Meanwhile, the process of step S 303  in the image transfer system  5  is also the same as the process of step S 303  in the image transfer system  2  of the second embodiment to the image transfer system  4  of the fourth embodiment. 
     Thereafter, also in the image transfer system  5 , in step S 304 , the display terminal  4200  generates a phase adjustment instruction for the phase adjustment unit, not shown in the drawing, which is provided in the round trip propagation-time measurement unit  1202  to adjust the phase of an imaging synchronization signal, on the basis of the round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  4100  and the display terminal  4200  which is calculated in step S 303 , and updates a round trip propagation-time determination value. Meanwhile, the process of step S 304  in the image transfer system  5  is also the same as the process of step S 304  in each of the image transfer system  2  of the second embodiment to the image transfer system  4  of the fourth embodiment. 
     Thereafter, in the image transfer system  5 , in step S 306 , the display terminal  4200  adjusts the period of an imaging synchronization signal on the basis of information of a plurality of round trip propagation-time determination values updated in step S 304 . More specifically, the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  4200  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. In addition, the cycle-adjustment determination unit  3104  included in the display terminal  4200  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value estimated (calculated) by the after-cycle-adjustment accuracy estimation unit  2105 . Further, in a case where the cycle-adjustment determination unit  3104  determines that cycle adjustment for an imaging synchronization signal is performed, the cycle adjustment unit  4101  included in the display terminal  4200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  4100  and generates a cycle adjustment instruction. 
     Thereafter, in the image transfer system  5 , in step S 312 , the display terminal  4200  generates an adjustment notification signal including information of the cycle adjustment instruction including the cycle adjustment amount calculated by the cycle adjustment unit  4101  and transmits the generated adjustment notification signal to the imaging terminal  4100 . More specifically, the cycle adjustment unit  4101  generates an adjustment notification signal including information of the cycle adjustment instruction including the calculated cycle adjustment amount and a phase adjustment instruction generated by the phase adjustment unit not shown in the drawing. In addition, the cycle adjustment unit  4101  outputs the generated adjustment notification signal to the wireless communication unit  201  and transmits the adjustment notification signal to the synchronization signal generation unit  102  provided in the imaging terminal  4100  through the wireless communication unit  201  and the antenna  220 . Thereby, the synchronization signal generation unit  102  acquires the information of the cycle adjustment instruction including the cycle adjustment amount from the cycle adjustment unit  4101  provided in the display terminal  4200 . More specifically, the wireless communication unit  108  receives the adjustment notification signal transmitted from the display terminal  4200  through the antenna  120 . In addition, the wireless communication unit  108  outputs each of the information of the cycle adjustment instruction and the information of the phase adjustment instruction which are included in the received adjustment notification signal to the round trip propagation-time measurement assistance unit  1106 . In addition, the round trip propagation-time measurement assistance unit  1106  outputs the information of the cycle adjustment instruction included in the adjustment notification signal which is output from the wireless communication unit  108  to the synchronization signal generation unit  102 . Thereby, the synchronization signal generation unit  102  performs cycle adjustment for adjusting the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  4101 . 
     Meanwhile, the round trip propagation-time measurement assistance unit  1106  outputs the phase adjustment instruction, which is output from the wireless communication unit  108 , to the synchronization signal generation unit  102 . Thereby, the synchronization signal generation unit  102  performs phase adjustment for an imaging synchronization signal to be generated, in response to the phase adjustment instruction which is output from the round trip propagation-time measurement assistance unit  1106 , that is, the phase adjustment instruction transmitted from the phase adjustment unit, not shown in the drawing, which is provided in the round trip propagation-time measurement unit  1202  included in the display terminal  4200 . 
     In this manner, in the image transfer system  5 , the display terminal  4200  transmits a round-trip-propagation-time-measurement outgoing signal and calculates a round trip propagation-time in wireless transfer between the imaging terminal  4100  and the display terminal  4200 . Further, in the image transfer system  5 , the display terminal  4200  performs a process of updating a round trip propagation-time determination value, a process of estimating (calculating) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed, a process of determining whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  4100  is performed, and a process of calculating a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction. Further, in the image transfer system  5 , the display terminal  4200  transmits information of the cycle adjustment instruction, including the calculated cycle adjustment amount, and a phase adjustment instruction to the imaging terminal  4100 . Thereby, in the image transfer system  5 , the imaging terminal  4100  adjusts the periods of an imaging synchronization signal and a display synchronization signal so as not to be shifted with the elapse of time, in response to the cycle adjustment instruction transmitted from the display terminal  4200 . 
     Meanwhile, similarly to the image transfer system  1  of the first embodiment to the image transfer system  4  of the fourth embodiment, also in the image transfer system  5 , when at least a process of updating a round trip propagation-time determination value is performed in step S 304 , the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  4200  can estimate the accuracy of an imaging synchronization signal (that is, calculate a period-adjusted accuracy estimation value). Therefore, similarly to the image transfer system  1  of the first embodiment to the image transfer system  4  of the fourth embodiment, also in the image transfer system  5 , the phase adjustment unit not shown in the drawing does not necessarily need to adjust the phase of an imaging synchronization signal in step S 304  and may be able to update a round trip propagation-time calculated by the round trip propagation-time measurement unit  1202  as a round trip propagation-time determination value. 
     With such a configuration and processing procedure, in the image transfer system  5 , the display terminal  4200  transmits a round-trip-propagation-time-measurement outgoing signal and measures (calculates) a round trip propagation-time in wireless transfer between the imaging terminal  4100  and the display terminal  4200  to update a round trip propagation-time determination value. Further, in the image transfer system  5 , the display terminal  4200  further estimates (calculates) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed, determines whether or not cycle adjustment for an imaging synchronization signal is performed in the imaging terminal  4100 , and calculates a cycle adjustment amount for adjusting the period of an imaging synchronization signal to generate a cycle adjustment instruction. Further, in the image transfer system  5 , the imaging terminal  4100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated, in response to the phase adjustment instruction generated by the display terminal  4200 . Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  4  of the fourth embodiment, also in the image transfer system  5 , the phase or period (at least a period) of an imaging synchronization signal which is generated by the imaging terminal  4100  (more specifically, the synchronization signal generation unit  102 ) is adjusted so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  4200 . That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  4  of the fourth embodiment, also in the image transfer system  5 , a timing when the imaging terminal  4100  wirelessly transfers captured image data of an image captured by an imaging unit not shown in the drawing to the display terminal  4200  is matched to a timing when the display terminal  4200  displays an image corresponding to the captured image data on a display unit not shown in the drawing. Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  4  of the fourth embodiment, also in the image transfer system  5 , the display terminal  4200  can stably display an image corresponding to captured image data wirelessly transferred from the imaging terminal  4100  on the display unit not shown in the drawing. 
     As described above, the image transfer system  5  of the fifth embodiment performs transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal between the round trip propagation-time measurement unit  1202  included in the display terminal  4200  and the round trip propagation-time measurement assistance unit  1106  included in the imaging terminal  4100  after wireless connection between the imaging terminal  4100  and the display terminal  4200  is established. Further, in the image transfer system  5  of the fifth embodiment, the round trip propagation-time measurement unit  1202  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  4100  and the display terminal  4200 , on the basis of a transmission time of a round-trip-propagation-time-measurement outgoing signal and a reception time of a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the round trip propagation-time measurement assistance unit  1106 . Further, in the image transfer system  5  of the fifth embodiment, the round trip propagation-time measurement unit  1202  (more specifically, the phase adjustment unit not shown in the drawing) updates the round trip propagation-time calculated by the round trip propagation-time measurement unit  1202  as a round trip propagation-time determination value. Further, in the image transfer system  5  of the fifth embodiment, the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  4200  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  5  of the fifth embodiment, the cycle-adjustment determination unit  3104  included in the display terminal  4200  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  4100  is performed, on the basis of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  2105 . Further, in the image transfer system  5  of the fifth embodiment, in a case where a cycle adjustment execution determination result indicates that cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  4100  is performed, the cycle adjustment unit  4101  included in the display terminal  4200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction. Further, in the image transfer system.  5  of the fifth embodiment, the cycle adjustment unit  4101  transmits information of a cycle adjustment instruction including a cycle adjustment amount for adjusting the period of an imaging synchronization signal in the imaging terminal  4100  to the imaging terminal  4100 . Thereby, in the image transfer system  5  of the fifth embodiment, the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is transmitted from the cycle adjustment unit  4101 . 
     Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  4  of the fourth embodiment, also in the image transfer system  5  of the fifth embodiment, the phase or period (at least a period) of an imaging synchronization signal can be matched to the phase or period (at least a period) of a display synchronization signal. That is, also in the image transfer system  5  of the fifth embodiment, even when the periods of an imaging synchronization signal and a display synchronization signal are shifted with the elapse of time due to an error of a phase or a period between an imaging reference clock signal and a display reference clock signal, the phase or period (at least a period) of the imaging synchronization signal can be matched to the phase or period (at least a period) of the display synchronization signal. Thus, also in the image transfer system  5  of the fifth embodiment, the same effects as those in the image transfer system  1  of the first embodiment to the image transfer system  4  of the fourth embodiment can be obtained. That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  4  of the fourth embodiment, also in the image transfer system  5  of the fifth embodiment, it is possible to wirelessly transfer captured image data from the imaging terminal  4100  to the display terminal  4200  without exceeding a validity period of a display image and to stably display a display image corresponding to the captured image data transmitted from the imaging terminal  4100  in the display terminal  4200 . 
     Moreover, in the image transfer system  5  of the fifth embodiment, the display terminal  4200  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  4100  and the display terminal  4200 , estimates (calculates) a period-adjusted accuracy estimation value, determines whether or not cycle adjustment for an imaging synchronization signal is performed, calculates a cycle adjustment amount, and generates a cycle adjustment instruction. Thereby, in the image transfer system  5  of the fifth embodiment, the imaging terminal  4100  may only perform cycle adjustment for an imaging synchronization signal in response to the cycle adjustment instruction. That is, in the image transfer system  5  of the fifth embodiment, it is not necessary to calculate a round trip propagation-time, estimate (calculate) a period-adjusted accuracy estimation value, determine whether or not cycle adjustment for an imaging synchronization signal is performed, calculate a cycle adjustment amount, and generate a cycle adjustment instruction in the imaging terminal  4100 , and thus it is possible to reduce a load of processing performed in the imaging terminal  4100 . 
     Sixth Embodiment 
     Hereinafter, an image transfer system of a sixth embodiment of the present invention will be described.  FIG. 24  is a block diagram showing a schematic configuration of the image transfer system in the sixth embodiment of the present invention. An image transfer system  6  includes an imaging terminal  5100  and a display terminal  5200 . The imaging terminal  5100  includes a synchronization signal generation unit  102 , a source oscillation clock generation unit  103 , a wireless communication unit  108 , a cycle adjustment determination unit  104 , a round trip propagation-time measurement assistance unit  1106 , a clocking unit  107 , and an antenna  120 . In addition, the display terminal  5200  includes a synchronous signal generation unit  204 , a source oscillation clock generation unit  205 , a wireless communication unit  201 , a round trip propagation-time measurement unit  1202 , a clocking unit  203 , an after-cycle-adjustment accuracy estimation unit  2105 , a cycle adjustment unit  4101 , and an antenna  220 . 
     Meanwhile, also in  FIG. 24 , in components of the image transfer system  6 , the same components as the components included in the image transfer system  1  of the first embodiment to the image transfer system  5  of the fifth embodiment are denoted by the same reference numerals and signs. 
     Similarly to the image transfer system  1  of the first embodiment to the image transfer system  5  of the fifth embodiment, the image transfer system  6  is also an image transfer system configured such that transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between the imaging terminal  5100  and the display terminal  5200 , and the imaging terminal  5100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  5200 . Meanwhile, similarly the image transfer system  2  of the second embodiment to the image transfer system  5  of the fifth embodiment, the image transfer system  6  is an image transfer system in which the display terminal  5200  transmits a round-trip-propagation-time-measurement outgoing signal to the imaging terminal  5100  to calculate a round trip propagation-time in wireless transfer between the imaging terminal  5100  and the display terminal  5200 , and the imaging terminal  5100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated. 
     However, in the image transfer system  6 , some components included in the imaging terminal  2100  in the image transfer system  3  of the third embodiment are moved to the display terminal  5200 . More specifically, in the image transfer system  6 , the cycle adjustment unit  4101  that replaces the cycle adjustment unit  101  included in the imaging terminal  2100  in the image transfer system  3  of the third embodiment is included in the display terminal  5200 . Meanwhile, it can be said that a configuration of the image transfer system  6  is a configuration in which the cycle-adjustment determination unit  3104  included in the display terminal  4200  in the image transfer system  5  of the fifth embodiment is returned to the imaging terminal  4100  as the cycle adjustment determination unit  104 . 
     For this reason, in the image transfer system  6 , the display terminal  5200  estimates (calculates) a period-adjusted accuracy estimation value in a case where the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  5100  is temporarily adjusted, and transmits the estimated period-adjusted accuracy estimation value to the imaging terminal  5100 . Further, in the image transfer system  6 , the imaging terminal  5100  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value transmitted from the display terminal  5200 , and transmits a cycle adjustment execution determination result indicating a determination result to the display terminal  5200 . Further, in the image transfer system  6 , in a case where the cycle adjustment execution determination result transmitted from the imaging terminal  5100  indicates that cycle adjustment for an imaging synchronization signal is performed, the display terminal  5200  calculates a cycle adjustment amount and transmits the calculated cycle adjustment amount to the imaging terminal  5100  together with a cycle adjustment instruction. Further, in the image transfer system  6 , the imaging terminal  5100  adjusts the period of an imaging synchronization signal to be generated, in response to the cycle adjustment instruction transmitted from the display terminal  5200 . 
     However, also in the image transfer system  6 , although there is a process of exchanging signals through wireless transfer in association with a change in the disposition of components, functions and operations of the components are the same as the functions and operations of the corresponding components in the image transfer system  1  of the first embodiment to the image transfer system  5  of the fifth embodiment. That is, the functions and operations of the components included in the imaging terminal  5100  and the display terminal  5200  in the image transfer system  6  can be easily understood from the above description of the components included in the imaging terminal and the display terminal in each of the image transfer system  1  of the first embodiment to the image transfer system  5  of the fifth embodiment. Therefore, a detailed description related to the components included in the image transfer system  6  will be omitted. 
     Next, operations of processing in the image transfer system  6  will be described. Meanwhile, in the image transfer system  6 , it is assumed that a phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  100  in the image transfer system  1  of the first embodiment is included in the round trip propagation-time measurement unit  1202 . That is, in the image transfer system  6 , a configuration in which the phase adjustment unit not shown in the drawing is disposed at the same position as those in the image transfer system  2  of the second embodiment to the image transfer system  5  of the fifth embodiment is adopted.  FIG. 25  is a flowchart showing a processing procedure of the image transfer system  6  in the sixth embodiment of the present invention. 
     In the image transfer system  6 , a process of transmitting information of a period-adjusted accuracy estimation value to the imaging terminal  5100  as information used to adjust the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102 , a process of transmitting information of a cycle adjustment execution determination result to the display terminal  5200 , and a process of transmitting a cycle adjustment instruction including a cycle adjustment amount to the imaging terminal  5100  are added, in association with a change to a configuration in which the after-cycle-adjustment accuracy estimation unit  2105  and the cycle adjustment unit  4101  are included in the display terminal  5200 . However, an outline of the overall operation in the image transfer system  6  is the same as those of the image transfer system  1  of the first embodiment to the image transfer system  5  of the fifth embodiment. Therefore, also in the image transfer system  6 , a description of an outline of the overall operation will be omitted, and a more specific operation of performing cycle adjustment for an imaging synchronization signal in the image transfer system  6  will be described. 
     Also in the image transfer system  6 , when a cycle adjustment process is started, the display terminal  5200  transmits a round-trip-propagation-time-measurement outgoing signal to the imaging terminal  5100 , and the imaging terminal  5100  transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal to the display terminal  5200  in step S 302 . Meanwhile, the process of step S 302  in the image transfer system  6  is the same as the process of step S 302  in each of the image transfer system  2  of the second embodiment to the image transfer system  5  of the fifth embodiment. 
     Thereafter, also in the image transfer system  6 , in step S 303 , the display terminal  5200  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  5100  and the display terminal  5200 . Meanwhile, the process of step S 303  in the image transfer system  6  is also the same as the process of step S 303  in each of the image transfer system  2  of the second embodiment to the image transfer system  5  of the fifth embodiment. 
     Thereafter, also in the image transfer system  6 , in step S 304 , the display terminal  5200  generates a phase adjustment instruction for the phase adjustment unit, not shown in the drawing, which is provided in the round trip propagation-time measurement unit  1202  to adjust the phase of an imaging synchronization signal, on the basis of the round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  5100  and the display terminal  5200  which is calculated in step S 303 , and updates a round trip propagation-time determination value. Meanwhile, the process of step S 304  in the image transfer system  6  is also the same as the process of step S 304  in each of the image transfer system  2  of the second embodiment to the image transfer system  5  of the fifth embodiment. 
     Thereafter, in the image transfer system  6 , in step S 306 , the period of an imaging synchronization signal is adjusted on the basis of the information of the plurality of round trip propagation-time determination values updated in step S 304 . However, in the image transfer system  6 , the display terminal  5200  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  6 , the imaging terminal  5100  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value. Further, in the image transfer system  6 , the display terminal  5200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal and generates a cycle adjustment instruction. 
     For this reason, in the image transfer system  6 , in step S 305  included in step S 306 , the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  5200  determines whether or not a predetermined period of time determined in advance has elapsed after the period of a previous imaging synchronization signal is adjusted. In a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has not elapsed after the period of the previous imaging synchronization signal is adjusted (“NO” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  2105  determines that cycle adjustment is not performed and terminates the process of step S 305 . 
     On the other hand, in a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has elapsed after the period of the previous imaging synchronization signal is adjusted (“YES” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  2105  determines that cycle adjustment is performed and calculates a period-adjusted accuracy estimation value in step S 2106  included in step S 306 . Meanwhile, the process of step S 305  and the process of step S 2106  which are included in step S 306  in the image transfer system  6  are the same as the process of step S 305  in step S 306  and the process of step S 2106  in the image transfer system  3  of the third embodiment and the image transfer system  4  of the fourth embodiment. 
     Thereafter, in the image transfer system  6 , in step S 319  included in step S 306 , the display terminal  5200  generates an estimation notification signal including information of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  2105  and transmits the generated estimation notification signal to the imaging terminal  5100 . Meanwhile, the process of step S 319  included in step S 306  in the image transfer system  6  and the estimation notification signal generated in the process of step S 319  and transmitted to the imaging terminal  5100  are the same as the process of step S 319  included in step S 306  and the estimation notification signal in the image transfer system  3  of the third embodiment. Thereby, the cycle adjustment determination unit  104  provided in the imaging terminal  5100  acquires information of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  2105  provided in the display terminal  5200  through the round trip propagation-time measurement assistance unit  1106 . 
     Further, in the image transfer system  6 , in step S 1004  included in step S 306 , the imaging terminal  5100  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value transmitted from the display terminal  5200 . More specifically, the cycle adjustment determination unit  104  included in the imaging terminal  5100  determines that cycle adjustment for an imaging synchronization signal is not performed in a case where the accuracy of an imaging synchronization signal indicated by the period-adjusted accuracy estimation value transmitted from the display terminal  5200 , that is, calculated by the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  5200  is equal to the current accuracy of the imaging synchronization signal or has not been improved, and terminates the process of step S 1004 . On the other hand, in a case where the estimated accuracy of the imaging synchronization signal has been improved compared with the current accuracy of the imaging synchronization signal, the cycle adjustment determination unit  104  determines that cycle adjustment for an imaging synchronization signal is performed, and generates a cycle adjustment execution determination result indicating a determination result. Meanwhile, the process of step S 1004  included in step S 306  in the image transfer system  6  is the same as the process of step S 1004  included in step S 306  in the image transfer system  1  of the first embodiment. 
     Thereafter, in the image transfer system  6 , in step S 311  included in step S 306 , the imaging terminal  5100  generates a determination notification signal including information of the cycle adjustment execution determination result obtained by the cycle adjustment determination unit  104  and transmits the generated determination notification signal to the display terminal  5200 . More specifically, the cycle adjustment determination unit  104  generates a determination notification signal including the information of the obtained cycle adjustment execution determination result and outputs the generated determination notification signal to the round trip propagation-time measurement assistance unit  1106 . Thereby, the round trip propagation-time measurement assistance unit  1106  outputs the determination notification signal output from the cycle adjustment determination unit  104  to the wireless communication unit  108  and transmits the determination notification signal to the cycle adjustment unit  4101  provided in the display terminal  5200  through the wireless communication unit  108  and the antenna  120 . Thereby, the cycle adjustment unit  4101  acquires information of the cycle adjustment execution determination result obtained by the cycle adjustment determination unit  104  provided in the imaging terminal  5100 . More specifically, the wireless communication unit  201  receives the determination notification signal transmitted from the imaging terminal  5100  through the antenna  220 . In addition, the wireless communication unit  201  outputs the information of the cycle adjustment execution determination result included in the received determination notification signal to the cycle adjustment unit  4101 . 
     Further, in the image transfer system  6 , in step S 1206  included in step S 306 , the display terminal  5200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal in accordance with the cycle adjustment execution determination result transmitted from the imaging terminal  5100 , and generates a cycle adjustment instruction. More specifically, in a case where the information of the cycle adjustment execution determination result transmitted from the imaging terminal  5100  indicates that cycle adjustment for an imaging synchronization signal is performed, the cycle adjustment unit  4101  included in the display terminal  5200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  5100  and generates a cycle adjustment instruction. Meanwhile, the process of step S 1206  included in step S 306  in the image transfer system  6  is the same as the process of step S 1206  included in step S 306  in the image transfer system  3  of the third embodiment. 
     Thereafter, in the image transfer system  6 , in step S 312 , the display terminal  5200  generates an adjustment notification signal including information of the cycle adjustment instruction including the cycle adjustment amount calculated by the cycle adjustment unit  4101  and a phase adjustment instruction generated by the phase adjustment unit not shown in the drawing, and transmit s the generated adjustment notification signal to the imaging terminal  5100 . Meanwhile, the process of step S 312  in the image transfer system  6  and the adjustment notification signal generated in the process of step S 312  and transmitted to the imaging terminal  5100  are the same as the process of step S 312  and the adjustment notification signal in the image transfer system  5  of the fifth embodiment. Thereby, the synchronization signal generation unit  102  provided in the imaging terminal  5100  acquires information of the cycle adjustment instruction including the cycle adjustment amount transmitted from the cycle adjustment unit  4101  provided in the display terminal  5200  through the round trip propagation-time measurement assistance unit  1106 . Thereby, the synchronization signal generation unit  102  performs cycle adjustment for adjusting the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  4101 . 
     Meanwhile, the phase adjustment instruction transmitted from the cycle adjustment unit  4101  provided in the display terminal  5200  is also output to the synchronization signal generation unit  102  from the round trip propagation-time measurement assistance unit  1106 . Thereby, the synchronization signal generation unit  102  performs phase adjustment for an imaging synchronization signal to be generated in response to the phase adjustment instruction which is output from the round trip propagation-time measurement assistance unit  1106 , that is, the phase adjustment instruction generated by the phase adjustment unit, not shown in the drawing, which is provided in the round trip propagation-time measurement unit  1202  included in the display terminal  5200 . 
     In this manner, in the image transfer system  6 , the display terminal  5200  transmits a round-trip-propagation-time-measurement outgoing signal and calculates a round trip propagation-time in wireless transfer between the imaging terminal  5100  and the display terminal  5200 . Further, in the image transfer system  6 , the display terminal  5200  performs a process of updating a round trip propagation-time determination value and a process of estimating (calculating) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed. Further, in the image transfer system  6 , the display terminal  5200  transmits information of a calculated period-adjusted accuracy estimation value, information of a plurality of round trip propagation-time determination values updated, information of a round trip propagation-time, information of a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, information of a scheduled transmission time included in a received round-trip-propagation-time-measurement returning signal, and a phase adjustment instruction to the imaging terminal  5100 . Thereby, in the image transfer system  6 , the imaging terminal  5100  performs a process of determining whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  5100  is performed, on the basis of the pieces of information transmitted from the display terminal  5200 . Further, in the image transfer system  6 , the imaging terminal  5100  transmits information of a cycle adjustment execution determination result which is a determination result to the display terminal  5200 . Thereby, in the image transfer system  6 , the display terminal  5200  performs a process of calculating a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction in accordance with the information of the cycle adjustment execution determination result transmitted from the imaging terminal  5100 . Further, in the image transfer system  6 , the display terminal  5200  transmits information of the cycle adjustment instruction, including the calculated cycle adjustment amount, and a phase adjustment instruction to the imaging terminal  5100 . Thereby, in the image transfer system  6 , the imaging terminal  5100  adjusts the periods of an imaging synchronization signal and a display synchronization signal so as not to be shifted with the elapse of time, in response to the cycle adjustment instruction transmitted from the display terminal  5200 . 
     Meanwhile, similarly to the image transfer system  1  of the first embodiment to the image transfer system  5  of the fifth embodiment, also in the image transfer system  6 , when at least a process of updating a round trip propagation-time determination value is performed in step S 304 , the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  5200  can estimate the accuracy of an imaging synchronization signal (that is, calculate a period-adjusted accuracy estimation value). Therefore, similarly to the image transfer system  1  of the first embodiment to the image transfer system  5  of the fifth embodiment, also in the image transfer system  6 , the phase adjustment unit not shown in the drawing does not necessarily need to adjust the phase of an imaging synchronization signal in step S 304  and may be able to update a round trip propagation-time calculated by the round trip propagation-time measurement unit  1202  as a round trip propagation-time determination value. 
     With such a configuration and processing procedure, in the image transfer system  6 , the display terminal  5200  transmits a round-trip-propagation-time-measurement outgoing signal and measures (calculates) a round trip propagation-time in wireless transfer between the imaging terminal  5100  and the display terminal  5200  to update a round trip propagation-time determination value. Further, in the image transfer system  6 , the display terminal  5200  further estimates (calculates) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed. Further, in the image transfer system  6 , the imaging terminal  5100  determines whether or not cycle adjustment for an imaging synchronization signal is performed, on the basis of the period-adjusted accuracy estimation value estimated (calculated) by the display terminal  5200 . Further, in the image transfer system  6 , the display terminal  5200  calculates a cycle adjustment amount for adjusting the period of an imaging synchronization signal to generate a cycle adjustment instruction in accordance with the cycle adjustment execution determination result obtained by the imaging terminal  5100 . Further, in the image transfer system  6 , the imaging terminal  5100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated, in response to the phase adjustment instruction generated by the display terminal  5200 . Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  5  of the fifth embodiment, also in the image transfer system  6 , the phase or period (at least a period) of an imaging synchronization signal generated by the imaging terminal  5100  (more specifically, the synchronization signal generation unit  102 ) is adjusted so as to match the phase or period (at least a period) of a display synchronization signal generated by the display terminal  5200 . That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  5  of the fifth embodiment, also in the image transfer system  6 , a timing when the imaging terminal  5100  wirelessly transfers captured image data of an image captured by an imaging unit not shown in the drawing to the display terminal  5200  is matched to a timing when the display terminal  5200  displays an image corresponding to the captured image data on a display unit not shown in the drawing. Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  5  of the fifth embodiment, also in the image transfer system  6 , the display terminal  5200  can stably display an image corresponding to captured image data wirelessly transferred from the imaging terminal  5100  on the display unit not shown in the drawing. 
     As described above, the image transfer system  6  of the sixth embodiment performs transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal between the round trip propagation-time measurement unit  1202  included in the display terminal  5200  and the round trip propagation-time measurement assistance unit  1106  included in the imaging terminal  5100  after wireless connection between the imaging terminal  5100  and the display terminal  5200  is established. Further, in the image transfer system  6  of the sixth embodiment, the round trip propagation-time measurement unit  1202  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  5100  and the display terminal  5200 , on the basis of a transmission time of a round-trip-propagation-time-measurement outgoing signal and a reception time of a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the round trip propagation-time measurement assistance unit  1106 . Further, in the image transfer system  6  of the sixth embodiment, the round trip propagation-time measurement unit  1202  (more specifically, the phase adjustment unit not shown in the drawing) updates the round trip propagation-time calculated by the round trip propagation-time measurement unit  1202  as a round trip propagation-time determination value. Further, in the image transfer system  6  of the sixth embodiment, the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  5200  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  6  of the sixth embodiment, the after-cycle-adjustment accuracy estimation unit  2105  transmits information for determining whether or not the period of an imaging synchronization signal is adjusted in the imaging terminal  5100  to the imaging terminal  5100 , inclusive of the period-adjusted accuracy estimation value. Further, in the image transfer system  6  of the sixth embodiment, the cycle adjustment determination unit  104  included in the imaging terminal  5100  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is performed, on the basis of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  2105 . Further, in the image transfer system  6  of the sixth embodiment, the cycle adjustment determination unit  104  transmits information of a cycle adjustment execution determination result which is a result obtained by determining whether or not the period of an imaging synchronization signal is adjusted to the display terminal  5200 . Further, in the image transfer system  6  of the sixth embodiment, in a case where the cycle adjustment execution determination result indicates that cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  5100  is performed, the cycle adjustment unit  4101  included in the display terminal  5200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction. Further, in the image transfer system  6  of the sixth embodiment, the cycle adjustment unit  4101  transmits information of a cycle adjustment instruction including a cycle adjustment amount for adjusting the period of an imaging synchronization signal in the imaging terminal  5100  to the imaging terminal  5100 . Thereby, in the image transfer system  6  of the sixth embodiment, the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is transmitted from the cycle adjustment unit  4101 . 
     Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  5  of the fifth embodiment, also in the image transfer system  6  of the sixth embodiment, the phase or period (at least a period) of an imaging synchronization signal can be matched to the phase or period (at least a period) of a display synchronization signal. That is, also in the image transfer system  6  of the sixth embodiment, even when the periods of an imaging synchronization signal and a display synchronization signal are shifted with the elapse of time due to an error of a phase or a period between an imaging reference clock signal and a display reference clock signal, the phase or period (at least a period) of the imaging synchronization signal can be matched to the phase or period (at least a period) of the display synchronization signal. Thus, also in the image transfer system  6  of the sixth embodiment, the same effects as those in the image transfer system  1  of the first embodiment to the image transfer system  5  of the fifth embodiment can be obtained. That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  5  of the fifth embodiment, also in the image transfer system  6  of the sixth embodiment, it is possible to wirelessly transfer captured image data from the imaging terminal  5100  to the display terminal  5200  without exceeding a validity period of a display image and to stably display a display image corresponding to the captured image data transmitted from the imaging terminal  5100  in the display terminal  5200 . 
     Moreover, in the image transfer system  6  of the sixth embodiment, the display terminal  5200  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  5100  and the display terminal  5200 , estimates (calculates) a period-adjusted accuracy estimation value, calculates a cycle adjustment amount, and generates a cycle adjustment instruction. Thereby, in the image transfer system  6  of the sixth embodiment, the imaging terminal  5100  may only determine whether or not cycle adjustment for an imaging synchronization signal is performed and perform cycle adjustment for an imaging synchronization signal in response to the cycle adjustment instruction. That is, in the image transfer system  6  of the sixth embodiment, it is not necessary to calculate a round trip propagation-time, estimate (calculate) a period-adjusted accuracy estimation value, calculate a cycle adjustment amount, and generate a cycle adjustment instruction in the imaging terminal  5100 , and thus it is possible to reduce a load of processing performed in the imaging terminal  5100 . 
     Seventh Embodiment 
     Hereinafter, an image transfer system of a seventh embodiment of the present invention will be described.  FIG. 26  is a block diagram showing a schematic configuration of the image transfer system in the seventh embodiment of the present invention. The image transfer system  7  includes an imaging terminal  6100  and a display terminal  6200 . The imaging terminal  6100  includes a synchronization signal generation unit  102 , a source oscillation clock generation unit  103 , a wireless communication unit  108 , a cycle adjustment determination unit  104 , an after-cycle-adjustment accuracy estimation unit  105 , a round trip propagation-time measurement assistance unit  1106 , a clocking unit  107 , and an antenna  120 . In addition, the display terminal  6200  includes a synchronous signal generation unit  204 , a source oscillation clock generation unit  205 , a wireless communication unit  201 , a round trip propagation-time measurement unit  1202 , a clocking unit  203 , a cycle adjustment unit  4101 , and an antenna  220 . 
     Meanwhile, also in  FIG. 26 , in components of the image transfer system  7 , the same components as the components included in the image transfer system  1  of the first embodiment to the image transfer system  6  of the sixth embodiment are denoted by the same reference numerals and signs. 
     Similarly to the image transfer system  1  of the first embodiment to the image transfer system  6  of the sixth embodiment, the image transfer system  7  is also an image transfer system configured such that transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between the imaging terminal  6100  and the display terminal  6200 , and the imaging terminal  6100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  6200 . Meanwhile, similarly to the image transfer system  2  of the second embodiment to the image transfer system  6  of the sixth embodiment, the image transfer system  7  is an image transfer system in which the display terminal  6200  transmits a round-trip-propagation-time-measurement outgoing signal to the imaging terminal  6100  to calculate a round trip propagation-time in wireless transfer between the imaging terminal  6100  and the display terminal  6200 , and the imaging terminal  6100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated. 
     However, in the image transfer system  7 , some components included in the imaging terminal  1100  in the image transfer system  2  of the second embodiment are moved to the display terminal  6200 . More specifically, in the image transfer system  7 , the cycle adjustment unit  4101  that replaces the cycle adjustment unit  101  included in the imaging terminal  1100  in the image transfer system  2  of the second embodiment is included in the display terminal  6200 . Meanwhile, it can be said that a configuration of the image transfer system  7  is a configuration in which the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  5200  in the image transfer system  6  of the sixth embodiment is returned to the imaging terminal  5100  as the after-cycle-adjustment accuracy estimation unit  105 . 
     For this reason, in the image transfer system  7 , the display terminal  6200  measures (calculates) around trip propagation-time required for transmission and reception at the time of performing wireless transfer between the display terminal  6200  and the imaging terminal  6100  and transmits the measured round trip propagation-time to the imaging terminal  6100 . Further, in the image transfer system  7 , the imaging terminal  6100  estimates (calculates) a period-adjusted accuracy estimation value in a case where the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is temporarily adjusted, and transmits a cycle adjustment execution determination result indicating a result obtained by determining whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value to the imaging terminal  6100 . Further, in the image transfer system  7 , in a case where the cycle adjustment execution determination result transmitted from the imaging terminal  6100  indicates that cycle adjustment for an imaging synchronization signal is performed, the display terminal  6200  calculates a cycle adjustment amount and transmits the calculated cycle adjustment amount to the imaging terminal  6100  together with a cycle adjustment instruction. Further, in the image transfer system  7 , the imaging terminal  6100  adjusts the period of an imaging synchronization signal to be generated, in response to the cycle adjustment instruction transmitted from the display terminal  6200 . 
     However, also in the image transfer system  7 , although there is a process of exchanging signals through wireless transfer in association with a change in the disposition of components, functions and operations of the components are the same as the functions and operations of the corresponding components in the image transfer system  1  of the first embodiment to the image transfer system  6  of the sixth embodiment. That is, the functions and operations of the components included in the imaging terminal  6100  and the display terminal  6200  in the image transfer system  7  can be easily understood from the above description of the components included in the imaging terminal and the display terminal in each of the image transfer system  1  of the first embodiment to the image transfer system  6  of the sixth embodiment. Therefore, a detailed description related to the components included in the image transfer system  7  will be omitted. 
     Next, operations of processing in the image transfer system  7  will be described. Meanwhile, in the image transfer system  7 , it is assumed that a phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  100  in the image transfer system  1  of the first embodiment is included in the round trip propagation-time measurement unit  1202 . That is, in the image transfer system  7 , a configuration in which the phase adjustment unit not shown in the drawing is disposed at the same position as those in the image transfer system  2  of the second embodiment to the image transfer system  6  of the sixth embodiment is adopted.  FIG. 27  is a flowchart showing a processing procedure of the image transfer system  7  in the seventh embodiment of the present invention. 
     In the image transfer system  7 , a process of transmitting information used to adjust the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  to the imaging terminal  6100 , a process of transmitting information of a cycle adjustment execution determination result to the display terminal  6200 , and a process of transmitting a cycle adjustment instruction including a cycle adjustment amount to the imaging terminal  6100  are added, in association with a change to a configuration in which the cycle adjustment unit  4101  is included in the display terminal  6200 . However, an outline of the overall operation in the image transfer system  7  is the same as those of the image transfer system  1  of the first embodiment to the image transfer system  6  of the sixth embodiment. Therefore, also in the image transfer system  7 , a description of an outline of the overall operation will be omitted, and a more specific operation of performing cycle adjustment for an imaging synchronization signal in the image transfer system  7  will be described. 
     Also in the image transfer system  7 , when a cycle adjustment process is started, the display terminal  6200  transmits a round-trip-propagation-time-measurement outgoing signal to the imaging terminal  6100 , and the imaging terminal  6100  transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal to the display terminal  6200  in step S 302 . Meanwhile, the process of step S 302  in the image transfer system  7  is the same as the process of step S 302  in each of the image transfer system  2  of the second embodiment to the image transfer system  6  of the sixth embodiment. 
     Thereafter, also in the image transfer system  7 , in step S 303 , the display terminal  6200  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  6100  and the display terminal  6200 . Meanwhile, the process of step S 303  in the image transfer system  7  is also the same as the process of step S 303  in each of the image transfer system  2  of the second embodiment to the image transfer system  6  of the sixth embodiment. 
     Thereafter, also in the image transfer system  7 , in step S 304 , the display terminal  6200  generates a phase adjustment instruction for the phase adjustment unit, not shown in the drawing, which is provided in the round trip propagation-time measurement unit  1202  to adjust the phase of an imaging synchronization signal, on the basis of the round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  6100  and the display terminal  6200  which is calculated in step S 303 , and updates a round trip propagation-time determination value. Meanwhile, the process of step S 304  in the image transfer system  7  is also the same as the process of step S 304  in each of the image transfer system  2  of the second embodiment to the image transfer system  6  of the sixth embodiment. 
     Thereafter, in the image transfer system  7 , in step S 307 , the display terminal  6200  generates a measurement notification signal including information of the calculated round trip propagation-time and transmits the generated measurement notification signal to the imaging terminal  6100 . Meanwhile, the process of step S 307  in the image transfer system  7  and the measurement notification signal generated in the process of step S 307  and transmitted to the imaging terminal  6100  are the same as the process of step S 307  and the measurement notification signal in the image transfer system  2  of the second embodiment. Thereby, the after-cycle-adjustment accuracy estimation unit  105  provided in the imaging terminal  6100  acquires information of the round trip propagation-time determination value transmitted from the display terminal  6200  through the round trip propagation-time measurement assistance unit  1106 . 
     Thereafter, in the image transfer system  7 , in step S 306 , the period of an imaging synchronization signal is adjusted on the basis of the information of the plurality of round trip propagation-time determination values updated in step S 304 . However, in the image transfer system  7 , the imaging terminal  6100  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed, and determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the estimated (calculated) period-adjusted accuracy estimation value. Further, in the image transfer system  7 , the display terminal  6200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal and generates a cycle adjustment instruction. 
     For this reason, in the image transfer system  7 , in step S 305  included in step S 306 , the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  6100  determines whether or not a predetermined period of time determined in advance has elapsed after the period of a previous imaging synchronization signal is adjusted. In a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has not elapsed after the period of the previous imaging synchronization signal is adjusted (“NO” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  105  determines that cycle adjustment is not performed and terminates the process of step S 305 . 
     On the other hand, in a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has elapsed after the period of the previous imaging synchronization signal is adjusted (“YES” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  105  determines that cycle adjustment is performed and calculates a period-adjusted accuracy estimation value in step S 2106  included in step S 306 . Meanwhile, the process of step S 305  and the process of step S 2106  which are included in step S 306  in the image transfer system  7  are the same as the process of step S 305  and the process of step S 2106  which are included in step S 306  in the image transfer system  1  of the first embodiment. 
     Further, in the image transfer system  7 , in step S 1004  included in step S 306 , the cycle adjustment determination unit  104  included in the imaging terminal  6100  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the calculated period-adjusted accuracy estimation value. Further, in a case where the cycle adjustment determination unit  104  determines that cycle adjustment for an imaging synchronization signal is performed because an estimated accuracy of the estimated imaging synchronization signal has been improved compared with the current accuracy of the imaging synchronization signal, the cycle adjustment execution determination unit generates a cycle adjustment execution determination result indicating a determination result. Meanwhile, the process of step S 1004  included in step S 306  in the image transfer system  7  is the same as the process of step S 1004  included in step S 306  in the image transfer system  1  of the first embodiment. 
     Thereafter, in the image transfer system  7 , in step S 311  included in step S 306 , the imaging terminal  6100  generates a determination notification signal including information of the cycle adjustment execution determination result obtained by the cycle adjustment determination unit  104  and transmits the generated determination notification signal to the display terminal  6200 . Meanwhile, the process of step S 311  included in step S 306  in the image transfer system  7  and the determination notification signal generated in the process of step S 311  and transmitted to the display terminal  6200  are the same as the process of step S 311  included in step S 306  and the determination notification signal in the image transfer system  6  of the sixth embodiment. Thereby, the cycle adjustment unit  4101  provided in the display terminal  6200  acquires information of the cycle adjustment execution determination result obtained by the cycle adjustment determination unit  104  provided in the imaging terminal  6100 . 
     Further, in the image transfer system  7 , in step S 1206  included in step S 306 , the display terminal  6200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal in accordance with the cycle adjustment execution determination result transmitted from the imaging terminal  6100 , and generates a cycle adjustment instruction. Meanwhile, the process of step S 1206  included in step S 306  in the image transfer system  7  is the same as the process of step S 1206  included in step S 306  in the image transfer system  6  of the sixth embodiment. 
     Thereafter, in the image transfer system  7 , in step S 312 , the display terminal  6200  generates an adjustment notification signal including information of the cycle adjustment instruction including the cycle adjustment amount calculated by the cycle adjustment unit  4101  and a phase adjustment instruction generated by the phase adjustment unit not shown in the drawing, and transmit s the generated adjustment notification signal to the imaging terminal  6100 . Meanwhile, the process of step S 312  in the image transfer system  7  and the adjustment notification signal generated in the process of step S 312  and transmitted to the imaging terminal  6100  are the same as the process of step S 312  and the adjustment notification signal in the image transfer system  6  of the sixth embodiment. Thereby, the synchronization signal generation unit  102  provided in the imaging terminal  6100  acquires information of the cycle adjustment instruction including the cycle adjustment amount transmitted from the cycle adjustment unit  4101  provided in the display terminal  6200  through the round trip propagation-time measurement assistance unit  1106 . Thereby, the synchronization signal generation unit  102  performs cycle adjustment for adjusting the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  4101 . 
     Meanwhile, the phase adjustment instruction transmitted from the cycle adjustment unit  4101  provided in the display terminal  6200  is also output to the synchronization signal generation unit  102  from the round trip propagation-time measurement assistance unit  1106 . Thereby, the synchronization signal generation unit  102  performs phase adjustment for an imaging synchronization signal to be generated in response to the phase adjustment instruction which is output from the round trip propagation-time measurement assistance unit  1106 , that is, the phase adjustment instruction generated by the phase adjustment unit, not shown in the drawing, which is provided in the round trip propagation-time measurement unit  1202  included in the display terminal  6200 . 
     In this manner, in the image transfer system  7 , the display terminal  6200  transmits a round-trip-propagation-time-measurement outgoing signal and calculates a round trip propagation-time in wireless transfer between the imaging terminal  6100  and the display terminal  6200 . Further, in the image transfer system  7 , the display terminal  6200  performs a process of updating a round trip propagation-time determination value and transmits information of a plurality of round trip propagation-time determination values updated, information of a round trip propagation-time, information of a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, information of a scheduled transmission time included in a received round-trip-propagation-time-measurement returning signal, and a phase adjustment instruction to the imaging terminal  6100 . Thereby, in the image transfer system  7 , the imaging terminal  6100  performs a process of estimating (calculating) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed and a process of determining whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is performed, on the basis of the pieces of information transmitted from the display terminal  6200 . Further, in the image transfer system  7 , the imaging terminal  6100  transmits information of a cycle adjustment execution determination result which is a determination result to the display terminal  6200 . Thereby, in the image transfer system  7 , the display terminal  6200  performs a process of calculating a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction in accordance with the information of the cycle adjustment execution determination result transmitted from the imaging terminal  6100 . Further, in the image transfer system  7 , the display terminal  6200  transmits information of the cycle adjustment instruction, including the calculated cycle adjustment amount, and a phase adjustment instruction to the imaging terminal  6100 . Thereby, in the image transfer system  7 , the imaging terminal  6100  adjusts the periods of an imaging synchronization signal and a display synchronization signal so as not to be shifted with the elapse of time, in response to the cycle adjustment instruction transmitted from the display terminal  6200 . 
     Meanwhile, similarly to the image transfer system  1  of the first embodiment to the image transfer system  6  of the sixth embodiment, also in the image transfer system  7 , when at least a process of updating a round trip propagation-time determination value is performed in step S 304 , the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  6100  can estimate the accuracy of an imaging synchronization signal (that is, calculate a period-adjusted accuracy estimation value). Therefore, similarly to the image transfer system  1  of the first embodiment to the image transfer system  6  of the sixth embodiment, also in the image transfer system  7 , the phase adjustment unit not shown in the drawing does not necessarily need to adjust the phase of an imaging synchronization signal in step S 304  and may be able to update a round trip propagation-time calculated by the round trip propagation-time measurement unit  1202  as a round trip propagation-time determination value. 
     With such a configuration and processing procedure, in the image transfer system  7 , the display terminal  6200  transmits a round-trip-propagation-time-measurement outgoing signal and measures (calculates) a round trip propagation-time in wireless transfer between the imaging terminal  6100  and the display terminal  6200  to update a round trip propagation-time determination value. Further, in the image transfer system  7 , the imaging terminal  6100  estimates (calculates) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed and determines whether or not cycle adjustment for an imaging synchronization signal is performed. Further, in the image transfer system  7 , the display terminal  6200  calculates a cycle adjustment amount for adjusting the period of an imaging synchronization signal to generate a cycle adjustment instruction in accordance with the cycle adjustment execution determination result obtained by the imaging terminal  6100 . Further, in the image transfer system  7 , the imaging terminal  6100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated, in response to the phase adjustment instruction generated by the display terminal  6200 . Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  6  of the sixth embodiment, also in the image transfer system  7 , the phase or period (at least a period) of an imaging synchronization signal generated by the imaging terminal  6100  (more specifically, the synchronization signal generation unit  102 ) is adjusted so as to match the phase or period (at least a period) of a display synchronization signal generated by the display terminal  6200 . That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  6  of the sixth embodiment, also in the image transfer system  7 , a timing when the imaging terminal  6100  wirelessly transfers captured image data of an image captured by an imaging unit not shown in the drawing to the display terminal  6200  is matched to a timing when the display terminal  6200  displays an image corresponding to the captured image data on a display unit not shown in the drawing. Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  6  of the sixth embodiment, also in the image transfer system  7 , the display terminal  6200  can stably display an image corresponding to captured image data wirelessly transferred from the imaging terminal  6100  on the display unit not shown in the drawing. 
     As described above, the image transfer system  7  of the seventh embodiment performs transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal between the round trip propagation-time measurement unit  1202  included in the display terminal  6200  and the round trip propagation-time measurement assistance unit  1106  included in the imaging terminal  6100  after wireless connection between the imaging terminal  6100  and the display terminal  6200  is established. Further, in the image transfer system  7  of the seventh embodiment, the round trip propagation-time measurement unit  1202  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  6100  and the display terminal  6200 , on the basis of a transmission time of a round-trip-propagation-time-measurement outgoing signal and a reception time of a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the round trip propagation-time measurement assistance unit  1106 . Further, in the image transfer system  7  of the seventh embodiment, the round trip propagation-time measurement unit  1202  (more specifically, the phase adjustment unit not shown in the drawing) updates the round trip propagation-time calculated by the round trip propagation-time measurement unit  1202  as a round trip propagation-time determination value. Further, in the image transfer system  7  of the seventh embodiment, the round trip propagation-time measurement unit  1202  transmits information for adjusting the period of an imaging synchronization signal in the imaging terminal  6100  to the imaging terminal  6100 . Further, in the image transfer system  7  of the seventh embodiment, the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  6100  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  7  of the seventh embodiment, the cycle adjustment determination unit  104  included in the imaging terminal  6100  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is performed, on the basis of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  105 . Further, in the image transfer system  7  of the seventh embodiment, the cycle adjustment determination unit  104  transmits information of a cycle adjustment execution determination result which is a result obtained by determining whether or not the period of an imaging synchronization signal is adjusted to the display terminal  6200 . Further, in the image transfer system  7  of the seventh embodiment, in a case where the cycle adjustment execution determination result indicates that cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  6100  is performed, the cycle adjustment unit  4101  included in the display terminal  6200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction. Further, in the image transfer system  7  of the seventh embodiment, the cycle adjustment unit  4101  transmits information of a cycle adjustment instruction including a cycle adjustment amount for adjusting the period of an imaging synchronization signal in the imaging terminal  6100  to the imaging terminal  6100 . Thereby, in the image transfer system  7  of the seventh embodiment, the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is transmitted from the cycle adjustment unit  4101 . 
     Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  6  of the sixth embodiment, also in the image transfer system  7  of the seventh embodiment, the phase or period (at least a period) of an imaging synchronization signal can be matched to the phase or period (at least a period) of a display synchronization signal. That is, also in the image transfer system  7  of the seventh embodiment, even when the periods of an imaging synchronization signal and a display synchronization signal are shifted with the elapse of time due to an error of a phase or a period between an imaging reference clock signal and a display reference clock signal, the phase or period (at least a period) of the imaging synchronization signal can be matched to the phase or period (at least a period) of the display synchronization signal. Thus, also in the image transfer system  7  of the seventh embodiment, the same effects as those in the image transfer system  1  of the first embodiment to the image transfer system  6  of the sixth embodiment can be obtained. That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  6  of the sixth embodiment, also in the image transfer system  7  of the seventh embodiment, it is possible to wirelessly transfer captured image data from the imaging terminal  6100  to the display terminal  6200  without exceeding a validity period of a display image and to stably display a display image corresponding to the captured image data transmitted from the imaging terminal  6100  in the display terminal  6200 . 
     Moreover, in the image transfer system  7  of the seventh embodiment, the display terminal  6200  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  6100  and the display terminal  6200 , calculates a cycle adjustment amount, and generates a cycle adjustment instruction. Thereby, in the image transfer system  7  of the seventh embodiment, the imaging terminal  6100  may only estimate (calculate) a period-adjusted accuracy estimation value, determine whether or not cycle adjustment for an imaging synchronization signal is performed, and perform cycle adjustment for an imaging synchronization signal in response to the cycle adjustment instruction. That is, in the image transfer system  7  of the seventh embodiment, it is not necessary to calculate a round trip propagation-time, calculate a cycle adjustment amount, and generate a cycle adjustment instruction in the imaging terminal  6100 , and thus it is possible to reduce a load of processing performed in the imaging terminal  6100 . 
     Eighth Embodiment 
     Hereinafter, an image transfer system of an eighth embodiment of the present invention will be described.  FIG. 28  is a block diagram showing a schematic configuration of the image transfer system in the eighth embodiment of the present invention. An image transfer system  8  includes an imaging terminal  7100  and a display terminal  7200 . The imaging terminal  7100  includes a synchronization signal generation unit  102 , a source oscillation clock generation unit  103 , a wireless communication unit  108 , an after-cycle-adjustment accuracy estimation unit  105 , a round trip propagation-time measurement assistance unit  1106 , a clocking unit  107 , and an antenna  120 . In addition, the display terminal  7200  includes a synchronous signal generation unit  204 , a source oscillation clock generation unit  205 , a wireless communication unit  201 , a round trip propagation-time measurement unit  1202 , a clocking unit  203 , a cycle-adjustment determination unit  3104 , a cycle adjustment unit  4101 , and an antenna  220 . 
     Meanwhile, also in  FIG. 28 , in components of the image transfer system  8 , the same components as the components included in the image transfer system  1  of the first embodiment to the image transfer system  7  of the seventh embodiment are denoted by the same reference numerals and signs. 
     Similarly to the image transfer system  1  of the first embodiment to the image transfer system  7  of the seventh embodiment, the image transfer system  8  is also an image transfer system configured such that transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between the imaging terminal  7100  and the display terminal  7200 , and the imaging terminal  7100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  7200 . Meanwhile, similarly to the image transfer system  2  of the second embodiment to the image transfer system  7  of the seventh embodiment, the image transfer system  8  is an image transfer system in which the display terminal  7200  transmits a round-trip-propagation-time-measurement outgoing signal to the imaging terminal  7100  to calculate a round trip propagation-time in wireless transfer between the imaging terminal  7100  and the display terminal  7200 , and the imaging terminal  7100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated. 
     However, in the image transfer system  8 , some components included in the imaging terminal  6100  in the image transfer system  7  of the seventh embodiment are moved to the display terminal  7200 . More specifically, in the image transfer system  8 , the cycle-adjustment determination unit  3104  that replaces the cycle adjustment determination unit  104  included in the imaging terminal  6100  in the image transfer system  7  of the seventh embodiment is included in the display terminal  7200 . Meanwhile, it can be said that a configuration of the image transfer system  8  is a configuration in which the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  4200  in the image transfer system  5  of the fifth embodiment is returned to the imaging terminal  4100  as the after-cycle-adjustment accuracy estimation unit  105 . 
     For this reason, in the image transfer system  8 , the display terminal  7200  measures (calculates) a roundtrip propagation-time required for transmission and reception at the time of performing wireless transfer between the display terminal  7200  and the imaging terminal  7100  and transmits the measured round trip propagation-time to the imaging terminal  7100 . Further, in the image transfer system  8 , the imaging terminal  7100  estimates (calculates) a period-adjusted accuracy estimation value in a case where the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is temporarily adjusted, and transmits the estimated period-adjusted accuracy estimation value to the display terminal  7200 . Further, in the image transfer system  8 , the display terminal  7200  determines whether or not cycle adjustment for an imaging synchronization signal is performed, on the basis of the period-adjusted accuracy estimation value transmitted from the imaging terminal  7100 , calculates a cycle adjustment amount in a case where it is determined that cycle adjustment for an imaging synchronization signal is performed, and transmits the calculated cycle adjustment amount to the imaging terminal  7100  together with a cycle adjustment instruction. Further, in the image transfer system  8 , the imaging terminal  7100  adjusts the period of an imaging synchronization signal to be generated, in response to the cycle adjustment instruction transmitted from the display terminal  7200 . 
     However, also in the image transfer system  8 , although there is a process of exchanging signals through wireless transfer in association with a change in the disposition of components, functions and operations of the components are the same as the functions and operations of the corresponding components in the image transfer system  1  of the first embodiment to the image transfer system  7  of the seventh embodiment. That is, the functions and operations of the components included in the imaging terminal  7100  and the display terminal  7200  in the image transfer system  8  can be easily understood from the above description of the components included in the imaging terminal and the display terminal in each of the image transfer system  1  of the first embodiment to the image transfer system  7  of the seventh embodiment. Therefore, a detailed description related to the components included in the image transfer system  8  will be omitted. 
     Next, operations of processing in the image transfer system  8  will be described. Meanwhile, in the image transfer system  8 , it is assumed that a phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  100  in the image transfer system  1  of the first embodiment is included in the round trip propagation-time measurement unit  1202 . That is, in the image transfer system  8 , a configuration in which the phase adjustment unit not shown in the drawing is disposed at the same position as those in the image transfer system  2  of the second embodiment to the image transfer system  7  of the seventh embodiment is adopted.  FIG. 29  is a flowchart showing a processing procedure of the image transfer system  8  in the eighth embodiment of the present invention. 
     In the image transfer system  8 , a process of transmitting information used to adjust the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  to the imaging terminal  7100 , a process of transmitting information of a period-adjusted accuracy estimation value to the display terminal  7200 , and a process of transmitting a cycle adjustment instruction including a cycle adjustment amount to the imaging terminal  7100  are added, in association with a change to a configuration in which the cycle-adjustment determination unit  3104  and the cycle adjustment unit  4101  are included in the display terminal  7200 . However, an outline of the overall operation in the image transfer system  8  is the same as those of the image transfer system  1  of the first embodiment to the image transfer system  7  of the seventh embodiment. Therefore, also in the image transfer system  8 , a description of an outline of the overall operation will be omitted, and a more specific operation of performing cycle adjustment for an imaging synchronization signal in the image transfer system  8  will be described. 
     Also in the image transfer system  8 , when a cycle adjustment process is started, the display terminal  7200  transmits a round-trip-propagation-time-measurement outgoing signal to the imaging terminal  7100 , and the imaging terminal  7100  transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal to the display terminal  7200  in step S 302 . Meanwhile, the process of step S 302  in the image transfer system  8  is the same as the process of step S 302  in each of the image transfer system.  2  of the second embodiment to the image transfer system  7  of the seventh embodiment. 
     Thereafter, also in the image transfer system  8 , in step S 303 , the display terminal  7200  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  7100  and the display terminal  7200 . Meanwhile, the process of step S 303  in the image transfer system  8  is also the same as the process of step S 303  in each of the image transfer system  2  of the second embodiment to the image transfer system  7  of the seventh embodiment. 
     Thereafter, also in the image transfer system  8 , in step S 304 , the display terminal  7200  generates a phase adjustment instruction for the phase adjustment unit, not shown in the drawing, which is provided in the round trip propagation-time measurement unit  1202  to adjust the phase of an imaging synchronization signal, on the basis of the round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  7100  and the display terminal  7200  which is calculated in step S 303 , and updates a round trip propagation-time determination value. Meanwhile, the process of step S 304  in the image transfer system  8  is also the same as the process of step S 304  in each of the image transfer system  2  of the second embodiment to the image transfer system  7  of the seventh embodiment. 
     Thereafter, in the image transfer system  8 , in step S 307 , the display terminal  7200  generates a measurement notification signal including information of the calculated round trip propagation-time and transmits the generated measurement notification signal to the imaging terminal  7100 . Meanwhile, the process of step S 307  in the image transfer system  8  and the measurement notification signal generated in the process of step S 307  and transmitted to the imaging terminal  7100  are the same as the process of step S 307  and the measurement notification signal in each of the image transfer system  2  of the second embodiment and the image transfer system  7  of the seventh embodiment. Thereby, the after-cycle-adjustment accuracy estimation unit  105  provided in the imaging terminal  7100  acquires information of the round trip propagation-time determination value transmitted from the display terminal  7200  through the round trip propagation-time measurement assistance unit  1106 . 
     Thereafter, in the image transfer system  8 , in step S 306 , the period of an imaging synchronization signal is adjusted on the basis of the information of the plurality of round trip propagation-time determination values updated in step S 304 . However, in the image transfer system  8 , the imaging terminal  7100  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  8 , the display terminal  7200  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value and calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction. 
     For this reason, in the image transfer system  8 , in step S 305  included in step S 306 , the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  7100  determines whether or not a predetermined period of time determined in advance has elapsed after the period of a previous imaging synchronization signal is adjusted. In a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has not elapsed after the period of the previous imaging synchronization signal is adjusted (“NO” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  105  determines that cycle adjustment is not performed and terminates the process of step S 305 . 
     On the other hand, in a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has elapsed after the period of the previous imaging synchronization signal is adjusted (“YES” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  105  determines that cycle adjustment is performed and calculates a period-adjusted accuracy estimation value in step S 2106  included in step S 306 . Meanwhile, the process of step S 305  and the process of step S 2106  which are included in step S 306  in the image transfer system  8  are the same as the process of step S 305  and the process of step S 2106  which are included in step S 306  in the image transfer system  1  of the first embodiment. 
     Thereafter, in the image transfer system  8 , in step S 319  included in step S 306 , the imaging terminal  7100  generates an estimation notification signal including information of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  105  and transmits the generated estimation notification signal to the display terminal  7200 . More specifically, the after-cycle-adjustment accuracy estimation unit  105  generates an estimation notification signal including information of the calculated period-adjusted accuracy estimation value and outputs the generated estimation notification signal to the round trip propagation-time measurement assistance unit  1106 . Thereby, the round trip propagation-time measurement assistance unit  1106  outputs an estimation notification signal which is output from the after-cycle-adjustment accuracy estimation unit  105  to the wireless communication unit  108  and transmits the estimation notification signal to the cycle-adjustment determination unit  3104  provided in the display terminal  7200  through the wireless communication unit  108  and the antenna  120 . Thereby, the cycle-adjustment determination unit  3104  acquires information of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  105  provided in the imaging terminal  7100 . More specifically, the wireless communication unit  201  receives the estimation notification signal transmitted from the imaging terminal  7100  through the antenna  220 . In addition, the wireless communication unit  201  outputs information of the period-adjusted accuracy estimation value included in the received estimation notification signal to the cycle-adjustment determination unit  3104 . 
     Further, in the image transfer system  8 , in step S 2107  included in step S 306 , the display terminal  7200  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the information of the period-adjusted accuracy estimation value transmitted from the imaging terminal  7100 , similar to the process of step S 2107  included in step S 306  in the image transfer system  1  of the first embodiment, and calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction. More specifically, the cycle-adjustment determination unit  3104  determines whether or not the cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  7100  is performed, on the basis of the period-adjusted accuracy estimation value calculated and transmitted by the after-cycle-adjustment accuracy estimation unit  105  provided in the imaging terminal  7100  (step S 1004 ). Further, in a case where the cycle-adjustment determination unit  3104  determines that cycle adjustment for an imaging synchronization signal is performed, the cycle adjustment unit  4101  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal and generates a cycle adjustment instruction (step S 1206 ). 
     Thereafter, in the image transfer system  8 , in step S 312 , the display terminal  7200  generates an adjustment notification signal including information of the cycle adjustment instruction, including the cycle adjustment amount calculated by the cycle adjustment unit  4101 , and a phase adjustment instruction generated by the phase adjustment unit not shown in the drawing, and transmits the generated adjustment notification signal to the imaging terminal  7100 . Meanwhile, the process of step S 312  in the image transfer system  8  and the adjustment notification signal generated in the process of step S 312  and transmitted to the imaging terminal  7100  are the same as the process of step S 312  and the adjustment notification signal in the image transfer system  7  of the seventh embodiment. Thereby, the synchronization signal generation unit  102  provided in the imaging terminal  7100  acquires information of the cycle adjustment instruction including the cycle adjustment amount transmitted from the cycle adjustment unit  4101  provided in the display terminal  7200  through the round trip propagation-time measurement assistance unit  1106 . Thereby, the synchronization signal generation unit  102  performs cycle adjustment for adjusting the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  4101 . 
     Meanwhile, the phase adjustment instruction transmitted from the cycle adjustment unit  4101  provided in the display terminal  7200  is also output to the synchronization signal generation unit  102  from the round trip propagation-time measurement assistance unit  1106 . Thereby, the synchronization signal generation unit  102  performs phase adjustment for an imaging synchronization signal to be generated in response to the phase adjustment instruction which is output from the round trip propagation-time measurement assistance unit  1106 , that is, the phase adjustment instruction generated by the phase adjustment unit, not shown in the drawing, which is provided in the round trip propagation-time measurement unit  1202  included in the display terminal  7200 . 
     In this manner, in the image transfer system  8 , the display terminal  7200  transmits a round-trip-propagation-time-measurement outgoing signal and calculates a round trip propagation-time in wireless transfer between the imaging terminal  7100  and the display terminal  7200 . Further, in the image transfer system  8 , the display terminal  7200  performs a process of updating a round trip propagation-time determination value and transmits information of a plurality of round trip propagation-time determination values updated, information of a round trip propagation-time, information of a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, information of a scheduled transmission time included in a received round-trip-propagation-time-measurement returning signal, and a phase adjustment instruction to the imaging terminal  7100 . Thereby, in the image transfer system  8 , the imaging terminal  7100  performs a process of estimating (calculating) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed, on the basis of the pieces of information transmitted from the display terminal  7200 . Further, in the image transfer system  8 , the imaging terminal  7100  transmits information of the calculated period-adjusted accuracy estimation value to the display terminal  7200 . Thereby, in the image transfer system  8 , the display terminal  7200  performs a process of determining whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  7100  is performed, on the basis of the information of the period-adjusted accuracy estimation value transmitted from the imaging terminal  7100 , and performs a process of calculating a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction in accordance with a cycle adjustment execution determination result. Further, in the image transfer system  8 , the display terminal  7200  transmits information of the cycle adjustment instruction, including the calculated cycle adjustment amount, and a phase adjustment instruction to the imaging terminal  7100 . Thereby, in the image transfer system  8 , the imaging terminal  7100  adjusts the periods of an imaging synchronization signal and a display synchronization signal so as not to be shifted with the elapse of time, in response to the cycle adjustment instruction transmitted from the display terminal  7200 . 
     Meanwhile, similarly to the image transfer system  1  of the first embodiment to the image transfer system  7  of the seventh embodiment, also in the image transfer system  8 , when at least a process of updating around trip propagation-time determination value is performed in step S 304 , the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  7100  can estimate the accuracy of an imaging synchronization signal (that is, calculate a period-adjusted accuracy estimation value). Therefore, similarly to the image transfer system  1  of the first embodiment to the image transfer system  7  of the seventh embodiment, also in the image transfer system  8 , the phase adjustment unit not shown in the drawing does not necessarily need to adjust the phase of an imaging synchronization signal in step S 304  and may be able to update a round trip propagation-time calculated by the round trip propagation-time measurement unit  1202  as a round trip propagation-time determination value. 
     With such a configuration and processing procedure, in the image transfer system  8 , the display terminal  7200  transmits a round-trip-propagation-time-measurement outgoing signal and measures (calculates) a round trip propagation-time in wireless transfer between the imaging terminal  7100  and the display terminal  7200  to update a round trip propagation-time determination value. Further, in the image transfer system  8 , the imaging terminal  7100  estimates (calculates) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed. Further, in the image transfer system  8 , the display terminal  7200  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value calculated by the imaging terminal  7100 , and calculates a cycle adjustment amount for adjusting the period of an imaging synchronization signal in accordance with the obtained cycle adjustment execution determination result to generate a cycle adjustment instruction. Further, in the image transfer system  8 , the imaging terminal  7100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated, in response to the phase adjustment instruction generated by the display terminal  7200 . Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  7  of the seventh embodiment, also in the image transfer system  8 , the phase or period (at least a period) of an imaging synchronization signal generated by the imaging terminal  7100  (more specifically, the synchronization signal generation unit  102 ) is adjusted so as to match the phase or period (at least a period) of a display synchronization signal generated by the display terminal  7200 . That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  7  of the seventh embodiment, also in the image transfer system  8 , a timing when the imaging terminal  7100  wirelessly transfers captured image data of an image captured by an imaging unit not shown in the drawing to the display terminal  7200  is matched to a timing when the display terminal  7200  displays an image corresponding to the captured image data on a display unit not shown in the drawing. Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  7  of the seventh embodiment, also in the image transfer system  8 , the display terminal  7200  can stably display an image corresponding to captured image data wirelessly transferred from the imaging terminal  7100  on the display unit not shown in the drawing. 
     As described above, the image transfer system  8  of the eighth embodiment performs transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal between the round trip propagation-time measurement unit  1202  included in the display terminal  7200  and the round trip propagation-time measurement assistance unit  1106  included in the imaging terminal  7100  after wireless connection between the imaging terminal  7100  and the display terminal  7200  is established. Further, in the image transfer system  8  of the eighth embodiment, the round trip propagation-time measurement unit  1202  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  7100  and the display terminal  7200 , on the basis of a transmission time of a round-trip-propagation-time-measurement outgoing signal and a reception time of a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the round trip propagation-time measurement assistance unit  1106 . In addition, in the image transfer system  8  of the eighth embodiment, the round trip propagation-time measurement unit  1202  (more specifically, the phase adjustment unit not shown in the drawing) updates the round trip propagation-time calculated by the round trip propagation-time measurement unit  1202  as a round trip propagation-time determination value. Further, in the image transfer system  8  of the eighth embodiment, the round trip propagation-time measurement unit  1202  transmits information for adjusting the period of an imaging synchronization signal in the imaging terminal  7100  to the imaging terminal  7100 . Further, in the image transfer system  8  of the eighth embodiment, the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  7100  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  8  of the eighth embodiment, the after-cycle-adjustment accuracy estimation unit  105  transmits information of the estimated (calculated) period-adjusted accuracy estimation value of the imaging synchronization signal to the display terminal  7200 . Further, in the image transfer system  8  of the eighth embodiment, the cycle-adjustment determination unit  3104  included in the display terminal  7200  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is performed, on the basis of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  105 . Further, in the image transfer system  8  of the eighth embodiment, in a case where a cycle adjustment execution determination result indicates that cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  7100  is performed, the cycle adjustment unit  4101  included in the display terminal  7200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction. Further, in the image transfer system  8  of the eighth embodiment, the cycle adjustment unit  4101  transmits information of a cycle adjustment instruction including a cycle adjustment amount for adjusting the period of an imaging synchronization signal in the imaging terminal  7100  to the imaging terminal  7100 . Thereby, in the image transfer system  8  of the eighth embodiment, the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is transmitted from the cycle adjustment unit  4101 . 
     Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  7  of the seventh embodiment, also in the image transfer system  8  of the eighth embodiment, the phase or period (at least a period) of an imaging synchronization signal can be matched to the phase or period (at least a period) of a display synchronization signal. That is, also in the image transfer system  8  of the eighth embodiment, even when the periods of an imaging synchronization signal and a display synchronization signal are shifted with the elapse of time due to an error of a phase or a period between an imaging reference clock signal and a display reference clock signal, the phase or period (at least a period) of the imaging synchronization signal can be matched to the phase or period (at least a period) of the display synchronization signal. Thus, also in the image transfer system  8  of the eighth embodiment, the same effects as those in the image transfer system  1  of the first embodiment to the image transfer system  7  of the seventh embodiment can be obtained. That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  7  of the seventh embodiment, also in the image transfer system  8  of the eighth embodiment, it is possible to wirelessly transfer captured image data from the imaging terminal  7100  to the display terminal  7200  without exceeding a validity period of a display image and to stably display a display image corresponding to the captured image data transmitted from the imaging terminal  7100  in the display terminal  7200 . 
     Moreover, in the image transfer system  8  of the eighth embodiment, the display terminal  7200  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  7100  and the display terminal  7200 , determines whether or not cycle adjustment for an imaging synchronization signal is performed, calculates a cycle adjustment amount, and generates a cycle adjustment instruction. Thereby, in the image transfer system  8  of the eighth embodiment, the imaging terminal  7100  may only estimate (calculate) a period-adjusted accuracy estimation value and perform cycle adjustment for an imaging synchronization signal in response to the cycle adjustment instruction. That is, in the image transfer system  8  of the eighth embodiment, it is not necessary to calculate a round trip propagation-time, determine whether or not cycle adjustment for an imaging synchronization signal is performed, calculate a cycle adjustment amount, and generate a cycle adjustment instruction in the imaging terminal  7100 , and thus it is possible to reduce a load of processing performed in the imaging terminal  7100 . 
     Ninth Embodiment 
     Hereinafter, an image transfer system of a ninth embodiment of the present invention will be described.  FIG. 30  is a block diagram showing a schematic configuration of the image transfer system in the ninth embodiment of the present invention. An image transfer system  9  includes an imaging terminal  8100  and a display terminal  8200 . The imaging terminal  8100  includes a synchronization signal generation unit  102 , a cycle adjustment unit  101 , a source oscillation clock generation unit  103 , a wireless communication unit  108 , an after-cycle-adjustment accuracy estimation unit  105 , a round trip propagation-time measurement assistance unit  1106 , a clocking unit  107 , and an antenna  120 . In addition, the display terminal  8200  includes a synchronous signal generation unit  204 , a source oscillation clock generation unit  205 , a wireless communication unit  201 , a round trip propagation-time measurement unit  1202 , a clocking unit  203 , a cycle-adjustment determination unit  3104 , and an antenna  220 . 
     Meanwhile, also in  FIG. 30 , in components of the image transfer system  9 , the same components as the components included in the image transfer system  1  of the first embodiment to the image transfer system  8  of the eighth embodiment are denoted by the same reference numerals and signs. 
     Similarly to the image transfer system  1  of the first embodiment to the image transfer system  8  of the eighth embodiment, the image transfer system  9  is also an image transfer system configured such that transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between the imaging terminal  8100  and the display terminal  8200 , and the imaging terminal  8100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  8200 . Meanwhile, similarly to the image transfer system  2  of the second embodiment to the image transfer system  8  of the eighth embodiment, the image transfer system  9  is an image transfer system in which the display terminal  8200  transmits a round-trip-propagation-time-measurement outgoing signal to the imaging terminal  8100  to calculate a round trip propagation-time in wireless transfer between the imaging terminal  8100  and the display terminal  8200 , and the imaging terminal  8100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated. 
     However, in the image transfer system  9 , some components included in the imaging terminal  1100  of the image transfer system  2  of the second embodiment are moved to the display terminal  8200 . More specifically, in the image transfer system  9 , the cycle-adjustment determination unit  3104  that replaces the cycle adjustment determination unit  104  included in the imaging terminal  1100  in the image transfer system  2  of the second embodiment is included in the display terminal  8200 . Meanwhile, it can be said that a configuration of the image transfer system  9  is a configuration in which the cycle adjustment unit  4101  included in the display terminal  7200  in the image transfer system  8  of the eighth embodiment is returned to the imaging terminal  7100  as the cycle adjustment unit  101 . 
     For this reason, in the image transfer system  9 , the display terminal  8200  estimates (calculates) a round trip propagation-time required for transmission and reception at the time of performing wirelessly transfer between the display terminal  8200  and the imaging terminal  8100  and transmits the measured round trip propagation-time to the imaging terminal  8100 . Further, in the image transfer system  9 , the imaging terminal  8100  estimates (calculates) a period-adjusted accuracy estimation value in a case where the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is temporarily adjusted, and transmits the estimated period-adjusted accuracy estimation value to the display terminal  8200 . Further, in the image transfer system  9 , the display terminal  8200  transmits a cycle adjustment execution determination result indicating a result obtained by determining whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value transmitted from the imaging terminal  8100  to the imaging terminal  8100 . Further, in the image transfer system  9 , in a case where the cycle adjustment execution determination result transmitted from the display terminal  8200  indicates that cycle adjustment for an imaging synchronization signal is performed, the imaging terminal  8100  adjusts the period of an imaging synchronization signal to be generated. 
     However, also in the image transfer system  9 , although there is a process of exchanging signals through wireless transfer in association with a change in the disposition of components, functions and operations of the components are the same as the functions and operations of the corresponding components in the image transfer system  1  of the first embodiment to the image transfer system  8  of the eighth embodiment. That is, the functions and operations of the components included in the imaging terminal  8100  and the display terminal  8200  in the image transfer system  9  can be easily understood from the above description of the components included in the imaging terminal and the display terminal in each of the image transfer system  1  of the first embodiment to the image transfer system  8  of the eighth embodiment. Therefore, a detailed description related to the components included in the image transfer system  9  will be omitted. 
     Next, operations of processing in the image transfer system  9  will be described. Meanwhile, in the image transfer system  9 , it is assumed that a phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  100  in the image transfer system  1  of the first embodiment is included in the round trip propagation-time measurement unit  1202 . That is, in the image transfer system  9 , a configuration in which the phase adjustment unit not shown in the drawing is disposed at the same position as those in the image transfer system  2  of the second embodiment to the image transfer system  8  of the eighth embodiment is adopted.  FIG. 31  is a flowchart showing a processing procedure of the image transfer system  9  in the ninth embodiment of the present invention. 
     In the image transfer system  9 , a process of transmitting information used to adjust the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  to the imaging terminal  8100 , a process of transmitting information of a period-adjusted accuracy estimation value to the display terminal  8200 , and a process of transmitting information of a cycle adjustment execution determination result to the imaging terminal  8100  are added, in association with a change to a configuration in which the cycle-adjustment determination unit  3104  is included in the display terminal  8200 . However, an outline of the overall operation in the image transfer system  9  is the same as those of the image transfer system  1  of the first embodiment to the image transfer system  8  of the eighth embodiment. Therefore, also in the image transfer system  9 , a description of an outline of the overall operation will be omitted, and a more specific operation of performing cycle adjustment for an imaging synchronization signal in the image transfer system  9  will be described. 
     Also in the image transfer system  9 , when a cycle adjustment process is started, the display terminal  8200  transmits a round-trip-propagation-time-measurement outgoing signal to the imaging terminal  8100 , and the imaging terminal  8100  transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal to the display terminal  8200  in step S 302 . Meanwhile, the process of step S 302  in the image transfer system  9  is the same as the process of step S 302  in each of the image transfer system  2  of the second embodiment to the image transfer system  8  of the eighth embodiment. 
     Thereafter, also in the image transfer system  9 , in step S 303 , the display terminal  8200  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  8100  and the display terminal  8200 . Meanwhile, the process of step S 303  in the image transfer system  9  is also the same as the process of step S 303  in each of the image transfer system  2  of the second embodiment to the image transfer system  8  of the eighth embodiment. 
     Thereafter, also in the image transfer system  9 , in step S 304 , the display terminal  8200  generates a phase adjustment instruction for the phase adjustment unit, not shown in the drawing, which is provided in the round trip propagation-time measurement unit  1202  to adjust the phase of an imaging synchronization signal, on the basis of the round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  8100  and the display terminal  8200  which is calculated in step S 303 , and updates a round trip propagation-time determination value. Meanwhile, the process of step S 304  in the image transfer system  9  is also the same as the process of step S 304  in each of the image transfer system  2  of the second embodiment to the image transfer system  8  of the eighth embodiment. 
     Thereafter, in the image transfer system  9 , in step S 307 , the display terminal  8200  generates a measurement notification signal including information of the calculated round trip propagation-time and transmits the generated measurement notification signal to the imaging terminal  8100 . Meanwhile, the process of step S 307  in the image transfer system  9  and the measurement notification signal generated in the process of step S 307  and transmitted to the imaging terminal  8100  are the same as the process of step S 307  and the measurement notification signal in each of the image transfer system  2  of the second embodiment, the image transfer system  7  of the seventh embodiment, and the image transfer system  8  of the eighth embodiment. Thereby, the after-cycle-adjustment accuracy estimation unit  105  provided in the imaging terminal  8100  acquires information of the round trip propagation-time determination value transmitted from the display terminal  8200  through the round trip propagation-time measurement assistance unit  1106 . 
     Thereafter, in the image transfer system  9 , in step S 306 , the period of an imaging synchronization signal is adjusted on the basis of the information of the plurality of round trip propagation-time determination values updated in step S 304 . However, in the image transfer system  9 , the imaging terminal  8100  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  9 , the display terminal  8200  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value. Further, in the image transfer system  9 , the imaging terminal  8100  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal and outputs a cycle adjustment instruction to the synchronization signal generation unit  102 . 
     For this reason, in the image transfer system  9 , in step S 305  included in step S 306 , the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  8100  determines whether or not a predetermined period of time determined in advance has elapsed after the period of a previous imaging synchronization signal is adjusted. In a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has not elapsed after the period of the previous imaging synchronization signal is adjusted (“NO” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  105  determines that cycle adjustment is not performed and terminates the process of step S 305 . 
     On the other hand, in a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has elapsed after the period of the previous imaging synchronization signal is adjusted (“YES” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  105  determines that cycle adjustment is performed and calculates a period-adjusted accuracy estimation value in step S 2106  included in step S 306 . Meanwhile, the process of step S 305  and the process of step S 2106  which are included in step S 306  in the image transfer system  9  are the same as the process of step S 305  and the process of step S 2106  which are included in step S 306  in the image transfer system  1  of the first embodiment. 
     Thereafter, in the image transfer system  9 , in step S 319  included in step S 306 , the imaging terminal  8100  generates an estimation notification signal including information of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  105  and transmits the generated estimation notification signal to the display terminal  8200 . Meanwhile, the process of step S 319  included in step S 306  in the image transfer system  9  and the estimation notification signal generated in the process of step S 319  and transmitted to the display terminal  8200  are the same as the process of step S 319  included in step S 306  and the estimation notification signal in the image transfer system  8  of the eighth embodiment. Thereby, the cycle-adjustment determination unit  3104  provided in the display terminal  8200  acquires information of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  105  provided in the imaging terminal  8100 . 
     Further, in the image transfer system  9 , in step S 1004  included in step S 306 , the display terminal  8200  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value transmitted from the imaging terminal  8100 . More specifically, the cycle-adjustment determination unit  3104  included in the display terminal  8200  determines that cycle adjustment for an imaging synchronization signal is not performed in a case where the accuracy of an imaging synchronization signal indicated by the period-adjusted accuracy estimation value transmitted from the imaging terminal  8100 , that is, calculated by the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  8100  is equal to the current accuracy of the imaging synchronization signal or has not been improved, and terminates the process of step S 1004 . On the other hand, in a case where the estimated accuracy of the imaging synchronization signal has been improved compared with the current accuracy of the imaging synchronization signal, the cycle-adjustment determination unit  3104  determines that cycle adjustment for an imaging synchronization signal is performed, and generates a cycle adjustment execution determination result indicating a determination result. Meanwhile, the process of step S 1004  included in step S 306  in the image transfer system  9  is the same as the process of step S 1004  included in step S 306  in the image transfer system  4  of the fourth embodiment. 
     Thereafter, in the image transfer system  9 , in step S 311  included in step S 306 , the display terminal  8200  generates a determination notification signal including information of the cycle adjustment execution determination result obtained by the cycle-adjustment determination unit  3104  and transmits the generated determination notification signal to the imaging terminal  8100 . Meanwhile, the process of step S 311  included in step S 306  in the image transfer system  9  and the determination notification signal generated in the process of step S 311  and transmitted to the imaging terminal  8100  are the same as the process of step S 311  included in step S 306  and the determination notification signal in the image transfer system  4  of the fourth embodiment. Thereby, the cycle adjustment unit  101  provided in the imaging terminal  8100  acquires information of the cycle adjustment execution determination result obtained by the cycle-adjustment determination unit  3104  provided in the display terminal  8200  through the round trip propagation-time measurement assistance unit  1106 . 
     Further, in the image transfer system  9 , in step S 1206  included in step S 306 , the imaging terminal  8100  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal on the basis of information such as a round trip propagation-time determination value transmitted from the display terminal  8200  in accordance with the cycle adjustment execution determination result transmitted from the display terminal  8200 , and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, the synchronization signal generation unit  102  performs cycle adjustment for adjusting the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  101 . Meanwhile, the process of step S 1206  included in step S 306  in the image transfer system  9  is the same as the process of step S 1206  included in step S 306  in the image transfer system  4  of the fourth embodiment. 
     Meanwhile, the phase adjustment instruction transmitted from the cycle-adjustment determination unit  3104  provided in the display terminal  8200  is also output to the synchronization signal generation unit  102  from the round trip propagation-time measurement assistance unit  1106 . Thereby, the synchronization signal generation unit  102  performs phase adjustment for an imaging synchronization signal to be generated in response to the phase adjustment instruction which is output from the round trip propagation-time measurement assistance unit  1106 , that is, the phase adjustment instruction generated by the phase adjustment unit, not shown in the drawing, which is provided in the round trip propagation-time measurement unit  1202  included in the display terminal  8200 . 
     In this manner, in the image transfer system  9 , the display terminal  8200  transmits a round-trip-propagation-time-measurement outgoing signal and calculates a round trip propagation-time in wireless transfer between the imaging terminal  8100  and the display terminal  8200 . Further, in the image transfer system  9 , the display terminal  8200  performs a process of updating a round trip propagation-time determination value and transmits information of a plurality of round trip propagation-time determination values updated, information of a round trip propagation-time, information of a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, information of a scheduled transmission time included in a received round-trip-propagation-time-measurement returning signal, and a phase adjustment instruction to the imaging terminal  8100 . Thereby, in the image transfer system  9 , the imaging terminal  8100  performs a process of estimating (calculating) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed, on the basis of the pieces of information transmitted from the display terminal  8200 . Further, in the image transfer system  9 , the imaging terminal  8100  transmits information of the calculated period-adjusted accuracy estimation value to the display terminal  8200 . Thereby, in the image transfer system  9 , the display terminal  8200  performs a process of determining whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  8100  is performed, on the basis of the information of the period-adjusted accuracy estimation value transmitted from the imaging terminal  8100 . Further, in the image transfer system  9 , the display terminal  8200  transmits information of a cycle adjustment execution determination result which is a determination result to the imaging terminal  8100 . Thereby, in the image transfer system  9 , the imaging terminal  8100  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal in accordance with the information of the cycle adjustment execution determination result transmitted from the display terminal  8200 , and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, in the image transfer system  9 , the imaging terminal  8100  adjusts the periods of an imaging synchronization signal and a display synchronization signal so as not to be shifted with the elapse of time. 
     Meanwhile, similarly to the image transfer system  1  of the first embodiment to the image transfer system  8  of the eighth embodiment, also in the image transfer system  9 , when at least a process of updating a round trip propagation-time determination value is performed in step S 304 , the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  8100  can estimate the accuracy of an imaging synchronization signal (that is, calculate a period-adjusted accuracy estimation value). Therefore, similarly to the image transfer system  1  of the first embodiment to the image transfer system  8  of the eighth embodiment, also in the image transfer system  9 , the phase adjustment unit not shown in the drawing does not necessarily need to adjust the phase of an imaging synchronization signal in step S 304  and may be able to update a round trip propagation-time calculated by the round trip propagation-time measurement unit  1202  as a round trip propagation-time determination value. 
     With such a configuration and processing procedure, in the image transfer system  9 , the display terminal  8200  transmits a round-trip-propagation-time-measurement outgoing signal and measures (calculates) a round trip propagation-time in wireless transfer between the imaging terminal  8100  and the display terminal  8200  to update a round trip propagation-time determination value. Further, in the image transfer system  9 , the imaging terminal  8100  estimates (calculates) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed. Further, in the image transfer system  9 , the display terminal  8200  determines whether or not cycle adjustment for an imaging synchronization signal is performed, on the basis of the period-adjusted accuracy estimation value calculated by the imaging terminal  8100 . Further, in the image transfer system  9 , the imaging terminal  8100  calculates a cycle adjustment amount for adjusting the period of an imaging synchronization signal in accordance with the cycle adjustment execution determination result obtained by the display terminal  8200 , outputs a cycle adjustment instruction, and adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated. Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  8  of the eighth embodiment, also in the image transfer system  9 , the phase or period (at least a period) of an imaging synchronization signal generated by the imaging terminal  8100  (more specifically, the synchronization signal generation unit  102 ) is adjusted so as to match the phase or period (at least a period) of a display synchronization signal generated by the display terminal  8200 . That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  8  of the eighth embodiment, also in the image transfer system  9 , a timing when the imaging terminal  8100  wirelessly transfers captured image data of an image captured by an imaging unit not shown in the drawing to the display terminal  8200  is matched to a timing when the display terminal  8200  displays an image corresponding to the captured image data on a display unit not shown in the drawing. Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  8  of the eighth embodiment, also in the image transfer system  9 , the display terminal  8200  can stably display an image corresponding to captured image data wirelessly transferred from the imaging terminal  8100  on the display unit not shown in the drawing. 
     As described above, the image transfer system  9  of the ninth embodiment performs transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal between the round trip propagation-time measurement unit  1202  included in the display terminal  8200  and the round trip propagation-time measurement assistance unit  1106  included in the imaging terminal  8100  after wireless connection between the imaging terminal  8100  and the display terminal  8200  is established. Further, in the image transfer system  9  of the ninth embodiment, the round trip propagation-time measurement unit  1202  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  8100  and the display terminal  8200 , on the basis of a transmission time of a round-trip-propagation-time-measurement outgoing signal and a reception time of a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the round trip propagation-time measurement assistance unit  1106 . Further, in the image transfer system  9  of the ninth embodiment, the round trip propagation-time measurement unit  1202  (more specifically, the phase adjustment unit not shown in the drawing) updates the round trip propagation-time calculated by the round trip propagation-time measurement unit  1202  as a round trip propagation-time determination value. Further, in the image transfer system  9  of the ninth embodiment, the round trip propagation-time measurement unit  1202  transmits information for adjusting the period of an imaging synchronization signal in the imaging terminal  8100  to the imaging terminal  8100 . Further, in the image transfer system  9  of the ninth embodiment, the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  8100  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  9  of the ninth embodiment, the after-cycle-adjustment accuracy estimation unit  105  transmits information of the estimated (calculated) period-adjusted accuracy estimation value of the imaging synchronization signal to the display terminal  8200 . Further, in the image transfer system  9  of the ninth embodiment, the cycle-adjustment determination unit  3104  included in the display terminal  8200  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is performed, on the basis of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  105 . Further, in the image transfer system  9  of the ninth embodiment, the cycle-adjustment determination unit  3104  transmits information of a cycle adjustment execution determination result which is a result obtained by determining whether or not the period of an imaging synchronization signal is adjusted to the imaging terminal  8100 . Further, in the image transfer system  9  of the ninth embodiment, in a case where the cycle adjustment execution determination result indicates that cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  8100  is performed, the cycle adjustment unit  101  included in the imaging terminal  8100  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, in the image transfer system  9  of the ninth embodiment, the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  101 . 
     Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  8  of the eighth embodiment, also in the image transfer system  9  of the ninth embodiment, the phase or period (at least a period) of an imaging synchronization signal can be matched to the phase or period (at least a period) of a display synchronization signal. That is, also in the image transfer system  9  of the ninth embodiment, even when the periods of an imaging synchronization signal and a display synchronization signal are shifted with the elapse of time due to an error of a phase or a period between an imaging reference clock signal and a display reference clock signal, the phase or period (at least a period) of the imaging synchronization signal can be matched to the phase or period (at least a period) of the display synchronization signal. Thus, also in the image transfer system  9  of the ninth embodiment, the same effects as those in the image transfer system  1  of the first embodiment to the image transfer system  8  of the eighth embodiment can be obtained. That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  8  of the eighth embodiment, also in the image transfer system  9  of the ninth embodiment, it is possible to wirelessly transfer captured image data from the imaging terminal  8100  to the display terminal  8200  without exceeding a validity period of a display image and to stably display a display image corresponding to the captured image data transmitted from the imaging terminal  8100  in the display terminal  8200 . 
     Moreover, in the image transfer system  9  of the ninth embodiment, the display terminal  8200  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  8100  and the display terminal  8200  and determines whether or not cycle adjustment for an imaging synchronization signal is performed. Thereby, in the image transfer system  9  of the ninth embodiment, the imaging terminal  8100  may only estimate (calculate) a period-adjusted accuracy estimation value, calculate a cycle adjustment amount, generate a cycle adjustment instruction, and perform cycle adjustment for an imaging synchronization signal in response to the cycle adjustment instruction. That is, in the image transfer system  9  of the ninth embodiment, it is not necessary to calculate a round trip propagation-time and determine whether or not cycle adjustment for an imaging synchronization signal is performed in the imaging terminal  8100 , and thus it is possible to reduce a load of processing performed in the imaging terminal  8100 . 
     Meanwhile, in the image transfer system  1  of the first embodiment, a description has been provided of a configuration in which a round trip propagation-time is calculated by the imaging terminal  100  transmitting a round-trip-propagation-time-measurement outgoing signal to the display terminal  200  and receiving a round-trip-propagation-time-measurement returning signal transmitted from the display terminal  200 . Further, contrary to the image transfer system  1  of the first embodiment, in the image transfer system  2  of the second embodiment to the image transfer system  9  of the ninth embodiment, a description has been provided of an example of a case where some components are moved to another terminal in a configuration in which a round trip propagation-time is calculated by a display terminal transmitting a round-trip-propagation-time-measurement outgoing signal to an imaging terminal and receiving a round-trip-propagation-time-measurement returning signal transmitted from the imaging terminal. However, as described above, in the image transfer system of the present invention, even in a configuration in which components for adjusting the period of an imaging synchronization signal which is generated by an imaging terminal are included in either one of the imaging terminal or a display terminal, a function of adjusting the period of an imaging synchronization signal which is generated by the imaging terminal can be realized similarly. That is, similarly to the image transfer system  1  of the first embodiment, even when some components are moved to another terminal in a configuration in which a round trip propagation-time is calculated by an imaging terminal transmitting a round-trip-propagation-time-measurement outgoing signal to a display terminal and receiving a round-trip-propagation-time-measurement returning signal transmitted from the display terminal, a function of adjusting the period of an imaging synchronization signal which is generated by the imaging terminal can be realized similarly. 
     Tenth Embodiment 
     Hereinafter, an image transfer system of a tenth embodiment of the present invention will be described.  FIG. 32  is a block diagram showing a schematic configuration of the image transfer system in the tenth embodiment of the present invention. An image transfer system  10  includes an imaging terminal  9100  and a display terminal  9200 . The imaging terminal  9100  includes a synchronization signal generation unit  102 , a cycle adjustment unit  101 , a source oscillation clock generation unit  103 , a wireless communication unit  108 , an after-cycle-adjustment accuracy estimation unit  105 , a round-trip-propagation-time measurement unit  106 , a clocking unit  107 , and an antenna  120 . In addition, the display terminal  200  includes a synchronous signal generation unit  204 , a source oscillation clock generation unit  205 , a wireless communication unit  201 , a round-trip-propagation-time-measurement assistance unit  202 , a clocking unit  203 , a cycle-adjustment determination unit  3104 , and an antenna  220 . 
     Meanwhile, also in  FIG. 32 , in the components of the image transfer system  10 , the same components as the components included in the image transfer system  1  of the first embodiment to the image transfer system  9  of the ninth embodiment are denoted by the same reference numerals and signs. 
     Similarly to the image transfer system  1  of the first embodiment to the image transfer system  9  of the ninth embodiment, the image transfer system  10  is also an image transfer system configured such that transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between the imaging terminal  9100  and the display terminal  9200 , and the imaging terminal  9100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  9200 . Meanwhile, similarly to the image transfer system  1  of the first embodiment, the image transfer system  10  is an image transfer system in which the imaging terminal  9100  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  9200  to calculate a round trip propagation-time in wireless transfer between the imaging terminal  9100  and the display terminal  9200 , and the imaging terminal  9100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated. 
     However, in the image transfer system  10 , some components included in the imaging terminal  100  in the image transfer system  1  of the first embodiment are moved to the display terminal  9200 . More specifically, in the image transfer system  10 , the cycle-adjustment determination unit  3104  that replaces the cycle adjustment determination unit  104  included in the imaging terminal  100  in the image transfer system  1  of the first embodiment is included in the display terminal  9200 . 
     For this reason, in the image transfer system  10 , the imaging terminal  9100  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  9200 , and the display terminal  9200  transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the imaging terminal  9100  to the imaging terminal  9100 . Further, in the image transfer system  10 , the imaging terminal  9100  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  9100  and the display terminal  9200  on the basis of a transmission time of a round-trip-propagation-time-measurement outgoing signal and a reception time of a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the display terminal  9200 . Further, in the image transfer system  10 , the imaging terminal  9100  estimates (calculates) a period-adjusted accuracy estimation value in a case where the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is temporarily adjusted, and transmits the estimated period-adjusted accuracy estimation value to the display terminal  9200 . Further, in the image transfer system  10 , the display terminal  9200  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  9100  is performed, on the basis of the period-adjusted accuracy estimation value transmitted from the imaging terminal  9100 , and transmits a cycle adjustment execution determination result indicating a determination result to the imaging terminal  9100 . Further, in the image transfer system  10 , in a case where the cycle adjustment execution determination result transmitted from the display terminal  9200  indicates that cycle adjustment for an imaging synchronization signal is performed, the imaging terminal  9100  adjusts the period of an imaging synchronization signal to be generated. 
     However, also in the image transfer system  10 , although there is a process of exchanging signals through wireless transfer in association with a change in the disposition of components, functions and operations of the components are the same as the functions and operations of the corresponding components in the image transfer system  1  of the first embodiment to the image transfer system  9  of the ninth embodiment. That is, the functions and operations of the components included in the imaging terminal  9100  and the display terminal  9200  in the image transfer system  10  can be easily understood from the above description of the components included in the imaging terminal and the display terminal in each of the image transfer system  1  of the first embodiment to the image transfer system  9  of the ninth embodiment. Therefore, a detailed description related to the components included in the image transfer system  10  will be omitted. 
     Next, operations of processing in the image transfer system  10  will be described. Meanwhile, similarly to the image transfer system  1  of the first embodiment, in the image transfer system  10 , it is assumed that a configuration in which a phase adjustment unit not shown in the drawing is included in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  9100  is adopted. That is, in the image transfer system  10 , a configuration in which the phase adjustment unit not shown in the drawing is disposed at the same position as that in the image transfer system  1  of the first embodiment is adopted.  FIG. 33  is a flowchart showing a processing procedure of the image transfer system  10  in the tenth embodiment of the present invention. 
     In the image transfer system  10 , a process of transmitting information of a period-adjusted accuracy estimation value to the display terminal  9200  and a process of transmitting information of a cycle adjustment execution determination result to the imaging terminal  9100  are added, in association with a change to a configuration in which the cycle-adjustment determination unit  3104  is included in the display terminal  9200 . However, an outline of the overall operation in the image transfer system  10  is the same as those of the image transfer system  1  of the first embodiment to the image transfer system  9  of the ninth embodiment. Therefore, also in the image transfer system  10 , a description of an outline of the overall operation will be omitted, and a more specific operation of performing cycle adjustment for an imaging synchronization signal in the image transfer system  10  will be described. 
     In the image transfer system  10 , when a cycle adjustment process is started, the imaging terminal  9100  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  9200 , and the display terminal  9200  transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal to the imaging terminal  9100  in step S 302 . Meanwhile, the process of step S 302  in the image transfer system  10  is the same as the process of step S 302  in the image transfer system  1  of the first embodiment. 
     Thereafter, in the image transfer system  10 , in step S 303 , the imaging terminal  9100  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  9100  and the display terminal  9200 . Meanwhile, the process of step S 303  in the image transfer system  10  is the same as the process of step S 303  in the image transfer system  1  of the first embodiment. 
     Thereafter, in the image transfer system  10 , in step S 304 , the imaging terminal  9100  generates a phase adjustment instruction for the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  to adjust the phase of an imaging synchronization signal, on the basis of the round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  9100  and the display terminal  9200  which is calculated in step S 303 , and updates a round trip propagation-time determination value. Meanwhile, the process of step S 304  in the image transfer system  10  is also the same as the process of step S 304  in the image transfer system  1  of the first embodiment. 
     Thereafter, in the image transfer system  10 , in step S 306 , the period of an imaging synchronization signal is adjusted on the basis of the information of the plurality of round trip propagation-time determination values updated in step S 304 . However, in the image transfer system  10 , the imaging terminal  9100  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  10 , the display terminal  9200  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value. Further, in the image transfer system  10 , the imaging terminal  9100  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal and outputs a cycle adjustment instruction to the synchronization signal generation unit  102 . 
     For this reason, in the image transfer system  10 , in step S 305  included in step S 306 , the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  9100  determines whether or not a predetermined period of time determined in advance has elapsed after the period of a previous imaging synchronization signal is adjusted. In a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has not elapsed after the period of the previous imaging synchronization signal is adjusted (“NO” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  105  determines that cycle adjustment is not performed and terminates the process of step S 305 . 
     On the other hand, in a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has elapsed after the period of the previous imaging synchronization signal is adjusted (“YES” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  105  determines that cycle adjustment is performed and calculates a period-adjusted accuracy estimation value in step S 2106  included in step S 306 . Meanwhile, the process of step S 305  and the process of step S 2106  which are included in step S 306  in the image transfer system  10  are the same as the process of step S 305  and the process of step S 2106  which are included in step S 306  in the image transfer system  1  of the first embodiment. 
     Thereafter, in the image transfer system  10 , in step S 319  included in step S 306 , the imaging terminal  9100  generates an estimation notification signal including information of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  105  and transmits the generated estimation notification signal to the display terminal  9200 . Meanwhile, the process of step S 319  included in step S 306  in the image transfer system  10  and the estimation notification signal generated in the process of step S 319  and transmitted to the display terminal  9200  are the same as the process of step S 319  included in step S 306  and the estimation notification signal in each of the image transfer system  8  of the eighth embodiment and the image transfer system  9  of the ninth embodiment. Thereby, the cycle-adjustment determination unit  3104  provided in the display terminal  9200  acquires information of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  105  provided in the imaging terminal  9100 . 
     Further, in the image transfer system  10 , in step S 1004  included in step S 306 , the display terminal  9200  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value transmitted from the imaging terminal  9100 . More specifically, the cycle-adjustment determination unit  3104  included in the display terminal  9200  determines that cycle adjustment for an imaging synchronization signal is not performed in a case where the accuracy of an imaging synchronization signal indicated by the period-adjusted accuracy estimation value transmitted from the imaging terminal  9100 , that is, calculated by the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  9100  is equal to the current accuracy of the imaging synchronization signal or has not been improved, and terminates the process of step S 1004 . On the other hand, in a case where the estimated accuracy of the imaging synchronization signal has been improved compared with the current accuracy of the imaging synchronization signal, the cycle-adjustment determination unit  3104  determines that cycle adjustment for an imaging synchronization signal is performed, and generates a cycle adjustment execution determination result indicating a determination result. Meanwhile, the process of step S 1004  included in step S 306  in the image transfer system  10  is the same as the process of step S 1004  included in step S 306  in each of the image transfer system  4  of the fourth embodiment and the image transfer system  9  of the ninth embodiment. 
     Thereafter, in the image transfer system  10 , in step S 311  included in step S 306 , the display terminal  9200  generates a determination notification signal including information of the cycle adjustment execution determination result obtained by the cycle-adjustment determination unit  3104  and transmits the generated determination notification signal to the imaging terminal  9100 . Meanwhile, the process of step S 311  included in step S 306  in the image transfer system  10  and the determination notification signal generated in the process of step S 311  and transmitted to the imaging terminal  9100  are the same as the process of step S 311  included in step S 306  and the determination notification signal in each of the image transfer system  4  of the fourth embodiment and the image transfer system  9  of the ninth embodiment. Thereby, the cycle adjustment unit  101  provided in the imaging terminal  9100  acquires information of the cycle adjustment execution determination result obtained by the cycle-adjustment determination unit  3104  provided in the display terminal  9200  through the round-trip-propagation-time measurement unit  106 . 
     Further, in the image transfer system  10 , in step S 1206  included in step S 306 , the imaging terminal  9100  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal on the basis of information such as a round trip propagation-time determination value transmitted from the display terminal  9200  in accordance with the cycle adjustment execution determination result transmitted from the display terminal  9200 , and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, the synchronization signal generation unit  102  performs cycle adjustment for adjusting the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  101 . Meanwhile, the process of step S 1206  included in step S 306  in the image transfer system  10  is the same as the process of step S 1206  included in step S 306  in each of the image transfer system  4  of the fourth embodiment and the image transfer system  9  of the ninth embodiment. 
     Meanwhile, the phase adjustment instruction transmitted from the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  9100  is also output to the synchronization signal generation unit  102 . Thereby, the synchronization signal generation unit  102  adjusts the phase of an imaging synchronization signal to be generated, in response to the phase adjustment instruction output from the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106 . 
     In this manner, in the image transfer system  10 , the imaging terminal  9100  transmits a round-trip-propagation-time-measurement outgoing signal and calculates a round trip propagation-time in wireless transfer between the imaging terminal  9100  and the display terminal  9200 . Further, in the image transfer system  10 , the imaging terminal  9100  performs a process of updating a round trip propagation-time determination value. Thereby, in the image transfer system  10 , the imaging terminal  9100  performs a process of estimating (calculating) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed. Further, in the image transfer system  10 , the imaging terminal  9100  transmits information of the calculated period-adjusted accuracy estimation value to the display terminal  9200 . Thereby, in the image transfer system  10 , the display terminal  9200  performs a process of determining whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  9100  is performed, on the basis of the information of the period-adjusted accuracy estimation value transmitted from the imaging terminal  9100 . Further, in the image transfer system  10 , the display terminal  9200  transmits information of a cycle adjustment execution determination result which is a determination result to the imaging terminal  9100 . Thereby, in the image transfer system  10 , the imaging terminal  9100  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal in accordance with the information of the cycle adjustment execution determination result transmitted from the display terminal  9200 , and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, in the image transfer system  10 , the imaging terminal  9100  adjusts the periods of an imaging synchronization signal and a display synchronization signal so as not to be shifted with the elapse of time. 
     Meanwhile, similarly to the image transfer system  1  of the first embodiment to the image transfer system  9  of the ninth embodiment, also in the image transfer system  10 , when at least a process of updating around trip propagation-time determination value is performed in step S 304 , the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  9100  can estimate the accuracy of an imaging synchronization signal (that is, calculate a period-adjusted accuracy estimation value). Therefore, similarly to the image transfer system  1  of the first embodiment to the image transfer system  9  of the ninth embodiment, also in the image transfer system  10 , the phase adjustment unit not shown in the drawing does not necessarily need to adjust the phase of an imaging synchronization signal in step S 304  and may be able to update a round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  as a round trip propagation-time determination value. 
     With such a configuration and processing procedure, in the image transfer system  10 , the imaging terminal  9100  transmits a round-trip-propagation-time-measurement outgoing signal and measures (calculates) a round trip propagation-time in wireless transfer between the imaging terminal  9100  and the display terminal  9200  to update a round trip propagation-time determination value. Further, in the image transfer system  10 , the imaging terminal  9100  estimates (calculates) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed. Further, in the image transfer system  10 , the display terminal  9200  determines whether or not cycle adjustment for an imaging synchronization signal is performed, on the basis of the period-adjusted accuracy estimation value calculated by the imaging terminal  9100 . Further, in the image transfer system  10 , the imaging terminal  9100  calculates a cycle adjustment amount for adjusting the period of an imaging synchronization signal in accordance with the cycle adjustment execution determination result obtained by the display terminal  9200 , outputs a cycle adjustment instruction, and adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated. Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  9  of the ninth embodiment, also in the image transfer system  10 , the phase or period (at least a period) of an imaging synchronization signal generated by the imaging terminal  9100  (more specifically, the synchronization signal generation unit  102 ) is adjusted so as to match the phase or period (at least a period) of a display synchronization signal generated by the display terminal  9200 . That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  9  of the ninth embodiment, also in the image transfer system  10 , a timing when the imaging terminal  9100  wirelessly transfers captured image data of an image captured by an imaging unit not shown in the drawing to the display terminal  9200  is matched to a timing when the display terminal  9200  displays an image corresponding to the captured image data on a display unit not shown in the drawing. Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  9  of the ninth embodiment, also in the image transfer system  10 , the display terminal  9200  can stably display an image corresponding to captured image data wirelessly transferred from the imaging terminal  9100  on the display unit not shown in the drawing. 
     As described above, the image transfer system  10  of the tenth embodiment performs transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal between the round-trip-propagation-time measurement unit  106  included in the imaging terminal  9100  and the round-trip-propagation-time-measurement assistance unit  202  included in the display terminal  9200  after wireless connection between the imaging terminal  9100  and the display terminal  9200  is established. Further, in the image transfer system  10  of the tenth embodiment, the round-trip-propagation-time measurement unit  106  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  9100  and the display terminal  9200 , on the basis of a transmission time of a round-trip-propagation-time-measurement outgoing signal and a reception time of a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the round-trip-propagation-time-measurement assistance unit  202 . Further, in the image transfer system  10  of the tenth embodiment, the round-trip-propagation-time measurement unit  106  (more specifically, the phase adjustment unit not shown in the drawing) updates the round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  as a round trip propagation-time determination value. Further, in the image transfer system  10  of the tenth embodiment, the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  9100  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  10  of the tenth embodiment, the after-cycle-adjustment accuracy estimation unit  105  transmits information of the estimated (calculated) period-adjusted accuracy estimation value of the imaging synchronization signal to the display terminal  9200 . Further, in the image transfer system  10  of the tenth embodiment, the cycle-adjustment determination unit  3104  included in the display terminal  9200  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is performed, on the basis of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  105 . Further, in the image transfer system  10  of the tenth embodiment, the cycle-adjustment determination unit  3104  transmits information of a cycle adjustment execution determination result which is a result obtained by determining whether or not the period of an imaging synchronization signal is adjusted to the imaging terminal  9100 . Further, in the image transfer system  10  of the tenth embodiment, in a case where the cycle adjustment execution determination result indicates that cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  9100  is performed, the cycle adjustment unit  101  included in the imaging terminal  9100  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, in the image transfer system  10  of the tenth embodiment, the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  101 . 
     Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  9  of the ninth embodiment, also in the image transfer system  10  of the tenth embodiment, the phase or period (at least a period) of an imaging synchronization signal can be matched to the phase or period (at least a period) of a display synchronization signal. That is, also in the image transfer system  10  of the tenth embodiment, even when the periods of an imaging synchronization signal and a display synchronization signal are shifted with the elapse of time due to an error of a phase or a period between an imaging reference clock signal and a display reference clock signal, the phase or period (at least a period) of the imaging synchronization signal can be matched to the phase or period (at least a period) of the display synchronization signal. Thus, also in the image transfer system  10  of the tenth embodiment, the same effects as those in the image transfer system  1  of the first embodiment to the image transfer system  9  of the ninth embodiment can be obtained. That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  9  of the ninth embodiment, also in the image transfer system  10  of the tenth embodiment, it is possible to wirelessly transfer captured image data from the imaging terminal  9100  to the display terminal  9200  without exceeding a validity period of a display image and to stably display a display image corresponding to the captured image data transmitted from the imaging terminal  9100  in the display terminal  9200 . 
     Moreover, in the image transfer system  10  of the tenth embodiment, the display terminal  9200  determines whether or not cycle adjustment for an imaging synchronization signal is performed. Thereby, in the image transfer system  10  of the tenth embodiment, it is not necessary to determine whether or not cycle adjustment for an imaging synchronization signal is performed in the imaging terminal  9100 , and thus it is possible to reduce a load of processing performed in the imaging terminal  9100 . 
     Eleventh Embodiment 
     Hereinafter, an image transfer system of an eleventh embodiment of the present invention will be described.  FIG. 34  is a block diagram showing a schematic configuration of the image transfer system in the eleventh embodiment of the present invention. An image transfer system  11  includes an imaging terminal  10100  and a display terminal  10200 . The imaging terminal  10100  includes a synchronization signal generation unit  102 , a source oscillation clock generation unit  103 , a wireless communication unit  108 , an after-cycle-adjustment accuracy estimation unit  105 , a round-trip-propagation-time measurement unit  106 , a clocking unit  107 , and an antenna  120 . In addition, the display terminal  10200  includes a synchronous signal generation unit  204 , a source oscillation clock generation unit  205 , a wireless communication unit  201 , a round-trip-propagation-time-measurement assistance unit  202 , a clocking unit  203 , a cycle-adjustment determination unit  3104 , a cycle adjustment unit  4101 , and an antenna  220 . 
     Meanwhile, also in  FIG. 34 , in the components of the image transfer system  11 , the same components as the components included in the image transfer system  1  of the first embodiment to the image transfer system  10  of the tenth embodiment are denoted by the same reference numerals and signs. 
     Similarly to the image transfer system  1  of the first embodiment to the image transfer system  10  of the tenth embodiment, the image transfer system  11  is also an image transfer system configured such that transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between the imaging terminal  10100  and the display terminal  10200 , and the imaging terminal  10100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  10200 . Meanwhile, similarly to the image transfer system  1  of the first embodiment and the image transfer system  10  of the tenth embodiment, the image transfer system  11  is an image transfer system in which the imaging terminal  10100  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  10200  to calculate a round trip propagation-time in wireless transfer between the imaging terminal  10100  and the display terminal  10200 , and the imaging terminal  10100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated. 
     However, in the image transfer system  11 , some components included in the imaging terminal  9100  in the image transfer system  10  of the tenth embodiment are moved to the display terminal  10200 . More specifically, in the image transfer system  11 , the cycle adjustment unit  4101  that replaces the cycle adjustment unit  101  included in the imaging terminal  9100  in the image transfer system  10  of the tenth embodiment is included in the display terminal  10200 . 
     For this reason, in the image transfer system  11 , the imaging terminal  10100  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  10100  and the display terminal  10200 . Further, in the image transfer system  11 , the imaging terminal  10100  estimates (calculates) a period-adjusted accuracy estimation value in a case where the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is temporarily adjusted, and transmits the estimated period-adjusted accuracy estimation value to the display terminal  10200 . Further, in the image transfer system  11 , the display terminal  10200  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  10100  is performed, on the basis of the period-adjusted accuracy estimation value transmitted from the imaging terminal  10100 , calculates a cycle adjustment amount in a case where it is determined that cycle adjustment for an imaging synchronization signal is performed, and transmits the calculated cycle adjustment amount to the imaging terminal  10100  together with a cycle adjustment instruction. Further, in the image transfer system  11 , the imaging terminal  10100  adjusts the period of an imaging synchronization signal to be generated, in response to the cycle adjustment instruction which is transmitted from the display terminal  10200 . 
     However, also in the image transfer system  11 , although there is a process of exchanging signals through wireless transfer in association with a change in the disposition of components, functions and operations of the components are the same as the functions and operations of the corresponding components in the image transfer system  1  of the first embodiment to the image transfer system  10  of the tenth embodiment. That is, the functions and operations of the components included in the imaging terminal  10100  and the display terminal  10200  in the image transfer system  11  can be easily understood from the above description of the components included in the imaging terminal and the display terminal in each of the image transfer system  1  of the first embodiment to the image transfer system  10  of the tenth embodiment. Therefore, a detailed description related to the components included in the image transfer system  11  will be omitted. 
     Next, operations of processing in the image transfer system  11  will be described. Meanwhile, similarly to the image transfer system  1  of the first embodiment, in the image transfer system  11 , it is assumed that a phase adjustment unit not shown in the drawing is included in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  10100 . That is, in the image transfer system  11 , a configuration in which the phase adjustment unit not shown in the drawing is disposed at the same position as that in each of the image transfer system  1  of the first embodiment and the image transfer system  10  of the tenth embodiment is adopted.  FIG. 35  is a flowchart showing a processing procedure of the image transfer system  11  in the eleventh embodiment of the present invention. 
     In the image transfer system  11 , a process of transmitting information of a period-adjusted accuracy estimation value to the display terminal  10200 , which is used to adjust the period of an imaging synchronization signal generated by the synchronization signal generation unit  102 , to the display terminal  10200  and a process of transmitting a cycle adjustment instruction including a cycle adjustment amount to the imaging terminal  10100  are added, in association with a change to a configuration in which the cycle-adjustment determination unit  3104  and the cycle adjustment unit  4101  are included in the display terminal  10200 . However, an outline of the overall operation in the image transfer system  11  is the same as those of the image transfer system  1  of the first embodiment to the image transfer system  10  of the tenth embodiment. Therefore, also in the image transfer system  11 , a description of an outline of the overall operation will be omitted, and a more specific operation of performing cycle adjustment for an imaging synchronization signal in the image transfer system  11  will be described. 
     Also in the image transfer system  11 , when a cycle adjustment process is started, the imaging terminal  10100  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  10200 , and the display terminal  10200  transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal to the imaging terminal  10100  in step S 302 . Meanwhile, the process of step S 302  in the image transfer system  11  is the same as the process of step S 302  in each of the image transfer system  1  of the first embodiment and the image transfer system  10  of the tenth embodiment. 
     Thereafter, also in the image transfer system  11 , in step S 303 , the imaging terminal  10100  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  10100  and the display terminal  10200 . Meanwhile, the process of step S 303  in the image transfer system  11  is the same as the process of step S 303  in each of the image transfer system  1  of the first embodiment and the image transfer system  10  of the tenth embodiment. 
     Thereafter, also in the image transfer system  11 , in step S 304 , the imaging terminal  10100  generates a phase adjustment instruction for the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  to adjust the phase of an imaging synchronization signal, on the basis of the round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  10100  and the display terminal  10200  which is calculated in step S 303 , and updates a round trip propagation-time determination value. Meanwhile, the process of step S 304  in the image transfer system  11  is also the same as the process of step S 304  in each of the image transfer system  1  of the first embodiment and the image transfer system  10  of the tenth embodiment. 
     Thereafter, in the image transfer system  11 , in step S 306 , the period of an imaging synchronization signal is adjusted on the basis of the information of the plurality of round trip propagation-time determination values updated in step S 304 . However, in the image transfer system  11 , the imaging terminal  10100  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  11 , the display terminal  10200  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value and calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction. 
     For this reason, in the image transfer system  11 , in step S 305  included in step S 306 , the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  10100  determines whether or not a predetermined period of time determined in advance has elapsed after the period of a previous imaging synchronization signal is adjusted. In a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has not elapsed after the period of the previous imaging synchronization signal is adjusted (“NO” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  105  determines that cycle adjustment is not performed and terminates the process of step S 305 . 
     On the other hand, in a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has elapsed after the period of the previous imaging synchronization signal is adjusted (“YES” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  105  determines that cycle adjustment is performed and calculates a period-adjusted accuracy estimation value in step S 2106  included in step S 306 . Meanwhile, the process of step S 305  and the process of step S 2106  which are included in step S 306  in the image transfer system  11  are the same as the process of step S 305  and the process of step S 2106  which are included in step S 306  in each of the image transfer system  1  of the first embodiment and the image transfer system  10  of the tenth embodiment. 
     Thereafter, in the image transfer system  11 , in step S 319  included in step S 306 , the imaging terminal  10100  generates an estimation notification signal including information of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  105  and transmits the generated estimation notification signal to the display terminal  10200 . Meanwhile, the process of step S 319  included in step S 306  in the image transfer system  11  and the estimation notification signal generated in the process of step S 319  and transmitted to the display terminal  10200  are the same as the process of step S 319  included in step S 306  and the estimation notification signal in each of the image transfer system  8  of the eighth embodiment to the image transfer system  10  of the tenth embodiment. Thereby, the cycle-adjustment determination unit  3104  provided in the display terminal  10200  acquires information of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  105  provided in the imaging terminal  10100 . 
     Further, in the image transfer system  11 , in step S 2107  included in step S 306 , the display terminal  10200  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of information of the period-adjusted accuracy estimation value transmitted from the imaging terminal  10100  and calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction. Meanwhile, the process of step S 2107  included in step S 306  in the image transfer system  11  is the same as the process of step S 2107  included in step S 306  in each of the image transfer system  1  of the first embodiment and the image transfer system  8  of the eighth embodiment. 
     Thereafter, in the image transfer system  11 , in step S 312 , the display terminal  10200  generates an adjustment notification signal including information of the cycle adjustment instruction including the cycle adjustment amount calculated by the cycle adjustment unit  4101  and transmits the generated adjustment notification signal to the imaging terminal  10100 . Meanwhile, the process of step S 312  in the image transfer system  11  and the adjustment notification signal generated in the process of step S 312  and transmitted to the imaging terminal  10100  are the same as the process of step S 312  and the adjustment notification signal in the image transfer system  8  of the eighth embodiment. Thereby, the synchronization signal generation unit  102  provided in the imaging terminal  10100  acquires information of the cycle adjustment instruction including the cycle adjustment amount transmitted from the cycle adjustment unit  4101  provided in the display terminal  10200  through the round-trip-propagation-time measurement unit  106 . Thereby, the synchronization signal generation unit  102  performs cycle adjustment for adjusting the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  4101 . 
     Meanwhile, the phase adjustment instruction transmitted from the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  10100  is also output to the synchronization signal generation unit  102 . Thereby, the synchronization signal generation unit  102  adjusts the phase of an imaging synchronization signal to be generated, in response to the phase adjustment instruction output from the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106 . 
     In this manner, in the image transfer system  11 , the imaging terminal  10100  transmits a round-trip-propagation-time-measurement outgoing signal and calculates a round trip propagation-time in wireless transfer between the imaging terminal  10100  and the display terminal  10200 . Further, in the image transfer system  11 , the imaging terminal  10100  performs a process of updating a round trip propagation-time determination value. Thereby, in the image transfer system  11 , the imaging terminal  10100  estimates (calculates) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed. Further, in the image transfer system  11 , the imaging terminal  10100  transmits information of the calculated period-adjusted accuracy estimation value to the display terminal  10200 . Thereby, in the image transfer system  11 , the display terminal  10200  performs a process of determining whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  10100  is performed, on the basis of the information of the period-adjusted accuracy estimation value which is transmitted from the imaging terminal  10100 , and performs a process of calculating a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction in accordance with a cycle adjustment execution determination result. Further, in the image transfer system  11 , the display terminal  10200  transmits information of the cycle adjustment instruction including the calculated cycle adjustment amount to the imaging terminal  10100 . Thereby, in the image transfer system  11 , the imaging terminal  10100  adjusts the periods of an imaging synchronization signal and a display synchronization signal so as not to be shifted with the elapse of time, in response to the cycle adjustment instruction transmitted from the display terminal  10200 . 
     Meanwhile, similarly to the image transfer system  1  of the first embodiment to the image transfer system  10  of the tenth embodiment, also in the image transfer system  11 , when at least a process of updating a round trip propagation-time determination value is performed in step S 304 , the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  10100  can estimate the accuracy of an imaging synchronization signal (that is, calculate a period-adjusted accuracy estimation value). Therefore, similarly to the image transfer system  1  of the first embodiment to the image transfer system  10  of the tenth embodiment, also in the image transfer system  11 , the phase adjustment unit not shown in the drawing does not necessarily need to adjust the phase of an imaging synchronization signal in step S 304  and may be able to update a round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  as a round trip propagation-time determination value. 
     With such a configuration and processing procedure, in the image transfer system  11 , the imaging terminal  10100  transmits a round-trip-propagation-time-measurement outgoing signal and measures (calculates) a round trip propagation-time in wireless transfer between the imaging terminal  10100  and the display terminal  10200  to update a round trip propagation-time determination value. Further, in the image transfer system  11 , the imaging terminal  10100  estimates (calculates) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed. Further, in the image transfer system  11 , the display terminal  10200  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value calculated by the imaging terminal  10100 , and calculates a cycle adjustment amount for adjusting the period of an imaging synchronization signal in accordance with the obtained cycle adjustment execution determination result to generate a cycle adjustment instruction. Further, in the image transfer system  11 , the imaging terminal  10100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated, in response to the phase adjustment instruction generated by the display terminal  10200 . Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  10  of the tenth embodiment, also in the image transfer system  11 , the phase or period (at least a period) of an imaging synchronization signal which is generated by the imaging terminal  10100  (more specifically, the synchronization signal generation unit  102 ) is adjusted so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  10200 . That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  10  of the tenth embodiment, also in the image transfer system  11 , a timing when the imaging terminal  10100  wirelessly transfers captured image data of an image captured by an imaging unit not shown in the drawing to the display terminal  10200  is matched to a timing when the display terminal  10200  displays an image corresponding to the captured image data on a display unit not shown in the drawing. Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  10  of the tenth embodiment, also in the image transfer system  11 , the display terminal  10200  can stably display an image corresponding to captured image data wirelessly transferred from the imaging terminal  10100  on the display unit not shown in the drawing. 
     As described above, the image transfer system  11  of the eleventh embodiment performs transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal between the round-trip-propagation-time measurement unit  106  included in the imaging terminal  10100  and the round-trip-propagation-time-measurement assistance unit  202  included in the display terminal  10200  after wireless connection between the imaging terminal  10100  and the display terminal  10200  is established. Further, in the image transfer system  11  of the eleventh embodiment, the round-trip-propagation-time measurement unit  106  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  10100  and the display terminal  10200 , on the basis of a transmission time of a round-trip-propagation-time-measurement outgoing signal and a reception time of a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the round-trip-propagation-time-measurement assistance unit  202 . Further, in the image transfer system  11  of the eleventh embodiment, the round-trip-propagation-time measurement unit  106  (more specifically, the phase adjustment unit not shown in the drawing) updates the round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  as a round trip propagation-time determination value. Further, in the image transfer system  11  of the eleventh embodiment, the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  10100  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  11  of the eleventh embodiment, the after-cycle-adjustment accuracy estimation unit  105  transmits information of the estimate (calculated) period-adjusted accuracy estimation value of the imaging synchronization signal to the display terminal  10200 . Further, in the image transfer system  11  of the eleventh embodiment, the cycle-adjustment determination unit  3104  included in the display terminal  10200  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is performed, on the basis of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  105 . Further, in the image transfer system  11  of the eleventh embodiment, in a case where a cycle adjustment execution determination result indicates that cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  10100  is performed, the cycle adjustment unit  4101  included in the display terminal  10200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction. Further, in the image transfer system  11  of the eleventh embodiment, the cycle adjustment unit  4101  transmits information of a cycle adjustment instruction including a cycle adjustment amount for adjusting the period of an imaging synchronization signal in the imaging terminal  10100  to the imaging terminal  10100 . Thereby, in the image transfer system  11  of the eleventh embodiment, the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is transmitted from the cycle adjustment unit  4101 . 
     Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  10  of the tenth embodiment, also in the image transfer system  11  of the eleventh embodiment, the phase or period (at least a period) of an imaging synchronization signal can be matched to the phase or period (at least a period) of a display synchronization signal. That is, also in the image transfer system  11  of the eleventh embodiment, even when the periods of an imaging synchronization signal and a display synchronization signal are shifted with the elapse of time due to an error of a phase or a period between an imaging reference clock signal and a display reference clock signal, the phase or period (at least a period) of the imaging synchronization signal can be matched to the phase or period (at least a period) of the display synchronization signal. Thus, also in the image transfer system  11  of the eleventh embodiment, the same effects as those in the image transfer system  1  of the first embodiment to the image transfer system  10  of the tenth embodiment can be obtained. That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  10  of the tenth embodiment, also in the image transfer system  11  of the eleventh embodiment, it is possible to wirelessly transfer captured image data from the imaging terminal  10100  to the display terminal  10200  without exceeding a validity period of a display image and to stably display a display image corresponding to the captured image data transmitted from the imaging terminal  10100  in the display terminal  10200 . 
     Moreover, in the image transfer system  11  of the eleventh embodiment, the display terminal  10200  determines whether or not cycle adjustment for an imaging synchronization signal is performed, calculates a cycle adjustment amount, and generates a cycle adjustment instruction. Thereby, in the image transfer system  11  of the eleventh embodiment, the imaging terminal  10100  may only calculate a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  10100  and the display terminal  10200 , estimate (calculate) a period-adjusted accuracy estimation value, and perform cycle adjustment for an imaging synchronization signal in response to the cycle adjustment instruction. That is, in the image transfer system  11  of the eleventh embodiment, it is not necessary to determine whether or not cycle adjustment for an imaging synchronization signal is performed, calculate a cycle adjustment amount, and generate a cycle adjustment instruction in the imaging terminal  10100 , and thus it is possible to reduce a load of processing performed in the imaging terminal  10100 . 
     Twelfth Embodiment 
     Hereinafter, an image transfer system of a twelfth embodiment of the present invention will be described.  FIG. 36  is a block diagram showing a schematic configuration of the image transfer system in the twelfth embodiment of the present invention. An image transfer system  12  includes an imaging terminal  11100  and a display terminal  11200 . The imaging terminal  11100  includes a synchronization signal generation unit  102 , a source oscillation clock generation unit  103 , a wireless communication unit  108 , a cycle adjustment determination unit  104 , an after-cycle-adjustment accuracy estimation unit  105 , a round-trip-propagation-time measurement unit  106 , a clocking unit  107 , and an antenna  120 . In addition, the display terminal  11200  includes a synchronous signal generation unit  204 , a source oscillation clock generation unit  205 , a wireless communication unit  201 , a round-trip-propagation-time-measurement assistance unit  202 , a clocking unit  203 , a cycle adjustment unit  4101 , and an antenna  220 . 
     Meanwhile, also in  FIG. 36 , in components of the image transfer system  12 , the same components as the components included in the image transfer system  1  of the first embodiment to the image transfer system  11  of the eleventh embodiment are denoted by the same reference numerals and signs. 
     Similarly to the image transfer system  1  of the first embodiment to the image transfer system  11  of the eleventh embodiment, the image transfer system  12  is also an image transfer system configured such that transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between the imaging terminal  11100  and the display terminal  11200 , and the imaging terminal  11100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  11200 . Meanwhile, similarly to the image transfer system  1  of the first embodiment, the image transfer system  10  of the tenth embodiment, and the image transfer system  11  of the eleventh embodiment, the image transfer system  12  is an image transfer system in which the imaging terminal  11100  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  11200  to calculate a round trip propagation-time in wireless transfer between the imaging terminal  11100  and the display terminal  11200 , and the imaging terminal  11100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated. 
     However, in the image transfer system  12 , some components included in the imaging terminal  100  in the image transfer system  1  of the first embodiment are moved to the display terminal  11200 . More specifically, in the image transfer system  12 , the cycle adjustment unit  4101  that replaces the cycle adjustment unit  101  included in the imaging terminal  100  in the image transfer system  1  of the first embodiment is included in the display terminal  11200 . Meanwhile, it can be said that a configuration of the image transfer system  12  is a configuration in which the cycle-adjustment determination unit  3104  included in the display terminal  10200  in the image transfer system  11  of the eleventh embodiment is returned to the imaging terminal  11100  as a cycle adjustment determination unit  104 . 
     For this reason, in the image transfer system  12 , the imaging terminal  11100  calculates a round trip propagation-time required for wireless transfer between the imaging terminal  11100  and the display terminal  11200 . Further, in the image transfer system  12 , the imaging terminal  11100  estimates (calculates) a period-adjusted accuracy estimation value in a case where the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is temporarily adjusted, and transmits a cycle adjustment execution determination result indicating a result obtained by determining whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value to the display terminal  11200 . Further, in the image transfer system  12 , in a case where the cycle adjustment execution determination result transmitted from the imaging terminal  11100  indicates that cycle adjustment for an imaging synchronization signal is performed, the display terminal  11200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  11100  and transmits the calculated cycle adjustment amount to the imaging terminal  11100  together with a cycle adjustment instruction. Further, in the image transfer system  12 , the imaging terminal  11100  adjusts the period of an imaging synchronization signal to be generated, in response to the cycle adjustment instruction transmitted from the display terminal  11200 . 
     However, also in the image transfer system  12 , although there is a process of exchanging signals through wireless transfer in association with a change in the disposition of components, functions and operations of the components are the same as the functions and operations of the corresponding components in the image transfer system  1  of the first embodiment to the image transfer system  11  of the eleventh embodiment. That is, the functions and operations of the components included in the imaging terminal  11100  and the display terminal  11200  in the image transfer system  12  can be easily understood from the above description of the components included in the imaging terminal and the display terminal in each of the image transfer system  1  of the first embodiment to the image transfer system  11  of the eleventh embodiment. Therefore, a detailed description related to the components included in the image transfer system  12  will be omitted. 
     Next, operations of processing in the image transfer system  12  will be described. Meanwhile, similarly to the image transfer system  1  of the first embodiment, in the image transfer system  12 , it is assumed that a phase adjustment unit, not shown in the drawing is included in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  11100 . That is, in the image transfer system  12 , a configuration in which the phase adjustment unit not shown in the drawing is disposed at the same position as those in the image transfer system  1  of the first embodiment, the image transfer system  10  of the tenth embodiment, and the image transfer system  11  of the eleventh embodiment.  FIG. 37  is a flowchart showing a processing procedure of the image transfer system  12  in the twelfth embodiment of the present invention. 
     In the image transfer system  12 , a process of transmitting information including a cycle adjustment execution determination result, which is used to adjust the period of an imaging synchronization signal generated by the synchronization signal generation unit  102 , to the display terminal  11200  and a process of transmitting a cycle adjustment instruction including a cycle adjustment amount to the imaging terminal  11100  are added, in association with a change to a configuration in which the cycle adjustment unit  4101  is included in the display terminal  11200 . However, an outline of the overall operation in the image transfer system  12  is the same as those of the image transfer system  1  of the first embodiment to the image transfer system  11  of the eleventh embodiment. Therefore, also in the image transfer system  12 , a description of an outline of the overall operation will be omitted, and a more specific operation of performing cycle adjustment for an imaging synchronization signal in the image transfer system  12  will be described. 
     Also in the image transfer system  12 , when a cycle adjustment process is started, the imaging terminal  11100  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  11200 , and the display terminal  11200  transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal to the imaging terminal  11100  in step S 302 . Meanwhile, the process of step S 302  in the image transfer system  12  is the same as the process of step S 302  in each of the image transfer system  1  of the first embodiment, the image transfer system  10  of the tenth embodiment, and the image transfer system  11  of the eleventh embodiment. 
     Thereafter, also in the image transfer system  12 , in step S 303 , the imaging terminal  11100  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  11100  and the display terminal  11200 . Meanwhile, the process of step S 303  in the image transfer system  12  is the same as the process of step S 303  in each of the image transfer system  1  of the first embodiment, the image transfer system  10  of the tenth embodiment, and the image transfer system  11  of the eleventh embodiment. 
     Thereafter, also in the image transfer system  12 , in step S 304 , the imaging terminal  11100  generates a phase adjustment instruction for the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  to adjust the phase of an imaging synchronization signal, on the basis of the round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  11100  and the display terminal  11200  which is calculated in step S 303 , and updates a round trip propagation-time determination value. Meanwhile, the process of step S 304  in the image transfer system  12  is also the same as the process of step S 304  in each of the image transfer system  1  of the first embodiment, the image transfer system  10  of the tenth embodiment, and the image transfer system  11  of the eleventh embodiment. 
     Thereafter, in the image transfer system  12 , in step S 306 , the period of an imaging synchronization signal is adjusted on the basis of the information of the plurality of round trip propagation-time determination values updated in step S 304 . However, in the image transfer system  12 , the imaging terminal  11100  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed and determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the estimated (calculated) period-adjusted accuracy estimation value. Further, in the image transfer system  12 , the display terminal  11200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal and generates a cycle adjustment instruction. 
     For this reason, in the image transfer system  12 , in step S 305  included in step S 306 , the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  11100  determines whether or not a predetermined period of time determined in advance has elapsed after the period of a previous imaging synchronization signal is adjusted. In a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has not elapsed after the period of the previous imaging synchronization signal is adjusted (“NO” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  105  determines that cycle adjustment is not performed and terminates the process of step S 305 . 
     On the other hand, in a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has elapsed after the period of the previous imaging synchronization signal is adjusted (“YES” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  105  determines that cycle adjustment is performed and calculates a period-adjusted accuracy estimation value in step S 2106  included in step S 306 . Meanwhile, the process of step S 305  and the process of step S 2106  which are included in step S 306  in the image transfer system  12  are the same as the process of step S 305  and the process of step S 2106  which are included in step S 306  in each of the image transfer system  1  of the first embodiment, the image transfer system  10  of the tenth embodiment, and the image transfer system  11  of the eleventh embodiment. 
     Further, in the image transfer system  12 , in step S 1004  included in step S 306 , the cycle adjustment determination unit  104  included in the imaging terminal  11100  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of information of the calculated period-adjusted accuracy estimation value. Further, in a case where the cycle adjustment determination unit  104  determines that cycle adjustment for an imaging synchronization signal is performed because an estimated accuracy of the estimated imaging synchronization signal has been improved compared with the current accuracy of the imaging synchronization signal, the cycle adjustment execution determination unit generates a cycle adjustment execution determination result indicating a determination result. Meanwhile, the process of step S 1004  included in step S 306  in image transfer system  12  is the same as the process of step S 1004  included in step S 306  in the image transfer system  1  of the first embodiment. 
     Thereafter, in the image transfer system  12 , in step S 311  included in step S 306 , the imaging terminal  11100  generates a determination notification signal including information of the cycle adjustment execution determination result obtained by the cycle adjustment determination unit  104  and transmits the generated determination notification signal to the display terminal  11200 . Meanwhile, the process of step S 311  included in step S 306  in the image transfer system  12  and the determination notification signal generated in the process of step S 311  and transmitted to the display terminal  11200  are the same as the process of step S 311  included in step S 306  and the determination notification signal in the image transfer system  6  of the sixth embodiment and the image transfer system  7  of the seventh embodiment. Thereby, the cycle adjustment unit  4101  provided in the display terminal  11200  acquires information of the cycle adjustment execution determination result obtained by the cycle adjustment determination unit  104  provided in the imaging terminal  11100 . 
     Further, in the image transfer system  12 , in step S 1206  included in step S 306 , the display terminal  11200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal in accordance with the cycle adjustment execution determination result transmitted from the imaging terminal  11100 , and generates a cycle adjustment instruction. Meanwhile, the process of step S 1206  included in step S 306  in the image transfer system  12  is the same as the process of step S 1206  included in step S 306  in each of the image transfer system  6  of the sixth embodiment and the image transfer system  7  of the seventh embodiment. 
     Thereafter, in the image transfer system  12 , in step S 312 , the display terminal  11200  generates an adjustment notification signal including information of a cycle adjustment instruction including the cycle adjustment amount calculated by the cycle adjustment unit  4101 , and transmits the generated adjustment notification signal to the imaging terminal  11100 . Meanwhile, the process of step S 312  in the image transfer system  12  and the adjustment notification signal generated in the process of step S 312  and transmitted to the imaging terminal  11100  are the same as the process of step S 312  and the adjustment notification signal in each of the image transfer system  6  of the sixth embodiment and the image transfer system  7  of the seventh embodiment. Thereby, the synchronization signal generation unit  102  provided in the imaging terminal  11100  acquires information of the cycle adjustment instruction including the cycle adjustment amount transmitted from the cycle adjustment unit  4101  provided in the display terminal  11200  through the round-trip-propagation-time measurement unit  106 . Thereby, the synchronization signal generation unit  102  performs cycle adjustment for adjusting the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  4101 . 
     Meanwhile, the phase adjustment instruction transmitted from the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  11100  is also output to the synchronization signal generation unit  102 . Thereby, the synchronization signal generation unit  102  adjusts the phase of an imaging synchronization signal to be generated, in response to the phase adjustment instruction output from the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106 . 
     In this manner, in the image transfer system  12 , the imaging terminal  11100  transmits a round-trip-propagation-time-measurement outgoing signal and calculates a round trip propagation-time in wireless transfer between the imaging terminal  11100  and the display terminal  11200 . Further, in the image transfer system  12 , the imaging terminal  11100  performs a process of updating a round trip propagation-time determination value. Thereby, in the image transfer system  12 , the imaging terminal  11100  performs a process of estimating (calculating) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed and a process of determining whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is performed. Further, in the image transfer system  12 , the imaging terminal  11100  transmits information of a cycle adjustment execution determination result which is a determination result to the display terminal  11200 . Thereby, in the image transfer system  12 , the display terminal  11200  performs a process of calculating a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction in accordance with the information of the cycle adjustment execution determination result transmitted from the imaging terminal  11100 . Further, in the image transfer system  12 , the display terminal  11200  transmits information of the cycle adjustment instruction including the calculated cycle adjustment amount to the imaging terminal  11100 . Thereby, in the image transfer system  12 , the imaging terminal  11100  adjusts the periods of an imaging synchronization signal and a display synchronization signal so as not to be shifted with the elapse of time, in response to the cycle adjustment instruction transmitted from the display terminal  11200 . 
     Meanwhile, similarly to the image transfer system  1  of the first embodiment to the image transfer system  11  of the eleventh embodiment, also in the image transfer system  12 , when at least a process of updating around trip propagation-time determination value is performed in step S 304 , the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  11100  can estimate the accuracy of an imaging synchronization signal (that is, calculate a period-adjusted accuracy estimation value). Therefore, similarly to the image transfer system  1  of the first embodiment to the image transfer system  11  of the eleventh embodiment, also in the image transfer system  12 , the phase adjustment unit not shown in the drawing does not necessarily need to adjust the phase of an imaging synchronization signal in step S 304  and may be able to update a round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  as a round trip propagation-time determination value. 
     With such a configuration and processing procedure, in the image transfer system  12 , the imaging terminal  11100  transmits a round-trip-propagation-time-measurement outgoing signal and measures (calculates) a round trip propagation-time in wireless transfer between the imaging terminal  11100  and the display terminal  11200  to update a round trip propagation-time determination value. Further, in the image transfer system  12 , the imaging terminal  11100  estimates (calculates) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed and determines whether or not cycle adjustment for an imaging synchronization signal is performed. Further, in the image transfer system  12 , the display terminal  11200  calculates a cycle adjustment amount for adjusting the period of an imaging synchronization signal to generate a cycle adjustment instruction in accordance with the cycle adjustment execution determination result obtained by the imaging terminal  11100 . Further, in the image transfer system  12 , the imaging terminal  11100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated, in response to the phase adjustment instruction generated by the display terminal  11200 . Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  11  of the eleventh embodiment, also in the image transfer system  12 , the phase or period (at least a period) of an imaging synchronization signal generated by the imaging terminal  11100  (more specifically, the synchronization signal generation unit  102 ) is adjusted so as to match the phase or period (at least a period) of a display synchronization signal generated by the display terminal  11200 . That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  11  of the eleventh embodiment, also in the image transfer system  12 , a timing when the imaging terminal  11100  wirelessly transfers captured image data of an image captured by an imaging unit not shown in the drawing to the display terminal  11200  is matched to a timing when the display terminal  11200  displays an image corresponding to the captured image data on a display unit not shown in the drawing. Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  11  of the eleventh embodiment, also in the image transfer system  12 , the display terminal  11200  can stably display an image corresponding to captured image data wirelessly transferred from the imaging terminal  11100  on the display unit not shown in the drawing. 
     As described above, the image transfer system  12  of the twelfth embodiment performs transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal between the round-trip-propagation-time measurement unit  106  included in the imaging terminal  11100  and the round-trip-propagation-time-measurement assistance unit  202  included in the display terminal  11200  after wireless connection between the imaging terminal  11100  and the display terminal  11200  is established. Further, in the image transfer system  12  of the twelfth embodiment, the round-trip-propagation-time measurement unit  106  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  11100  and the display terminal  11200 , on the basis of a transmission time of a round-trip-propagation-time-measurement outgoing signal and a reception time of a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the round-trip-propagation-time-measurement assistance unit  202 . Further, in the image transfer system  12  of the twelfth embodiment, the round-trip-propagation-time measurement unit  106  (more specifically, the phase adjustment unit not shown in the drawing) updates the round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  as a round trip propagation-time determination value. Further, in the image transfer system  12  of the twelfth embodiment, the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  11100  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  12  of the twelfth embodiment, the cycle adjustment determination unit  104  included in the imaging terminal  11100  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is performed, on the basis of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  105 . Further, in the image transfer system  12  of the twelfth embodiment, the cycle adjustment determination unit  104  transmits information of a cycle adjustment execution determination result which is a result obtained by determining whether or not the period of an imaging synchronization signal is adjusted to the display terminal  11200 . Further, in the image transfer system  12  of the twelfth embodiment, in a case where the cycle adjustment execution determination result indicates that cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  11100  is performed, the cycle adjustment unit  4101  included in the display terminal  11200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction. Further, in the image transfer system  12  of the twelfth embodiment, the cycle adjustment unit  4101  transmits information of a cycle adjustment instruction including a cycle adjustment amount for adjusting the period of an imaging synchronization signal in the imaging terminal  11100  to the imaging terminal  11100 . Thereby, in the image transfer system  12  of the twelfth embodiment, the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is transmitted from the cycle adjustment unit  4101 . 
     Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  11  of the eleventh embodiment, also in the image transfer system  12  of the twelfth embodiment, the phase or period (at least a period) of an imaging synchronization signal can be matched to the phase or period (at least a period) of a display synchronization signal. That is, also in the image transfer system  12  of the twelfth embodiment, even when the periods of an imaging synchronization signal and a display synchronization signal are shifted with the elapse of time due to an error of a phase or a period between an imaging reference clock signal and a display reference clock signal, the phase or period (at least a period) of the imaging synchronization signal can be matched to the phase or period (at least a period) of the display synchronization signal. Thus, also in the image transfer system  12  of the twelfth embodiment, the same effects as those in the image transfer system  1  of the first embodiment to the image transfer system  11  of the eleventh embodiment can be obtained. That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  11  of the eleventh embodiment, also in the image transfer system  12  of the twelfth embodiment, it is possible to wirelessly transfer captured image data from the imaging terminal  11100  to the display terminal  11200  without exceeding a validity period of a display image and to stably display a display image corresponding to the captured image data transmitted from the imaging terminal  11100  in the display terminal  11200 . 
     Moreover, in the image transfer system  12  of the twelfth embodiment, the display terminal  11200  calculates a cycle adjustment amount and generates a cycle adjustment instruction. Thereby, in the image transfer system  12  of the twelfth embodiment, it is not necessary to calculate a cycle adjustment amount and generate a cycle adjustment instruction in the imaging terminal  11100 , and thus it is possible to reduce a load of processing performed in the imaging terminal  11100 . 
     Thirteenth Embodiment 
     Hereinafter, an image transfer system of a thirteenth embodiment of the present invention will be described.  FIG. 38  is a block diagram showing a schematic configuration of the image transfer system in the thirteenth embodiment of the present invention. An image transfer system  13  includes an imaging terminal  12100  and a display terminal  12200 . The imaging terminal  12100  includes a synchronization signal generation unit  102 , a source oscillation clock generation unit  103 , a wireless communication unit  108 , a cycle adjustment determination unit  104 , a round-trip-propagation-time measurement unit  106 , a clocking unit  107 , and an antenna  120 . In addition, the display terminal  12200  includes a synchronous signal generation unit  204 , a source oscillation clock generation unit  205 , a wireless communication unit  201 , a round-trip-propagation-time-measurement assistance unit  202 , a clocking unit  203 , an after-cycle-adjustment accuracy estimation unit  2105 , a cycle adjustment unit  4101 , and an antenna  220 . 
     Meanwhile, also in  FIG. 38 , in components of the image transfer system  13 , the same components as the components included in the image transfer system  1  of the first embodiment to the image transfer system  12  of the twelfth embodiment are denoted by the same reference numerals and signs. 
     Similarly to the image transfer system  1  of the first embodiment to the image transfer system  12  of the twelfth embodiment, the image transfer system  13  is also an image transfer system configured such that transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between the imaging terminal  12100  and the display terminal  12200 , and the imaging terminal  12100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  12200 . Meanwhile, similarly to the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  12  of the twelfth embodiment, the image transfer system  13  is an image transfer system in which the imaging terminal  12100  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  12200  to calculate a round trip propagation-time in wireless transfer between the imaging terminal  12100  and the display terminal  12200 , and the imaging terminal  12100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated. 
     However, in the image transfer system  13 , some components included in the imaging terminal  11100  in the image transfer system  12  of the twelfth embodiment are moved to the display terminal  12200 . More specifically, in the image transfer system  13 , the after-cycle-adjustment accuracy estimation unit  2105  that replaces the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  11100  in the image transfer system  12  of the twelfth embodiment is included in the display terminal  12200 . 
     For this reason, in the image transfer system  13 , the imaging terminal  12100  measures (calculates) a round trip propagation-time required for transmission and reception when wireless transfer is performed between the imaging terminal  12100  and the display terminal  12200 , and transmits the measured round trip propagation-time to the display terminal  12200 . Further, in the image transfer system  13 , the display terminal  12200  estimates (calculates) a period-adjusted accuracy estimation value in a case where the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  12100  is temporarily adjusted, and transmits the estimated period-adjusted accuracy estimation value to the imaging terminal  12100 . Further, in the image transfer system  13 , the imaging terminal  12100  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is performed, on the basis of the period-adjusted accuracy estimation value transmitted from the display terminal  12200 , and transmits a cycle adjustment execution determination result indicating a determination result to the display terminal  12200 . Further, in the image transfer system  13 , in a case where the cycle adjustment execution determination result transmitted from the imaging terminal  12100  indicates that cycle adjustment for an imaging synchronization signal is performed, the display terminal  12200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  12100  and transmits the calculated cycle adjustment amount to the imaging terminal  12100  together with a cycle adjustment instruction. Further, in the image transfer system  13 , the imaging terminal  12100  adjusts the period of an imaging synchronization signal to be generated, in response to the cycle adjustment instruction transmitted from the display terminal  12200 . 
     However, also in the image transfer system  13 , although there is a process of exchanging signals through wireless transfer in association with a change in the disposition of components, functions and operations of the components are the same as the functions and operations of the corresponding components in the image transfer system  1  of the first embodiment to the image transfer system  12  of the twelfth embodiment. That is, the functions and operations of the components included in the imaging terminal  12100  and the display terminal  12200  in the image transfer system  13  can be easily understood from the above description of the components included in the imaging terminal and the display terminal in each of the image transfer system  1  of the first embodiment to the image transfer system  12  of the twelfth embodiment. Therefore, a detailed description related to the components included in the image transfer system  13  will be omitted. 
     Next, operations of processing in the image transfer system  13  will be described. Meanwhile, similarly to the image transfer system  1  of the first embodiment, in the image transfer system  13 , it is assumed that a phase adjustment unit not shown in the drawing is included in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  12100 . That is, in the image transfer system  13 , a configuration in which the phase adjustment unit not shown in the drawing is disposed at the same position as those in the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  12  of the twelfth embodiment is adopted.  FIG. 39  is a flowchart showing a processing procedure of the image transfer system  13  in the thirteenth embodiment of the present invention. 
     In the image transfer system  13 , a process of transmitting information used to adjust the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  to the display terminal  12200 , a process of transmitting information of a period-adjusted accuracy estimation value to the imaging terminal  12100 , a process of transmitting information of a cycle adjustment execution determination result to the display terminal  12200 , and a process of transmitting a cycle adjustment instruction including a cycle adjustment amount to the imaging terminal  12100  are added, in association with a change to a configuration in which the after-cycle-adjustment accuracy estimation unit  2105  and the cycle adjustment unit  4101  are included in the display terminal  12200 . However, an outline of the overall operation in the image transfer system  13  is the same as those of the image transfer system  1  of the first embodiment to the image transfer system  12  of the twelfth embodiment. Therefore, also in the image transfer system  13 , a description of an outline of the overall operation will be omitted, and a more specific operation of performing cycle adjustment for an imaging synchronization signal in the image transfer system  13  will be described. 
     Also in the image transfer system  13 , when a cycle adjustment process is started, the imaging terminal  12100  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  12200 , and the display terminal  12200  transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal to the imaging terminal  12100  in step S 302 . Meanwhile, the process of step S 302  in the image transfer system  13  is the same as the process of step S 302  in each of the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  12  of the twelfth embodiment. 
     Thereafter, also in the image transfer system  13 , in step S 303 , the imaging terminal  12100  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  12100  and the display terminal  12200 . Meanwhile, the process of step S 303  in the image transfer system  13  is also the same as the process of step S 303  in each of the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  12  of the twelfth embodiment. 
     Thereafter, also in the image transfer system  13 , in step S 304 , the imaging terminal  12100  generates a phase adjustment instruction for the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  to adjust the phase of an imaging synchronization signal, on the basis of the round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  12100  and the display terminal  12200  which is calculated in step S 303 , and updates a round trip propagation-time determination value. Meanwhile, the process of step S 304  in the image transfer system  13  is also the same as the process of step S 304  in each of the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  12  of the twelfth embodiment. 
     Thereafter, in the image transfer system  13 , in step S 307 , the imaging terminal  12100  generates a measurement notification signal including information of the calculated round trip propagation-time and transmits the generated measurement notification signal to the display terminal  12200 . More specifically, the round-trip-propagation-time measurement unit  106  generates a measurement notification signal including information of the calculated round trip propagation-time, information of a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, information of a scheduled transmission time included in a received round-trip-propagation-time-measurement returning signal, a phase adjustment instruction generated by the phase adjustment unit not shown in the drawing, and information of a plurality of round trip propagation-time determination values updated by the phase adjustment unit not shown in the drawing. In addition, the round-trip-propagation-time measurement unit  106  outputs the generated measurement notification signal to the wireless communication unit  108  and transmits the measurement notification signal to the after-cycle-adjustment accuracy estimation unit  2105  provided in the display terminal  12200  through the wireless communication unit  108  and the antenna  120 . Thereby, the display terminal  12200  acquires information of the roundtrip propagation-time determination values from the imaging terminal  12100 . More specifically, the wireless communication unit  201  receives the measurement notification signal transmitted from the imaging terminal  12100  through the antenna  220 . In addition, the wireless communication unit  201  outputs each of the information of the round trip propagation-time included in the received measurement notification signal, the information of the scheduled transmission time of the round-trip-propagation-time-measurement outgoing signal, the information of the scheduled transmission time of the round-trip-propagation-time-measurement returning signal, the information of the phase adjustment instruction, and the information of the plurality of round trip propagation-time determination values to the round-trip-propagation-time-measurement assistance unit  202 . Thereby, the after-cycle-adjustment accuracy estimation unit  2105  provided in the display terminal  12200  acquires the information of the round trip propagation-time determination values transmitted from the imaging terminal  12100  through the round-trip-propagation-time-measurement assistance unit  202 . 
     Thereafter, in the image transfer system  13 , in step S 306 , the period of an imaging synchronization signal is adjusted on the basis of the information of the plurality of round trip propagation-time determination values updated in step S 304 . However, in the image transfer system  13 , the display terminal  12200  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  13 , the imaging terminal  12100  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value. Further, in the image transfer system  13 , the display terminal  12200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal and generates a cycle adjustment instruction. 
     For this reason, in the image transfer system  13 , in step S 305  included in step S 306 , the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  12200  determines whether or not a predetermined period of time determined in advance has elapsed after the period of a previous imaging synchronization signal is adjusted. In a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has not elapsed after the period of the previous imaging synchronization signal is adjusted (“NO” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  2105  determines that cycle adjustment is not performed and terminates the process of step S 305 . 
     On the other hand, in a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has elapsed after the period of the previous imaging synchronization signal is adjusted (“YES” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  2105  determines that cycle adjustment is performed and calculates a period-adjusted accuracy estimation value in step S 2106  included in step S 306 . Meanwhile, the process of step S 305  and the process of step S 2106  which are included in step S 306  in the image transfer system  13  are the same as the process of step S 305  and the process of step S 2106  which are included in step S 306  in each of the image transfer system  3  of the third embodiment, the image transfer system  4  of the fourth embodiment, and the image transfer system  6  of the sixth embodiment. 
     Thereafter, in the image transfer system  13 , in step S 319  included in step S 306 , the display terminal  12200  generates an estimation notification signal including information of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  2105  and transmits the generated estimation notification signal to the imaging terminal  12100 . Meanwhile, the process of step S 319  included in step S 306  in the image transfer system  13  and the estimation notification signal generated in the process of step S 319  and transmitted to the imaging terminal  12100  are the same as the process of step S 319  included in step S 306  and the estimation notification signal in each of the image transfer system  3  of the third embodiment and the image transfer system  6  of the sixth embodiment. Thereby, the cycle adjustment determination unit  104  provided in the imaging terminal  12100  acquires information of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  2105  provided in the display terminal  12200  through the round-trip-propagation-time measurement unit  106 . 
     Further, in the image transfer system  13 , in step S 1004  included in step S 306 , the imaging terminal  12100  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value transmitted from the display terminal  12200 . More specifically, the cycle adjustment determination unit  104  included in the imaging terminal  12100  determines that cycle adjustment for an imaging synchronization signal is not performed in a case where the accuracy of an imaging synchronization signal indicated by the period-adjusted accuracy estimation value transmitted from the display terminal  12200 , that is, calculated by the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  12200  is equal to the current accuracy of the imaging synchronization signal or has not been improved, and terminates the process of step S 1004 . On the other hand, in a case where the estimated accuracy of the imaging synchronization signal has been improved compared with the current accuracy of the imaging synchronization signal, the cycle adjustment determination unit  104  determines that cycle adjustment for an imaging synchronization signal is performed, and generates a cycle adjustment execution determination result indicating a determination result. Meanwhile, the process of step S 1004  included in step S 306  in the image transfer system  13  is the same as the process of step S 1004  included in step S 306  in each of the image transfer system  1  of the first embodiment and the image transfer system  6  of the sixth embodiment. 
     Thereafter, in the image transfer system  13 , in step S 311  included in step S 306 , the imaging terminal  12100  generates a determination notification signal including information of the cycle adjustment execution determination result obtained by the cycle adjustment determination unit  104  and transmits the generated determination notification signal to the display terminal  12200 . Meanwhile, the process of step S 311  included in step S 306  in the image transfer system  13  and the determination notification signal generated in the process of step S 311  and transmitted to the display terminal  12200  are the same as the process of step S 311  included in step S 306  and the determination notification signal in each of the image transfer system  6  of the sixth embodiment, the image transfer system  7  of the seventh embodiment, and the image transfer system  12  of the twelfth embodiment. Thereby, the cycle adjustment unit  4101  provided in the display terminal  12200  acquires information of the cycle adjustment execution determination result obtained by the cycle adjustment determination unit  104  provided in the imaging terminal  12100 . 
     Further, in the image transfer system  13 , in step S 1206  included in step S 306 , the display terminal  12200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal in accordance with the cycle adjustment execution determination result transmitted from the imaging terminal  12100 , and generates a cycle adjustment instruction. Meanwhile, the process of step S 1206  included in step S 306  in the image transfer system  13  is the same as the process of step S 1206  included in step S 306  in each of the image transfer system  6  of the sixth embodiment, the image transfer system  7  of the seventh embodiment, and the image transfer system  12  of the twelfth embodiment. 
     Thereafter, in the image transfer system  13 , in step S 312 , the display terminal  12200  generates an adjustment notification signal including information of the cycle adjustment instruction including the cycle adjustment amount calculated by the cycle adjustment unit  4101 , and transmits the generated adjustment notification signal to the imaging terminal  12100 . Meanwhile, the process of step S 312  in the image transfer system  13  and the adjustment notification signal generated in the process of step S 312  and transmitted to the imaging terminal  12100  are the same as the process of step S 312  and the adjustment notification signal in each of the image transfer system  6  of the sixth embodiment, the image transfer system  7  of the seventh embodiment, and the image transfer system  12  of the twelfth embodiment. Thereby, the synchronization signal generation unit  102  provided in the imaging terminal  12100  acquires information of the cycle adjustment instruction including the cycle adjustment amount transmitted from the cycle adjustment unit  4101  provided in the display terminal  12200  through the reciprocation time measurement unit  106 . Thereby, the synchronization signal generation unit  102  performs cycle adjustment for adjusting the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  4101 . 
     Meanwhile, the phase adjustment instruction transmitted from the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  12100  is also output to the synchronization signal generation unit  102 . Thereby, the synchronization signal generation unit  102  adjusts the phase of an imaging synchronization signal to be generated, in response to the phase adjustment instruction output from the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106 . 
     In this manner, in the image transfer system  13 , the imaging terminal  12100  transmits a round-trip-propagation-time-measurement outgoing signal and calculates a round trip propagation-time in wireless transfer between the imaging terminal  12100  and the display terminal  12200 . Further, in the image transfer system  13 , the imaging terminal  12100  performs a process of updating a round trip propagation-time determination value and transmits information of the plurality of round trip propagation-time determination values updated, information of a round trip propagation-time, information of a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, information of a scheduled transmission time included in a received round-trip-propagation-time-measurement returning signal, and a phase adjustment instruction to the display terminal  12200 . Thereby, in the image transfer system  13 , the display terminal  12200  performs a process of estimating (calculating) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed, on the basis of the pieces of information transmitted from the imaging terminal  12100 . Further, in the image transfer system  13 , the display terminal  12200  transmits information of the calculated period-adjusted accuracy estimation value to the imaging terminal  12100 . Thereby, in the image transfer system  13 , the imaging terminal  12100  performs a process of determining whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  12100  is performed, on the basis of the information of the period-adjusted accuracy estimation value transmitted from the display terminal  12200 . Further, in the image transfer system  13 , the imaging terminal  12100  transmits information of a cycle adjustment execution determination result which is a determination result to the display terminal  12200 . Thereby, in the image transfer system  13 , the display terminal  12200  performs a process of calculating a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction in accordance with the information of the cycle adjustment execution determination result transmitted from the imaging terminal  12100 . Further, in the image transfer system  13 , the display terminal  12200  transmits information of the cycle adjustment instruction including the calculated cycle adjustment amount to the imaging terminal  12100 . Thereby, in the image transfer system  13 , the imaging terminal  12100  adjusts the periods of an imaging synchronization signal and a display synchronization signal so as not to be shifted with the elapse of time, in response to the cycle adjustment instruction transmitted from the display terminal  12200 . 
     Meanwhile, similarly to the image transfer system  1  of the first embodiment to the image transfer system  12  of the twelfth embodiment, also in the image transfer system  13 , when at least a process of updating a round trip propagation-time determination value is performed in step S 304 , the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  12200  can estimate the accuracy of an imaging synchronization signal (that is, calculate a period-adjusted accuracy estimation value). Therefore, similarly to the image transfer system  1  of the first embodiment to the image transfer system  12  of the twelfth embodiment, also in the image transfer system  13 , the phase adjustment unit not shown in the drawing does not necessarily need to adjust the phase of an imaging synchronization signal in step S 304  and may be able to update a round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  as a round trip propagation-time determination value. 
     With such a configuration and processing procedure, in the image transfer system  13 , the imaging terminal  12100  transmits a round-trip-propagation-time-measurement outgoing signal and measures (calculates) a round trip propagation-time in wireless transfer between the imaging terminal  12100  and the display terminal  12200  to update a round trip propagation-time determination value. Further, in the image transfer system  13 , the display terminal  12200  estimates (calculates) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed. Further, in the image transfer system  13 , the imaging terminal  12100  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value estimated (calculated) by the display terminal  12200 . Further, in the image transfer system  13 , the display terminal  12200  calculates a cycle adjustment amount for adjusting the period of an imaging synchronization signal to generate a cycle adjustment instruction in accordance with the cycle adjustment execution determination result obtained by the imaging terminal  12100 . Further, in the image transfer system  13 , the imaging terminal  12100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated, in response to the phase adjustment instruction generated by the display terminal  12200 . Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  12  of the twelfth embodiment, also in the image transfer system  13 , the phase or period (at least a period) of an imaging synchronization signal generated by the imaging terminal  12100  (more specifically, the synchronization signal generation unit  102 ) is adjusted so as to match the phase or period (at least a period) of a display synchronization signal generated by the display terminal  12200 . That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  12  of the twelfth embodiment, also in the image transfer system  13 , a timing when the imaging terminal  12100  wirelessly transfers captured image data of an image captured by an imaging unit not shown in the drawing to the display terminal  12200  is matched to a timing when the display terminal  12200  displays an image corresponding to the captured image data on a display unit not shown in the drawing. Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  12  of the twelfth embodiment, also in the image transfer system  13 , the display terminal  12200  can stably display an image corresponding to captured image data wirelessly transferred from the imaging terminal  12100  on the display unit not shown in the drawing. 
     As described above, the image transfer system  13  of the thirteenth embodiment performs transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal between the round-trip-propagation-time measurement unit  106  included in the imaging terminal  12100  and the round-trip-propagation-time-measurement assistance unit  202  included in the display terminal  12200  after wireless connection between the imaging terminal  12100  and the display terminal  12200  is established. Further, in the image transfer system  13  of the thirteenth embodiment, the round-trip-propagation-time measurement unit  106  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  12100  and the display terminal  12200 , on the basis of a transmission time of a round-trip-propagation-time-measurement outgoing signal and a reception time of a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the round-trip-propagation-time-measurement assistance unit  202 . Further, in the image transfer system  13  of the thirteenth embodiment, the round-trip-propagation-time measurement unit  106  (more specifically, the phase adjustment unit not shown in the drawing) updates the round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  as a round trip propagation-time determination value. Further, in the image transfer system  13  of the thirteenth embodiment, the round-trip-propagation-time measurement unit  106  transmits information for adjusting the period of an imaging synchronization signal in the imaging terminal  12100  to the display terminal  12200 . Further, in the image transfer system  13  of the thirteenth embodiment, the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  12200  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  13  of the thirteenth embodiment, the after-cycle-adjustment accuracy estimation unit  2105  transmits information of the estimated (calculated) period-adjusted accuracy estimation value of the imaging synchronization signal to the imaging terminal  12100 . Further, in the image transfer system  13  of the thirteenth embodiment, the cycle adjustment determination unit  104  included in the imaging terminal  12100  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is performed, on the basis of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  2105 . Further, in the image transfer system  13  of the thirteenth embodiment, the cycle adjustment determination unit  104  transmits information of a cycle adjustment execution determination result which is a result obtained by determining whether or not the period of an imaging synchronization signal is adjusted to the display terminal  12200 . Further, in the image transfer system  13  of the thirteenth embodiment, in a case where the cycle adjustment execution determination result indicates that cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  12100  is performed, the cycle adjustment unit  4101  included in the display terminal  12200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction. Further, in the image transfer system  13  of the thirteenth embodiment, the cycle adjustment unit  4101  transmits information of a cycle adjustment instruction including a cycle adjustment amount for adjusting the period of an imaging synchronization signal in the imaging terminal  12100  to the imaging terminal  12100 . Thereby, in the image transfer system  13  of the thirteenth embodiment, the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is transmitted from the cycle adjustment unit  4101 . 
     Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  12  of the twelfth embodiment, also in the image transfer system  13  of the thirteenth embodiment the phase or period (at least a period) of an imaging synchronization signal can be matched to the phase or period (at least a period) of a display synchronization signal. That is, also in the image transfer system  13  of the thirteenth embodiment, even when the periods of an imaging synchronization signal and a display synchronization signal are shifted with the elapse of time due to an error of a phase or a period between an imaging reference clock signal and a display reference clock signal, the phase or period (at least a period) of the imaging synchronization signal can be matched to the phase or period (at least a period) of the display synchronization signal. Thus, also in the image transfer system  13  of the thirteenth embodiment, the same effects as those in the image transfer system  1  of the first embodiment to the image transfer system  12  of the twelfth embodiment can be obtained. That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  12  of the twelfth embodiment, also in the image transfer system  13  of the thirteenth embodiment, it is possible to wirelessly transfer captured image data from the imaging terminal  12100  to the display terminal  12200  without exceeding a validity period of a display image and to stably display a display image corresponding to the captured image data transmitted from the imaging terminal  12100  in the display terminal  12200 . 
     Moreover, in the image transfer system  13  of the thirteenth embodiment, the display terminal  12200  estimates (calculates) a period-adjusted accuracy estimation value, calculates a cycle adjustment amount, and generates a cycle adjustment instruction. Thereby, in the image transfer system  13  of the thirteenth embodiment, the imaging terminal  12100  may only calculate a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  12100  and the display terminal  12200 , determine whether or not cycle adjustment for an imaging synchronization signal is performed, and perform cycle adjustment for an imaging synchronization signal in response to the cycle adjustment instruction. That is, in the image transfer system  13  of the thirteenth embodiment, it is not necessary to estimate (calculate) a period-adjusted accuracy estimation value, calculate a cycle adjustment amount, and generate a cycle adjustment instruction in the imaging terminal  12100 , and thus it is possible to reduce a load of processing performed in the imaging terminal  12100 . 
     Fourteenth Embodiment 
     Hereinafter, an image transfer system of a fourteenth embodiment of the present invention will be described.  FIG. 40  is a block diagram showing a schematic configuration of the image transfer system in the fourteenth embodiment of the present invention. An image transfer system  14  includes an imaging terminal  13100  and a display terminal  13200 . The imaging terminal  13100  includes a synchronization signal generation unit  102 , a source oscillation clock generation unit  103 , a wireless communication unit  108 , a round-trip-propagation-time measurement unit  106 , a clocking unit  107 , and an antenna  120 . In addition, the display terminal  13200  includes a synchronous signal generation unit  204 , a source oscillation clock generation unit  205 , a wireless communication unit  201 , a round-trip-propagation-time-measurement assistance unit  202 , a clocking unit  203 , an after-cycle-adjustment accuracy estimation unit  2105 , a cycle-adjustment determination unit  3104 , a cycle adjustment unit  4101 , and an antenna  220 . 
     Meanwhile, also in  FIG. 40 , in components of the image transfer system  14 , the same components as the components included in the image transfer system  1  of the first embodiment to the image transfer system  13  of the thirteenth embodiment are denoted by the same reference numerals and signs. 
     Similarly to the image transfer system  1  of the first embodiment to the image transfer system  13  of the thirteenth embodiment, the image transfer system  14  is also an image transfer system configured such that transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between the imaging terminal  13100  and the display terminal  13200 , and the imaging terminal  13100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  13200 . Meanwhile, similarly to the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  13  of the thirteenth embodiment, the image transfer system  14  is an image transfer system in which the imaging terminal  13100  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  13200  to calculate a round trip propagation-time in wireless transfer between the imaging terminal  13100  and the display terminal  13200 , and the imaging terminal  13100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated. 
     However, in the image transfer system  14 , some components included in the imaging terminal  12100  in the image transfer system  13  of the thirteenth embodiment are moved to the display terminal  13200 . More specifically, in the image transfer system  14 , the cycle-adjustment determination unit  3104  that replaces the cycle adjustment determination unit  104  included in the imaging terminal  12100  in the image transfer system  13  of the thirteenth embodiment is included in the display terminal  13200 . 
     For this reason, in the image transfer system  14 , the imaging terminal  13100  measures (calculates) a round trip propagation-time required for transmission and reception when wireless transfer is performed between the imaging terminal  13100  and the display terminal  13200 , and transmits the measured round trip propagation-time to the display terminal  13200 . Further, in the image transfer system  14 , the display terminal  13200  estimates (calculates) a period-adjusted accuracy estimation value in a case where the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  13100  is temporarily adjusted, determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value, calculates a cycle adjustment amount in a case where it is determined that cycle adjustment for an imaging synchronization signal is performed, and transmits the calculated cycle adjustment amount to the imaging terminal  13100  together with a cycle adjustment instruction. Further, in the image transfer system  14 , the imaging terminal  13100  adjusts the period of an imaging synchronization signal to be generated, in response to the cycle adjustment instruction transmitted from the display terminal  13200 . 
     However, also in the image transfer system  14 , although there is a process of exchanging signals through wireless transfer in association with a change in the disposition of components, functions and operations of the components are the same as the functions and operations of the corresponding components in the image transfer system  1  of the first embodiment to the image transfer system  13  of the thirteenth embodiment. That is, the functions and operations of the components included in the imaging terminal  13100  and the display terminal  13200  in the image transfer system  14  can be easily understood from the above description of the components included in the imaging terminal and the display terminal in each of the image transfer system  1  of the first embodiment to the image transfer system  13  of the thirteenth embodiment. Therefore, a detailed description related to the components included in the image transfer system  14  will be omitted. 
     Next, operations of processing in the image transfer system  14  will be described. Meanwhile, similarly to the image transfer system  1  of the first embodiment, in the image transfer system  14 , it is assumed that a phase adjustment unit not shown in the drawing is provided in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  13100 . That is, in the image transfer system  14 , a configuration in which the phase adjustment unit not shown in the drawing is disposed at the same position as those in the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  13  of the thirteenth embodiment is adopted.  FIG. 41  is a flowchart showing a processing procedure of the image transfer system  14  in the fourteenth embodiment of the present invention. 
     In the image transfer system  14 , a process of transmitting information used to adjust the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  to the display terminal  13200  and a process of transmitting a cycle adjustment instruction including a cycle adjustment amount to the imaging terminal  13100  are added, in association with a change to a configuration in which the after-cycle-adjustment accuracy estimation unit  2105 , the cycle-adjustment determination unit  3104 , and the cycle adjustment unit  4101  are included in the display terminal  13200 . However, an outline of the overall operation in the image transfer system  14  is the same as those of the image transfer system  1  of the first embodiment to the image transfer system  13  of the thirteenth embodiment. Therefore, also in the image transfer system  14 , a description of an outline of the overall operation will be omitted, and a more specific operation of performing cycle adjustment for an imaging synchronization signal in the image transfer system  14  will be described. 
     Also in the image transfer system  14 , when a cycle adjustment process is started, the imaging terminal  13100  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  13200 , and the display terminal  13200  transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal to the imaging terminal  13100  in step S 302 . Meanwhile, the process of step S 302  in the image transfer system  14  is the same as the process of step S 302  in each of the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  13  of the thirteenth embodiment. 
     Thereafter, also in the image transfer system  14 , in step S 303 , the imaging terminal  13100  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  13100  and the display terminal  13200 . Meanwhile, the process of step S 303  in the image transfer system  14  is also the same as the process of step S 303  in each of the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  13  of the thirteenth embodiment. 
     Thereafter, also in the image transfer system  14 , in step S 304 , the imaging terminal  13100  generates a phase adjustment instruction for the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  to adjust the phase of an imaging synchronization signal, on the basis of the round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  13100  and the display terminal  13200  which is calculated in step S 303 , and updates a round trip propagation-time determination value. Meanwhile, the process of step S 304  in the image transfer system  14  is also the same as the process of step S 304  in each of the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  13  of the thirteenth embodiment. 
     Thereafter, also in the image transfer system  14 , in step S 307 , the imaging terminal  13100  generates a measurement notification signal including information of the calculated round trip propagation-time and transmits the generated measurement notification signal to the display terminal  13200 . Meanwhile, the process of step S 307  in the image transfer system  14  and the measurement notification signal generated in the process of step S 307  and transmitted to the display terminal  13200  are the same as the process of step S 307  and the measurement notification signal in the image transfer system  13  of the thirteenth embodiment. Thereby, the display terminal  13200  acquires information of the round trip propagation-time determination value transmitted from the imaging terminal  13100 . In addition, each of the after-cycle-adjustment accuracy estimation unit  2105 , the cycle-adjustment determination unit  3104 , and the cycle adjustment unit  4101  which are included in the display terminal  13200  acquires the pieces of information transmitted from the imaging terminal  13100  through the round-trip-propagation-time-measurement assistance unit  202 . 
     Thereafter, in the image transfer system  14 , in step S 306 , the display terminal  13200  adjusts the period of an imaging synchronization signal on the basis of information transmitted from the imaging terminal  13100 , that is, information of the plurality of round trip propagation-time determination values updated in step S 304  by the imaging terminal  13100 , similar to the processes in step S 306  in the image transfer system  1  of the first embodiment. More specifically, the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  13200  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed on the basis of the information of the plurality of round trip propagation-time determination values transmitted from the imaging terminal  13100  and outputs the estimated period-adjusted accuracy estimation value to the cycle-adjustment determination unit  3104  (step S 2106 ). In addition, the cycle-adjustment determination unit  3104  included in the display terminal  13200  determines whether or not cycle adjustment for an imaging synchronization signal is performed, on the basis of the period-adjusted accuracy estimation value which is estimated (calculated) by the after-cycle-adjustment accuracy estimation unit  2105  (step S 1004 ). Further, in a case where the cycle-adjustment determination unit  3104  determines that cycle adjustment for an imaging synchronization signal is performed, the cycle adjustment unit  4101  included in the display terminal  13200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  13100  and generates a cycle adjustment instruction (step S 1206 ). 
     Thereafter, in the image transfer system  14 , in step S 312 , the display terminal  13200  generates an adjustment notification signal including information of the cycle adjustment instruction including the cycle adjustment amount calculated by the cycle adjustment unit  4101  and transmits the generated adjustment notification signal to the imaging terminal  13100 . Meanwhile, the process of step S 312  in the image transfer system  14  and the adjustment notification signal generated in the process of step S 312  and transmitted to the imaging terminal  13100  are the same as the process of step S 312  and the adjustment notification signal in the image transfer system  5  of the fifth embodiment to the image transfer system  8  of the eighth embodiment, and the image transfer system  11  of the eleventh embodiment to the image transfer system  13  of the thirteenth embodiment. Thereby, the synchronization signal generation unit  102  provided in the imaging terminal  13100  acquires information of the cycle adjustment instruction including the cycle adjustment amount transmitted from the cycle adjustment unit  4101  provided in the display terminal  13200  through the round-trip-propagation-time measurement unit  106 . Thereby, the synchronization signal generation unit  102  performs cycle adjustment for adjusting the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  4101 . 
     Meanwhile, the phase adjustment instruction transmitted from the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  13100  is also output to the synchronization signal generation unit  102 . Thereby, the synchronization signal generation unit  102  adjusts the phase of an imaging synchronization signal to be generated, in response to the phase adjustment instruction output from the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106 . 
     In this manner, in the image transfer system  14 , the imaging terminal  13100  transmits a round-trip-propagation-time-measurement outgoing signal and calculates a round trip propagation-time in wireless transfer between the imaging terminal  13100  and the display terminal  13200 . Further, in the image transfer system  14 , the imaging terminal  13100  performs a process of updating a round trip propagation-time determination value and transmits information of a plurality of round trip propagation-time determination values updated, information of a round trip propagation-time, information of a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, information of a scheduled transmission time included in a received round-trip-propagation-time-measurement returning signal, and a phase adjustment instruction to the display terminal  13200 . Thereby, in the image transfer system  14 , the display terminal  13200  performs a process of estimating (calculating) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed, a process of determining whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  13100  is performed, and a process of calculating a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction, on the basis of the pieces of information transmitted from the imaging terminal  13100 . Further, in the image transfer system  14 , the display terminal  13200  transmits information of the cycle adjustment instruction, including the calculated cycle adjustment amount, to the imaging terminal  13100 . Thereby, in the image transfer system  14 , the imaging terminal  13100  adjusts the periods of an imaging synchronization signal and a display synchronization signal so as not to be shifted with the elapse of time, in response to the cycle adjustment instruction transmitted from the display terminal  13200 . 
     Meanwhile, similarly to the image transfer system  1  of the first embodiment to the image transfer system  13  of the thirteenth embodiment, also in the image transfer system  14 , when at least a process of updating a round trip propagation-time determination value is performed in step S 304 , the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  13200  can estimate the accuracy of an imaging synchronization signal (that is, calculate a period-adjusted accuracy estimation value). Therefore, similarly to the image transfer system  1  of the first embodiment to the image transfer system  13  of the thirteenth embodiment, also in the image transfer system  14 , the phase adjustment unit not shown in the drawing does not necessarily need to adjust the phase of an imaging synchronization signal in step S 304  and may be able to update a round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  as a round trip propagation-time determination value. 
     With such a configuration and processing procedure, in the image transfer system  14 , the imaging terminal  13100  transmits a round-trip-propagation-time-measurement outgoing signal and measures (calculates) a round trip propagation-time in wireless transfer between the imaging terminal  13100  and the display terminal  13200  to update a round trip propagation-time determination value. Further, in the image transfer system  14 , the display terminal  13200  estimates (calculates) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed, determines whether or not cycle adjustment for an imaging synchronization signal is performed in the imaging terminal  13100 , and calculates a cycle adjustment amount for adjusting the period of an imaging synchronization signal to generate a cycle adjustment instruction. Further, in the image transfer system  14 , the imaging terminal  13100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated, in response to the phase adjustment instruction generated by the display terminal  13200 . Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  13  of the thirteenth embodiment, also in the image transfer system  14 , the phase or period (at least a period) of an imaging synchronization signal which is generated by the imaging terminal  13100  (more specifically, the synchronization signal generation unit  102 ) is adjusted so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  13200 . That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  13  of the thirteenth embodiment, also in the image transfer system  14 , a timing when the imaging terminal  13100  wirelessly transfers captured image data of an image captured by an imaging unit not shown in the drawing to the display terminal  13200  is matched to a timing when the display terminal  13200  displays an image corresponding to the captured image data on a display unit not shown in the drawing. Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  13  of the thirteenth embodiment, also in the image transfer system  14 , the display terminal  13200  can stably display an image corresponding to captured image data wirelessly transferred from the imaging terminal  13100  on the display unit not shown in the drawing. 
     As described above, the image transfer system  14  of the fourteenth embodiment performs transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal between the round-trip-propagation-time measurement unit  106  included in the imaging terminal  13100  and the round-trip-propagation-time-measurement assistance unit  202  included in the display terminal  13200  after wireless connection between the imaging terminal  13100  and the display terminal  13200  is established. Further, in the image transfer system  14  of the fourteenth embodiment, the round-trip-propagation-time measurement unit  106  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  13100  and the display terminal  13200 , on the basis of a transmission time of a round-trip-propagation-time-measurement outgoing signal and a reception time of a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the round-trip-propagation-time-measurement assistance unit  202 . Further, in the image transfer system  14  of the fourteenth embodiment, the round-trip-propagation-time measurement unit  106  (more specifically, the phase adjustment unit not shown in the drawing) updates the round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  as a round trip propagation-time determination value. Further, in the image transfer system  14  of the fourteenth embodiment, the round-trip-propagation-time measurement unit  106  transmits information for adjusting the period of an imaging synchronization signal in the imaging terminal  13100  to the display terminal  13200 . Further, in the image transfer system  14  of the fourteenth embodiment, the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  13200  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  14  of the fourteenth embodiment, the cycle-adjustment determination unit  3104  included in the imaging terminal  13100  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  13100  is performed, on the basis of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  2105 . Further, in the image transfer system  14  of the fourteenth embodiment, in a case where a cycle adjustment execution determination result indicates that cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  13100  is performed, the cycle adjustment unit  4101  included in the display terminal  13200  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal to generate a cycle adjustment instruction. Further, in the image transfer system  14  of the fourteenth embodiment, the cycle adjustment unit  4101  transmits information of a cycle adjustment instruction including a cycle adjustment amount for adjusting the period of an imaging synchronization signal in the imaging terminal  13100  to the imaging terminal  13100 . Thereby, in the image transfer system  14  of the fourteenth embodiment, the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is transmitted from the cycle adjustment unit  4101 . 
     Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  13  of the thirteenth embodiment, also in the image transfer system  14  of the fourteenth embodiment, the phase or period (at least a period) of an imaging synchronization signal can be matched to the phase or period (at least a period) of a display synchronization signal. That is, also in the image transfer system  14  of the fourteenth embodiment, even when the periods of an imaging synchronization signal and a display synchronization signal are shifted with the elapse of time due to an error of a phase or a period between an imaging reference clock signal and a display reference clock signal, the phase or period (at least a period) of the imaging synchronization signal can be matched to the phase or period (at least a period) of the display synchronization signal. Thus, also in the image transfer system  14  of the fourteenth embodiment, the same effects as those in the image transfer system  1  of the first embodiment to the image transfer system  13  of the thirteenth embodiment can be obtained. That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  13  of the thirteenth embodiment, also in the image transfer system  14  of the fourteenth embodiment, it is possible to wirelessly transfer captured image data from the imaging terminal  13100  to the display terminal  13200  without exceeding a validity period of a display image and to stably display a display image corresponding to the captured image data transmitted from the imaging terminal  13100  in the display terminal  13200 . 
     Moreover, in the image transfer system  14  of the fourteenth embodiment, the display terminal  13200  estimates (calculates) a period-adjusted accuracy estimation value, determines whether or not cycle adjustment for an imaging synchronization signal is performed, calculates a cycle adjustment amount, and generates a cycle adjustment instruction. Thereby, in the image transfer system  14  of the fourteenth embodiment, the imaging terminal  13100  may only calculate a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  13100  and the display terminal  13200  and perform cycle adjustment for an imaging synchronization signal in response to the cycle adjustment instruction. That is, in the image transfer system  14  of the fourteenth embodiment, it is not necessary to estimate (calculate) a period-adjusted accuracy estimation value, determine whether or not cycle adjustment for an imaging synchronization signal is performed, calculate a cycle adjustment amount, and generate a cycle adjustment instruction in the imaging terminal  13100 , and thus it is possible to reduce a load of processing performed in the imaging terminal  13100 . 
     Fifteenth Embodiment 
     Hereinafter, an image transfer system of a fifteenth embodiment of the present invention will be described.  FIG. 42  is a block diagram showing a schematic configuration of the image transfer system in the fifteenth embodiment of the present invention. An image transfer system  15  includes an imaging terminal  14100  and a display terminal  14200 . The imaging terminal  14100  includes a synchronization signal generation unit  102 , a cycle adjustment unit  101 , a source oscillation clock generation unit  103 , a wireless communication unit  108 , a round-trip-propagation-time measurement unit  106 , a clocking unit  107 , and an antenna  120 . In addition, the display terminal  14200  includes a synchronous signal generation unit  204 , a source oscillation clock generation unit  205 , a wireless communication unit  201 , a round-trip-propagation-time-measurement assistance unit  202 , a clocking unit  203 , an after-cycle-adjustment accuracy estimation unit  2105 , a cycle-adjustment determination unit  3104 , and an antenna  220 . 
     Meanwhile, also in  FIG. 42 , in components of the image transfer system  15 , the same components as the components included in the image transfer system  1  of the first embodiment to the image transfer system  14  of the fourteenth embodiment are denoted by the same reference numerals and signs. 
     Similarly to the image transfer system  1  of the first embodiment to the image transfer system  14  of the fourteenth embodiment, the image transfer system  15  is also an image transfer system configured such that transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between the imaging terminal  14100  and the display terminal  14200 , and the imaging terminal  14100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  14200 . Meanwhile, similarly to the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  14  of the fourteenth embodiment, the image transfer system  15  is an image transfer system in which the imaging terminal  14100  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  14200  to calculate a round trip propagation-time in wireless transfer between the imaging terminal  14100  and the display terminal  14200 , and the imaging terminal  14100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated. 
     However, in the image transfer system  15 , some components included in the imaging terminal  9100  in the image transfer system  10  of the tenth embodiment are moved to the display terminal  14200 . More specifically, in the image transfer system  15 , the after-cycle-adjustment accuracy estimation unit  2105  that replaces the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  9100  in the image transfer system  10  of the tenth embodiment is included in the display terminal  14200 . Meanwhile, it can be said that a configuration of the image transfer system  15  is a configuration in which the cycle adjustment unit  4101  included in the display terminal  13200  in the image transfer system  14  of the fourteenth embodiment is returned to the imaging terminal  14100  as the cycle adjustment unit  101 . 
     For this reason, in the image transfer system  15 , the imaging terminal  14100  measures (calculates) a reciprocation time required for transmission and reception at the time of performing wireless transfer between the imaging terminal  14100  and the display terminal  14200  and transmits the measured round trip propagation-time to the display terminal  14200 . Further, in the image transfer system  15 , the display terminal  14200  estimates (calculates) a period-adjusted accuracy estimation value in a case where the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  14100  is temporarily adjusted, and transmits a cycle adjustment execution determination result indicating a result obtained by determining whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value to the imaging terminal  14100 . Further, in the image transfer system  15 , in a case where the cycle adjustment execution determination result transmitted from the display terminal  14200  indicates that cycle adjustment for an imaging synchronization signal is performed, the imaging terminal  14100  adjusts the period of an imaging synchronization signal to be generated. 
     However, also in the image transfer system  15 , although there is a process of exchanging signals through wireless transfer in association with a change in the disposition of components, functions and operations of the components are the same as the functions and operations of the corresponding components in the image transfer system  1  of the first embodiment to the image transfer system  14  of the fourteenth embodiment. That is, the functions and operations of the components included in the imaging terminal  14100  and the display terminal  14200  in the image transfer system  15  can be easily understood from the above description of the components included in the imaging terminal and the display terminal in each of the image transfer system  1  of the first embodiment to the image transfer system  14  of the fourteenth embodiment. Therefore, a detailed description related to the components included in the image transfer system  15  will be omitted. 
     Next, operations of processing in the image transfer system  15  will be described. Meanwhile, similarly to the image transfer system  1  of the first embodiment, in the image transfer system  15 , it is assumed that a phase adjustment unit not shown in the drawing is included in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  14100 . That is, in the image transfer system  15 , a configuration in which the phase adjustment unit not shown in the drawing is disposed at the same position as those in the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  14  of the fourteenth embodiment is adopted.  FIG. 43  is a flowchart showing a processing procedure of the image transfer system  15  in the fifteenth embodiment of the present invention. 
     In the image transfer system  15 , a process of transmitting information used to adjust the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  to the display terminal  14200  and a process of transmitting information of a period-adjusted accuracy estimation value and information of a cycle adjustment execution determination result to the imaging terminal  14100  are added, in association with a change to a configuration in which the after-cycle-adjustment accuracy estimation unit  2105  and the cycle-adjustment determination unit  3104  are included in the display terminal  14200 . However, an outline of the overall operation in the image transfer system  15  is the same as those of the image transfer system  1  of the first embodiment to the image transfer system  14  of the fourteenth embodiment. Therefore, also in the image transfer system  15 , a description of an outline of the overall operation will be omitted, and a more specific operation of performing cycle adjustment for an imaging synchronization signal in the image transfer system  15  will be described. 
     Also in the image transfer system  15 , when a cycle adjustment process is started, the imaging terminal  14100  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  14200 , and the display terminal  14200  transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal to the imaging terminal  14100  in step S 302 . Meanwhile, the process of step S 302  in the image transfer system  15  is the same as the process of step S 302  in each of the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  14  of the fourteenth embodiment. 
     Thereafter, also in the image transfer system  15 , in step S 303 , the imaging terminal  14100  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  14100  and the display terminal  14200 . Meanwhile, the process of step S 303  in the image transfer system  15  is also the same as the process of step S 303  in each of the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  14  of the fourteenth embodiment. 
     Thereafter, also in the image transfer system  15 , in step S 304 , the imaging terminal  14100  generates a phase adjustment instruction for the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  to adjust the phase of an imaging synchronization signal, on the basis of the round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  14100  and the display terminal  14200  which is calculated in step S 303 , and updates a round trip propagation-time determination value. Meanwhile, the process of step S 304  in the image transfer system  15  is also the same as the process of step S 304  in each of the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  14  of the fourteenth embodiment. 
     Thereafter, also in the image transfer system  15 , in step S 307 , the imaging terminal  14100  generates a measurement notification signal including information of the calculated round trip propagation-time and transmits the generated measurement notification signal to the display terminal  14200 . Meanwhile, the process of step S 307  in the image transfer system  15  and the measurement notification signal generated in the process of step S 307  and transmitted to the display terminal  13200  are the same as the process of step S 307  and the measurement notification signal in the image transfer system  13  of the thirteenth embodiment and the image transfer system  14  of the fourteenth embodiment. Thereby, each of the after-cycle-adjustment accuracy estimation unit  2105  and the cycle-adjustment determination unit  3104  which are included in the display terminal  14200  acquires the pieces of information transmitted from the imaging terminal  14100  through the round-trip-propagation-time-measurement assistance unit  202 . 
     Thereafter, in the image transfer system  15 , in step S 306 , the period of an imaging synchronization signal is adjusted on the basis of the information of the plurality of round trip propagation-time determination values updated in step S 304 . However, in the image transfer system  15 , the display terminal  14200  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed, and determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the estimated (calculated) period-adjusted accuracy estimation value. Further, in the image transfer system  15 , the imaging terminal  14100  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal and outputs a cycle adjustment instruction to the synchronization signal generation unit  102 . 
     For this reason, in the image transfer system  15 , in step S 305  included in step S 306 , the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  14200  determines whether or not a predetermined period of time determined in advance has elapsed after the period of a previous imaging synchronization signal is adjusted. In a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has not elapsed after the period of the previous imaging synchronization signal is adjusted (“NO” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  2105  determines that cycle adjustment is not performed and terminates the process of step S 305 . 
     On the other hand, in a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has elapsed after the period of the previous imaging synchronization signal is adjusted (“YES” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  2105  determines that cycle adjustment is performed and calculates a period-adjusted accuracy estimation value in step S 2106  included in step S 306 . Meanwhile, the process of step S 305  and the process of step S 2106  which are included in step S 306  in the image transfer system  15  are the same as the process of step S 305  and the process of step S 2106  which are included in step S 306  in each of the image transfer system  3  of the third embodiment, the image transfer system  4  of the fourth embodiment, the image transfer system  6  of the sixth embodiment, and the image transfer system  13  of the thirteenth embodiment. 
     Further, in the image transfer system  15 , in step S 1004  included in step S 306 , the cycle-adjustment determination unit  3104  included in the display terminal  14200  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  14100  is performed, on the basis of the calculated period-adjusted accuracy estimation value. Further, in a case where the cycle-adjustment determination unit  3104  determines that cycle adjustment for an imaging synchronization signal is performed because an estimated accuracy of the imaging synchronization signal has been improved compared with the current accuracy of the imaging synchronization signal, the cycle adjustment execution determination unit generates a cycle adjustment execution determination result indicating a determination result. Meanwhile, the process of step S 1004  included in step S 306  in the image transfer system  15  is the same as the process of step S 1004  included in step S 306  in the image transfer system  4  of the fourth embodiment. 
     Thereafter, in the image transfer system  15 , in step S 311  included in step S 306 , the display terminal  14200  generates a determination notification signal including information of the cycle adjustment execution determination result obtained by the cycle-adjustment determination unit  3104  and transmits the generated determination notification signal to the display terminal  14200 . Meanwhile, the process of step S 311  included in step S 306  in the image transfer system  15  and the determination notification signal generated in the process of step S 311  and transmitted to the imaging terminal  14100  are the same as the process of step S 311  included in step S 306  and the determination notification signal in each of the image transfer system  4  of the fourth embodiment, the image transfer system  9  of the ninth embodiment, and the image transfer system  10  of the tenth embodiment. Thereby, the cycle adjustment unit  101  provided in the imaging terminal  14100  acquires information of the cycle adjustment execution determination result obtained by the cycle-adjustment determination unit  3104  provided in the display terminal  14200  through the round-trip-propagation-time measurement unit  106 . 
     Further, in the image transfer system  15 , in step S 1206  included in step S 306 , the imaging terminal  14100  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal on the basis of information such as a round trip propagation-time determination value transmitted from the round-trip-propagation-time measurement unit  106  in accordance with the cycle adjustment execution determination result transmitted from the display terminal  14200 , and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, the synchronization signal generation unit  102  performs cycle adjustment for adjusting the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  101 . Meanwhile, the process of step S 1206  included in step S 306  in the image transfer system  15  is the same as the process of step S 1206  included in step S 306  in each of the image transfer system  4  of the fourth embodiment, the image transfer system  9  of the ninth embodiment, and the image transfer system  10  of the tenth embodiment. 
     Meanwhile, the phase adjustment instruction transmitted from the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  14100  is also output to the synchronization signal generation unit  102 . Thereby, the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated, in response to the phase adjustment instruction transmitted from the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106 . 
     In this manner, in the image transfer system  15 , the imaging terminal  14100  transmits a round-trip-propagation-time-measurement outgoing signal and calculates a round trip propagation-time in wireless transfer between the imaging terminal  14100  and the display terminal  14200 . Further, in the image transfer system  15 , the imaging terminal  14100  performs a process of updating a round trip propagation-time determination value and transmits information of a plurality of round trip propagation-time determination values updated, information of a round trip propagation-time, information of a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, information of a scheduled transmission time included in a received round-trip-propagation-time-measurement returning signal, and a phase adjustment instruction to the display terminal  14200 . Thereby, in the image transfer system  15 , the display terminal  14200  performs a process of estimating (calculating) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed and performs a process of determining whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  14100 , on the basis of the pieces of information transmitted from the imaging terminal  14100 . Further, in the image transfer system  15 , the display terminal  14200  transmits information of the calculated period-adjusted accuracy estimation value and information of a cycle adjustment execution determination result which is a determination result to the imaging terminal  14100 . Thereby, in the image transfer system  15 , the imaging terminal  14100  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal in accordance with the information of the cycle adjustment execution determination result transmitted from the display terminal  14200 , and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, in the image transfer system  15 , the imaging terminal  14100  adjusts the periods of an imaging synchronization signal and a display synchronization signal so as not to be shifted with the elapse of time. 
     Meanwhile, similarly to the image transfer system  1  of the first embodiment to the image transfer system  14  of the fourteenth embodiment, also in the image transfer system  15 , when at least a process of updating a round trip propagation-time determination value is performed in step S 304 , the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  14200  can estimate the accuracy of an imaging synchronization signal (that is, calculate a period-adjusted accuracy estimation value). Therefore, similarly to the image transfer system  1  of the first embodiment to the image transfer system  14  of the fourteenth embodiment, also in the image transfer system  15 , the phase adjustment unit not shown in the drawing does not necessarily need to adjust the phase of an imaging synchronization signal in step S 304  and may be able to update a round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  as a round trip propagation-time determination value. 
     With such a configuration and processing procedure, in the image transfer system  15 , the imaging terminal  14100  transmits a round-trip-propagation-time-measurement outgoing signal and measures (calculates) a round trip propagation-time in wireless transfer between the imaging terminal  14100  and the display terminal  14200  to update a round trip propagation-time determination value. Further, in the image transfer system  15 , the display terminal  14200  estimates (calculates) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed, and determines whether or not cycle adjustment for an imaging synchronization signal is performed in the imaging terminal  14100 . Further, in the image transfer system  15 , the imaging terminal  14100  calculates a cycle adjustment amount for adjusting the period of an imaging synchronization signal in accordance with the cycle adjustment execution determination result obtained by the display terminal  14200 , outputs a cycle adjustment instruction, and adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated. Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  14  of the fourteenth embodiment, also in the image transfer system  15 , the phase or period (at least a period) of an imaging synchronization signal generated by the imaging terminal  14100  (more specifically, the synchronization signal generation unit  102 ) is adjusted so as to match the phase or period (at least a period) of a display synchronization signal generated by the display terminal  14200 . That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  14  of the fourteenth embodiment, also in the image transfer system  15 , a timing when the imaging terminal  14100  wirelessly transfers captured image data of an image captured by an imaging unit not shown in the drawing to the display terminal  14200  is matched to a timing when the display terminal  14200  displays an image corresponding to the captured image data on a display unit not shown in the drawing. Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  14  of the fourteenth embodiment, also in the image transfer system  15 , the display terminal  14200  can stably display an image corresponding to captured image data wirelessly transferred from the imaging terminal  14100  on the display unit not shown in the drawing. 
     As described above, the image transfer system  15  of the fifteenth embodiment performs transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal between the round-trip-propagation-time measurement unit  106  included in the imaging terminal  14100  and the round-trip-propagation-time-measurement assistance unit  202  included in the display terminal  14200  after wireless connection between the imaging terminal  14100  and the display terminal  14200  is established. Further, in the image transfer system  15  of the fifteenth embodiment, the round-trip-propagation-time measurement unit  106  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  14100  and the display terminal  14200 , on the basis of a transmission time of a round-trip-propagation-time-measurement outgoing signal and a reception time of a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the round-trip-propagation-time-measurement assistance unit  202 . Further, in the image transfer system  15  of the fifteenth embodiment, the round-trip-propagation-time measurement unit  106  (more specifically, the phase adjustment unit not shown in the drawing) updates the round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  as a round trip propagation-time determination value. Further, in the image transfer system  15  of the fifteenth embodiment, the round-trip-propagation-time measurement unit  106  transmits information for adjusting the period of an imaging synchronization signal in the imaging terminal  14100  to the display terminal  14200 . Further, in the image transfer system  15  of the fifteenth embodiment, the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  14200  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  15  of the fifteenth embodiment, the cycle-adjustment determination unit  3104  included in the display terminal  14200  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  14100  is performed, on the basis of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  2105 . Further, in the image transfer system  15  of the fifteenth embodiment, the cycle-adjustment determination unit  3104  transmits information for adjusting the period of an imaging synchronization signal in the imaging terminal  14100  to the imaging terminal  14100 , inclusive of a cycle adjustment execution determination result and a period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  2105 . Further, in the image transfer system  15  of the fifteenth embodiment, the cycle adjustment unit  101  included in the imaging terminal  14100  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal in a case where the cycle adjustment execution determination result indicates that cycle adjustment for an imaging synchronization signal is performed, and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, in the image transfer system  15  of the fifteenth embodiment, the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  101 . 
     Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  14  of the fourteenth embodiment, also in the image transfer system  15  of the fifteenth embodiment, the phase or period (at least a period) of an imaging synchronization signal can be matched to the phase or period (at least a period) of a display synchronization signal. That is, also in the image transfer system  15  of the fifteenth embodiment, even when the periods of an imaging synchronization signal and a display synchronization signal are shifted with the elapse of time due to an error of a phase or a period between an imaging reference clock signal and a display reference clock signal, the phase or period (at least a period) of the imaging synchronization signal can be matched to the phase or period (at least a period) of the display synchronization signal. Thus, also in the image transfer system  15  of the fifteenth embodiment, the same effects as those in the image transfer system  1  of the first embodiment to the image transfer system  14  of the fourteenth embodiment can be obtained. That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  14  of the fourteenth embodiment, also in the image transfer system  15  of the fifteenth embodiment, it is possible to wirelessly transfer captured image data from the imaging terminal  14100  to the display terminal  14200  without exceeding a validity period of a display image and to stably display a display image corresponding to the captured image data transmitted from the imaging terminal  14100  in the display terminal  14200 . 
     Moreover, in the image transfer system  15  of the fifteenth embodiment, the display terminal  14200  estimates (calculates) a period-adjusted accuracy estimation value and determines whether or not cycle adjustment for an imaging synchronization signal is performed. Thereby, in the image transfer system  15  of the fifteenth embodiment, the imaging terminal  14100  may only calculate a roundtrip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  14100  and the display terminal  14200 , calculate a cycle adjustment amount, and generate a cycle adjustment instruction. That is, in the image transfer system  15  of the fifteenth embodiment, it is not necessary to estimate (calculate) a period-adjusted accuracy estimation value and determine whether or not cycle adjustment for an imaging synchronization signal is performed in the imaging terminal  14100 , and thus it is possible to reduce a load of processing performed in the imaging terminal  14100 . 
     Sixteenth Embodiment 
     Hereinafter, an image transfer system of a sixteenth embodiment of the present invention will be described.  FIG. 44  is a block diagram showing a schematic configuration of the image transfer system in the sixteenth embodiment of the present invention. An image transfer system  16  includes an imaging terminal  15100  and a display terminal  15200 . The imaging terminal  15100  includes a synchronization signal generation unit  102 , a cycle adjustment unit  101 , a source oscillation clock generation unit  103 , a wireless communication unit  108 , a cycle adjustment determination unit  104 , a round-trip-propagation-time measurement unit  106 , a clocking unit  107 , and an antenna  120 . In addition, the display terminal  15200  includes a synchronous signal generation unit  204 , a source oscillation clock generation unit  205 , a wireless communication unit  201 , a round-trip-propagation-time-measurement assistance unit  202 , a clocking unit  203 , an after-cycle-adjustment accuracy estimation unit  2105 , and an antenna  220 . 
     Meanwhile, also in  FIG. 44 , in components of the image transfer system  16 , the same components as the components included in the image transfer system  1  of the first embodiment to the image transfer system  15  of the fifteenth embodiment are denoted by the same reference numerals and signs. 
     Similarly to the image transfer system  1  of the first embodiment to the image transfer system  15  of the fifteenth embodiment, the image transfer system  16  is also an image transfer system configured such that transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are performed between the imaging terminal  15100  and the display terminal  15200 , and the imaging terminal  15100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated so as to match the phase or period (at least a period) of a display synchronization signal which is generated by the display terminal  15200 . Meanwhile, similarly to the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  15  of the fifteenth embodiment, the image transfer system  16  is an image transfer system in which the imaging terminal  15100  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  15200  to calculate a round trip propagation-time in wireless transfer between the imaging terminal  15100  and the display terminal  15200 , and the imaging terminal  15100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated. 
     However, in the image transfer system  16 , some components included in the imaging terminal  100  in the image transfer system  1  of the first embodiment are moved to the display terminal  15200 . More specifically, in the image transfer system  16 , the after-cycle-adjustment accuracy estimation unit  2105  that replaces the after-cycle-adjustment accuracy estimation unit  105  included in the imaging terminal  100  in the image transfer system  1  of the first embodiment is included in the display terminal  15200 . Meanwhile, it can be said that a configuration of the image transfer system  16  is a configuration in which the cycle-adjustment determination unit  3104  included in the display terminal  14200  in the image transfer system  15  of the fifteenth embodiment is returned to the imaging terminal  15100  as the cycle adjustment determination unit  104 . 
     For this reason, in the image transfer system  16 , the imaging terminal  15100  measures (calculates) a round trip propagation-time required for transmission and reception at the time of performing wireless transfer between the imaging terminal  15100  and the display terminal  15200  and transmits the measured round trip propagation-time to the display terminal  15200 . Further, in the image transfer system  16 , the display terminal  15200  estimates (calculates) a period-adjusted accuracy estimation value in a case where the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  15100  is temporarily adjusted, and transmits the estimated period-adjusted accuracy estimation value to the imaging terminal  15100 . Further, in the image transfer system  16 , the imaging terminal  15100  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  is performed on the basis of the period-adjusted accuracy estimation value transmitted from the display terminal  15200 , and adjusts the period of an imaging synchronization signal to be generated. 
     However, also in the image transfer system  16 , although there is a process of exchanging signals through wireless transfer in association with a change in the disposition of components, functions and operations of the components are the same as the functions and operations of the corresponding components in the image transfer system  1  of the first embodiment to the image transfer system  15  of the fifteenth embodiment. That is, the functions and operations of the components included in the imaging terminal  15100  and the display terminal  15200  in the image transfer system  16  can be easily understood from the above description of the components included in the imaging terminal and the display terminal in each of the image transfer system  1  of the first embodiment to the image transfer system  15  of the fifteenth embodiment. Therefore, a detailed description related to the components included in the image transfer system  16  will be omitted. 
     Next, operations of processing in the image transfer system  16  will be described. Meanwhile, similarly to the image transfer system  1  of the first embodiment, in the image transfer system  16 , it is assumed that a phase adjustment unit not shown in the drawing is provided in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  15100 . That is, in the image transfer system  16 , a configuration in which the phase adjustment unit not shown in the drawing is disposed at the same position as those in the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  15  of the fifteenth embodiment is adopted.  FIG. 45  is a flowchart showing a processing procedure of the image transfer system  16  in the sixteenth embodiment of the present invention. 
     In the image transfer system  16 , a process of transmitting information used to adjust the period of an imaging synchronization signal which is generated by the synchronization signal generation unit  102  to the display terminal  15200  and a process of transmitting information of a period-adjusted accuracy estimation value to the imaging terminal  15100  are added, in association with a change to a configuration in which the after-cycle-adjustment accuracy estimation unit  2105  is included in the display terminal  15200 . However, an outline of the overall operation in the image transfer system  16  is the same as those of the image transfer system  1  of the first embodiment to the image transfer system  15  of the fifteenth embodiment. Therefore, also in the image transfer system  16 , a description of an outline of the overall operation will be omitted, and a more specific operation of performing cycle adjustment for an imaging synchronization signal in the image transfer system  16  will be described. 
     Also in the image transfer system  16 , when a cycle adjustment process is started, the imaging terminal  15100  transmits a round-trip-propagation-time-measurement outgoing signal to the display terminal  15200 , and the display terminal  15200  transmits a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal to the imaging terminal  15100  in step S 302 . Meanwhile, the process of step S 302  in the image transfer system  16  is the same as the process of step S 302  in each of the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  15  of the fifteenth embodiment. 
     Thereafter, also in the image transfer system  16 , in step S 303 , the imaging terminal  15100  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  15100  and the display terminal  15200 . Meanwhile, the process of step S 303  in the image transfer system  16  is also the same as the process of step S 303  in each of the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  15  of the fifteenth embodiment. 
     Thereafter, also in the image transfer system  16 , in step S 304 , the imaging terminal  15100  generates a phase adjustment instruction for the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  to adjust the phase of an imaging synchronization signal, on the basis of the round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  15100  and the display terminal  15200  which is calculated in step S 303 , and updates a round trip propagation-time determination value. Meanwhile, the process of step S 304  in the image transfer system  16  is also the same as the process of step S 304  in each of the image transfer system  1  of the first embodiment, and the image transfer system  10  of the tenth embodiment to the image transfer system  15  of the fifteenth embodiment. 
     Thereafter, also in the image transfer system  16 , in step S 307 , the imaging terminal  15100  generates a measurement notification signal including information of the calculated round trip propagation-time and transmits the generated measurement notification signal to the display terminal  15200 . Meanwhile, the process of step S 307  in the image transfer system  16  and the measurement notification signal generated in the process of step S 307  and transmitted to the display terminal  15200  are the same as the process of step S 307  and the measurement notification signal in each of the image transfer system  13  of the thirteenth embodiment to the image transfer system  15  of the fifteenth embodiment. Thereby, the after-cycle-adjustment accuracy estimation unit  2105  provided in the display terminal  15200  acquires information of the round trip propagation-time determination value transmitted from the imaging terminal  15100  through the round-trip-propagation-time-measurement assistance unit  202 . 
     Thereafter, in the image transfer system  16 , in step S 306 , the period of an imaging synchronization signal is adjusted on the basis of the information of the plurality of round trip propagation-time determination values updated in step S 304 . However, in the image transfer system  16 , the display terminal  15200  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  16 , the imaging terminal  15100  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of the period-adjusted accuracy estimation value, calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal, and outputs a cycle adjustment instruction to the synchronization signal generation unit  102 . 
     For this reason, in the image transfer system  16 , in step S 305  included in step S 306 , the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  15200  determines whether or not a predetermined period of time determined in advance has elapsed after the period of a previous imaging synchronization signal is adjusted. In a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has not elapsed after the period of the previous imaging synchronization signal is adjusted (“NO” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  2105  determines that cycle adjustment is not performed and terminates the process of step S 305 . 
     On the other hand, in a result of the determination in step S 305 , in a case where a predetermined period of time determined in advance has elapsed after the period of the previous imaging synchronization signal is adjusted (“YES” in step S 305 ), the after-cycle-adjustment accuracy estimation unit  2105  determines that cycle adjustment is performed and calculates a period-adjusted accuracy estimation value in step S 2106  included in step S 306 . Meanwhile, the process of step S 305  and the process of step S 2106  which are included in step S 306  in the image transfer system  16  are the same as the process of step S 305  and the process of step S 2106  which are included in step S 306  in each of the image transfer system  3  of the third embodiment, the image transfer system  4  of the fourth embodiment, the image transfer system  6  of the sixth embodiment, the image transfer system  13  of the thirteenth embodiment, and the image transfer system  15  of the fifteenth embodiment. 
     Thereafter, in the image transfer system  16 , in step S 319  included in step S 306 , the display terminal  15200  generates an estimation notification signal including information of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  2105  and transmits the generated estimation notification signal to the imaging terminal  15100 . Meanwhile, the process of step S 319  included in step S 306  in the image transfer system  16  and the estimation notification signal generated in the process of step S 319  and transmitted to the imaging terminal  15100  are the same as the process of step S 319  included in step S 306  and the estimation notification signal in each of the image transfer system  3  of the third embodiment, the image transfer system  6  of the sixth embodiment, and the image transfer system  13  of the thirteenth embodiment. Thereby, the cycle adjustment determination unit  104  provided in the imaging terminal  15100  acquires information of the period-adjusted accuracy estimation value calculated by the after-cycle-adjustment accuracy estimation unit  2105  provided in the display terminal  15200  through the round-trip-propagation-time measurement unit  106 . 
     Further, in the image transfer system  16 , in step S 2107  included in step S 306 , the imaging terminal  15100  determines whether or not cycle adjustment for an imaging synchronization signal is performed on the basis of information of the period-adjusted accuracy estimation value transmitted from the display terminal  15200  and information such as a round trip propagation-time determination value which is output from the round-trip-propagation-time measurement unit  106 , calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal, and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, the synchronization signal generation unit  102  performs cycle adjustment for adjusting the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is output from the cycle adjustment unit  101 . Meanwhile, the process of step S 2107  included in step S 306  in the image transfer system  16  is the same as the process of step S 2107  included in step S 306  in each of the image transfer system  1  of the first embodiment and the image transfer system  3  of the third embodiment. 
     Meanwhile, the phase adjustment instruction transmitted from the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106  included in the imaging terminal  15100  is also output to the synchronization signal generation unit  102 . Thereby, the synchronization signal generation unit  102  adjusts the phase of an imaging synchronization signal to be generated, in response to the phase adjustment instruction output from the phase adjustment unit, not shown in the drawing, which is provided in the round-trip-propagation-time measurement unit  106 . 
     In this manner, in the image transfer system  16 , the imaging terminal  15100  transmits a round-trip-propagation-time-measurement outgoing signal and calculates a round trip propagation-time in wireless transfer between the imaging terminal  15100  and the display terminal  15200 . Further, in the image transfer system  16 , the imaging terminal  15100  performs a process of updating a round trip propagation-time determination value and transmits information of a plurality of round trip propagation-time determination values updated, information of a round trip propagation-time, information of a scheduled transmission time of a round-trip-propagation-time-measurement outgoing signal, information of a scheduled transmission time included in a received round-trip-propagation-time-measurement returning signal, and a phase adjustment instruction to the display terminal  15200 . Thereby, in the image transfer system  16 , the display terminal  15200  performs a process of estimating (calculating) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed on the basis of the pieces of information transmitted from the imaging terminal  15100 . Further, in the image transfer system  16 , the display terminal  15200  transmits information of the calculated period-adjusted accuracy estimation value to the imaging terminal  15100 . Thereby, in the image transfer system  16 , the imaging terminal  15100  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  15100  is performed, on the basis of the period-adjusted accuracy estimation value transmitted from the display terminal  15200 , calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal, and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, in the image transfer system  16 , the imaging terminal  15100  adjusts the periods of an imaging synchronization signal and a display synchronization signal so as not to be shifted with the elapse of time. 
     Meanwhile, similarly to the image transfer system  1  of the first embodiment to the image transfer system  15  of the fifteenth embodiment, also in the image transfer system  16 , when at least a process of updating a round trip propagation-time determination value is performed in step S 304 , the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  15200  can estimate the accuracy of an imaging synchronization signal (that is, calculate a period-adjusted accuracy estimation value). Therefore, similarly to the image transfer system  1  of the first embodiment to the image transfer system  15  of the fifteenth embodiment, also in the image transfer system  16 , the phase adjustment unit not shown in the drawing does not necessarily need to adjust the phase of an imaging synchronization signal in step S 304  and may be able to update a round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  as a round trip propagation-time determination value. 
     With such a configuration and processing procedure, in the image transfer system  16 , the imaging terminal  15100  transmits a round-trip-propagation-time-measurement outgoing signal and measures (calculates) a round trip propagation-time in wireless transfer between the imaging terminal  15100  and the display terminal  15200  to update a round trip propagation-time determination value. Further, in the image transfer system  16 , the display terminal  15200  estimates (calculates) a period-adjusted accuracy estimation value in a case where cycle adjustment for an imaging synchronization signal is temporarily executed. Further, in the image transfer system  16 , the imaging terminal  15100  adjusts the phase or period (at least a period) of an imaging synchronization signal to be generated, on the basis of the period-adjusted accuracy estimation value estimated (calculated) by the display terminal  15200 . Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  15  of the fifteenth embodiment, also in the image transfer system  16 , the phase or period (at least a period) of an imaging synchronization signal generated by the imaging terminal  15100  (more specifically, the synchronization signal generation unit  102 ) is adjusted so as to match the phase or period (at least a period) of a display synchronization signal generated by the display terminal  15200 . That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  15  of the fifteenth embodiment, also in the image transfer system  16 , a timing when the imaging terminal  15100  wirelessly transfers captured image data of an image captured by an imaging unit not shown in the drawing to the display terminal  15200  is matched to a timing when the display terminal  15200  displays an image corresponding to the captured image data on a display unit not shown in the drawing. Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  15  of the fifteenth embodiment, also in the image transfer system  16 , the display terminal  15200  can stably display an image corresponding to captured image data wirelessly transferred from the imaging terminal  15100  on the display unit not shown in the drawing. 
     As described above, the image transfer system  16  of the sixteenth embodiment performs transmission and reception of a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal between the round-trip-propagation-time measurement unit  106  included in the imaging terminal  15100  and the round-trip-propagation-time-measurement assistance unit  202  included in the display terminal  15200  after wireless connection between the imaging terminal  15100  and the display terminal  15200  is established. Further, in the image transfer system  16  of the sixteenth embodiment, the round-trip-propagation-time measurement unit  106  calculates a round trip propagation-time required for transmission and reception in wireless transfer between the imaging terminal  15100  and the display terminal  15200 , on the basis of a transmission time of a round-trip-propagation-time-measurement outgoing signal and a reception time of a round-trip-propagation-time-measurement returning signal corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the round-trip-propagation-time-measurement assistance unit  202 . Further, in the image transfer system  16  of the sixteenth embodiment, the round-trip-propagation-time measurement unit  106  (more specifically, the phase adjustment unit not shown in the drawing) updates the round trip propagation-time calculated by the round-trip-propagation-time measurement unit  106  as a round trip propagation-time determination value. Further, in the image transfer system  16  of the sixteenth embodiment, the round-trip-propagation-time measurement unit  106  transmits information for adjusting the period of an imaging synchronization signal in the imaging terminal  15100  to the display terminal  15200 . Further, in the image transfer system  16  of the sixteenth embodiment, the after-cycle-adjustment accuracy estimation unit  2105  included in the display terminal  15200  estimates (calculates) a period-adjusted accuracy estimation value of an imaging synchronization signal in a case where cycle adjustment is temporarily executed. Further, in the image transfer system  16  of the sixteenth embodiment, the after-cycle-adjustment accuracy estimation unit  2105  transmits information of the estimated (calculated) period-adjusted accuracy estimation value of the imaging synchronization signal to the imaging terminal  15100 . Further, in the image transfer system  16  of the sixteenth embodiment, the cycle adjustment determination unit  104  included in the imaging terminal  15100  determines whether or not cycle adjustment for an imaging synchronization signal which is generated by the synchronization signal generation unit  102  included in the imaging terminal  15100  is performed, on the basis of the period-adjusted accuracy estimation value. Further, in the image transfer system  16  of the sixteenth embodiment, in a case where the cycle adjustment determination unit  104  determines that cycle adjustment for an imaging synchronization signal is performed, the cycle adjustment unit  101  included in the imaging terminal  15100  calculates a cycle adjustment amount for performing cycle adjustment for an imaging synchronization signal and outputs the calculated cycle adjustment amount to the synchronization signal generation unit  102  together with a cycle adjustment instruction. Thereby, in the image transfer system  16  of the sixteenth embodiment, the synchronization signal generation unit  102  adjusts the period of an imaging synchronization signal to be generated by an amount of cycle adjustment, in response to the cycle adjustment instruction which is transmitted from the cycle adjustment unit  101 . 
     Thereby, similarly to the image transfer system  1  of the first embodiment to the image transfer system  15  of the fifteenth embodiment, also in the image transfer system  16  of the sixteenth embodiment, the phase or period (at least a period) of an imaging synchronization signal can be matched to the phase or period (at least a period) of a display synchronization signal. That is, also in the image transfer system  16  of the sixteenth embodiment, even when the periods of an imaging synchronization signal and a display synchronization signal are shifted with the elapse of time due to an error of a phase or a period between an imaging reference clock signal and a display reference clock signal, the phase or period (at least a period) of the imaging synchronization signal can be matched to the phase or period (at least a period) of the display synchronization signal. Thus, also in the image transfer system  16  of the sixteenth embodiment, the same effects as those in the image transfer system  1  of the first embodiment to the image transfer system  15  of the fifteenth embodiment can be obtained. That is, similarly to the image transfer system  1  of the first embodiment to the image transfer system  15  of the fifteenth embodiment, also in the image transfer system  16  of the sixteenth embodiment, it is possible to wirelessly transfer captured image data from the imaging terminal  15100  to the display terminal  15200  without exceeding a validity period of a display image and to stably display a display image corresponding to the captured image data transmitted from the imaging terminal  15100  in the display terminal  15200 . 
     Moreover, in the image transfer system  16  of the sixteenth embodiment, the display terminal  15200  estimates (calculates) a period-adjusted accuracy estimation value. Thereby, in the image transfer system  16  of the sixteenth embodiment, it is not necessary to estimate (calculate) a period-adjusted accuracy estimation value in the imaging terminal  15100 , and thus it is possible to reduce a load of processing performed in the imaging terminal  15100 . 
     According to the embodiments of the present invention, an image transfer system (for example, the image transfer system  1 ) is an image transfer system including an imaging terminal (for example, the imaging terminal  100 ) transmitting captured image data at a period of an imaging timing (a timing signal, for example, an imaging synchronization signal) and a display terminal (for example, the display terminal  200 ) receiving captured image data and displaying the captured image data at a period of a display timing (a timing signal, for example, a display synchronization signal). The image transfer system includes a communication delay time calculation function (for example, the function of the round-trip-propagation-time measurement unit  106 ), a communication delay time calculation assistance function (for example, the function of the round-trip-propagation-time-measurement assistance unit  202 ), an accuracy estimation function (for example, the function of the after-cycle-adjustment accuracy estimation unit  105 ), an adjustment execution determination function (for example, the function of the cycle adjustment determination unit  104 ), and a cycle adjustment function (for example, the function of the cycle adjustment unit  101 ). The communication delay time calculation function, which is included in any one terminal out of the imaging terminal and the display terminal, is a function of generating a first signal for measurement (round-trip-propagation-time-measurement outgoing signal) in synchronization with the period of an imaging timing (for example, an imaging synchronization signal), transmitting the generated first signal to the other terminal, receiving a second signal for measurement (round-trip-propagation-time-measurement returning signal) corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the other terminal, and calculating a communication delay time (round trip propagation-time=round trip propagation-time determination value) on the basis of a transmission timing of the round-trip-propagation-time-measurement outgoing signal, a reception timing of the round-trip-propagation-time-measurement returning signal, and elapsed time data (receiver elapsed time), included in the round-trip-propagation-time-measurement returning signal, which indicates an elapsed time from a reception timing of the round-trip-propagation-time-measurement outgoing signal in the other terminal to a transmission timing of the round-trip-propagation-time-measurement returning signal. The communication delay time calculation assistance function, which is included in the other terminal, is a function of receiving a round-trip-propagation-time-measurement outgoing signal, generating a round-trip-propagation-time-measurement returning signal including a receiver elapsed time, and transmitting the generated round-trip-propagation-time-measurement returning signal to one terminal. The accuracy estimation function, which is included in any one terminal out of the imaging terminal and the display terminal, is a function of setting any one of an imaging timing (for example, an imaging synchronization signal) and a display timing (for example, a display synchronization signal) to be an adjustment target timing (for example, an imaging synchronization signal) and calculating an accuracy estimation value (period-adjusted accuracy estimation value) obtained by estimating an accuracy after adjusting the period (cycle adjustment) of the adjustment target timing (for example, the imaging synchronization signal), on the basis of a plurality of communication delay times (round trip propagation-times=round trip propagation-time determination values) calculated for a period of time determined in advance and transmission timings of a plurality of outward path signals for round trip propagation-time measurement corresponding to the respective communication delay times (round trip propagation-times=round trip propagation-time determination values). The adjustment execution determination function, which is included in any one terminal out of the imaging terminal and the display terminal, is a function of determining whether or not a period-adjusted accuracy estimation value has been improved compared with the current accuracy of the adjustment target timing (for example, an imaging synchronization signal) on the basis of the period-adjusted accuracy estimation value and the current accuracy of the adjustment target timing (for example, the imaging synchronization signal) and determining whether or not the adjustment of the period (cycle adjustment) of an adjustment target timing (for example, an imaging synchronization signal) is performed in accordance with a determination result. The cycle adjustment function, which is included in any one terminal out of the imaging terminal and the display terminal, is a function of performing the adjustment of the period (cycle adjustment) of an adjustment target timing (for example, an imaging synchronization signal) in a case where it is determined that the adjustment of the period (cycle adjustment) of the adjustment target timing (for example, the imaging synchronization signal) is performed. 
     In addition, according to the embodiments of the present invention, the image transfer system (for example, the image transfer system  1 ) is configured such that the cycle adjustment function is a function of calculating an adjustment amount (cycle adjustment amount) for adjusting the period (cycle adjustment) of an adjustment target timing (for example, an imaging synchronization signal) on the basis of a period-adjusted accuracy estimation value, a transmission timing of a round-trip-propagation-time-measurement outgoing signal, and a transmission timing of a round-trip-propagation-time-measurement returning signal in a case where it is determined that the adjustment of the period (cycle adjustment) of the adjustment target timing (for example, the imaging synchronization signal) is performed, and adjusting the period (cycle adjustment) of the adjustment target timing (for example, the imaging synchronization signal) on the basis of the calculated cycle adjustment amount. 
     In addition, according to the embodiments of the present invention, the image transfer system (for example, the image transfer system  1 ) is configured such that the accuracy estimation function is a function of extracting a minimum communication delay time (minimum determination value) for each population by setting a plurality of communication delay times (round trip propagation-times=round trip propagation-time determination values) calculated within a period of time determined in advance to be a population, and calculating a period-adjusted accuracy estimation value on the basis of a difference between two minimum determination values extracted from a plurality of minimum determination values extracted from a plurality of populations and a difference between transmission timings of two outward path signals for round trip propagation-time measurement transmitted to respectively calculate the extracted two minimum determination values, the adjustment execution determination function is a function of determining that the period of an adjustment target timing (for example, an imaging synchronization signal) is adjusted in a case where the period-adjusted accuracy estimation value has been improved compared with the current accuracy of the adjustment target timing (for example, the imaging synchronization signal) and determining that the period of the adjustment target timing (for example, the imaging synchronization signal) is not adjusted in a case where the period-adjusted accuracy estimation value has not been improved compared with the current accuracy of the adjustment target timing (for example, the imaging synchronization signal), and the cycle adjustment function is a function of calculating a cycle adjustment amount on the basis of a period-adjusted accuracy estimation value, a difference between transmission timings of two outward path signals for round trip propagation-time measurement transmitted to respectively calculate two minimum determination values extracted at the time of calculating the period-adjusted accuracy estimation value, and a difference between transmission timings of two return path signals for round trip propagation-time measurement corresponding to the two outward path signals for roundtrip propagation-time measurement. 
     In addition, according to the embodiments of the present invention, the image transfer system (for example, the image transfer system  1 ) is configured such that the cycle adjustment function is a function of multiplying a ratio of a difference between transmission timings of two outward path signals for round trip propagation-time measurement to a difference between transmission timings of two return path signals for round trip propagation-time measurement by a period-adjusted accuracy estimation value to calculate a cycle adjustment amount. 
     In addition, according to the embodiments of the present invention, the image transfer system (for example, the image transfer system  1 ) is configured such that the accuracy estimation function is a function of calculating a period-adjusted accuracy estimation value by increasing the number of communication delay times (round trip propagation-times=round trip propagation-time determination values) included in a population in a case where it is determined that the period of an adjustment target timing (for example, an imaging synchronization signal) is not adjusted. 
     In addition, according to the embodiments of the present invention, the image transfer system (for example, the image transfer system  1 ) is configured such that the accuracy estimation function is a function of including communication delay times (round trip propagation-times=round trip propagation-time determination values), which are included in a population before increasing the number of communication delay times (round trip propagation-times=round trip propagation-time determination values), in the population including the increased number of communication delay times (round trip propagation-times=round trip propagation-time determination values). 
     In addition, according to the embodiments of the present invention, the image transfer system (for example, the image transfer system  1 ) is configured to further include a phase adjustment execution determination function (for example, the function of a phase adjustment unit not shown in the drawing) of determining whether or not the adjustment of a phase shift (phase adjustment) between an imaging timing (for example, an imaging synchronization signal) and a display timing (for example, a display synchronization signal) is performed on the basis of a communication delay time (round trip propagation-time=round trip propagation-time determination value). 
     In addition, according to the embodiments of the present invention, the image transfer system (for example, the image transfer system  1 ) is configured to further include a phase adjustment function (for example, the function of a phase adjustment unit not shown in the drawing) of calculating a phase adjustment amount for adjusting a phase shift (phase adjustment) between an imaging timing (for example, an imaging synchronization signal) and a display timing (for example, a display synchronization signal) on the basis of a communication delay time (round trip propagation-time=round trip propagation-time determination value) and adjusting the phase shift (phase adjustment) between the imaging timing (for example, he imaging synchronization signal) and the display timing (for example, the display synchronization signal) on the basis of the calculated phase adjustment amount. 
     In addition, according to the embodiments of the present invention, an imaging terminal (for example, the imaging terminal  100 ) is an imaging terminal in an image transfer system (for example, the image transfer system  1 ) including the imaging terminal (for example, the imaging terminal  100 ) transmitting captured image data at a period of an imaging timing (a timing signal, for example, an imaging synchronization signal) and a display terminal (for example, the display terminal  200 ) receiving captured image data and displaying the captured image data at a period of a display timing (a timing signal, for example, a display synchronization signal). The imaging terminal is configured to include a communication delay time calculation function (for example, the function of the round-trip-propagation-time measurement unit  106 ), an accuracy estimation function (for example, the function of the after-cycle-adjustment accuracy estimation unit  105 ), an adjustment execution determination function (for example, the function of the cycle adjustment determination unit  104 ), and a cycle adjustment function (for example, the function of the cycle adjustment unit  101 ). The communication delay time calculation function is a function of generating a first signal for measurement in synchronization with the period of an imaging timing (for example, an imaging synchronization signal), transmitting the generated first signal to a display terminal, receiving a second signal for measurement (round-trip-propagation-time-measurement returning signal) corresponding to the round-trip-propagation-time-measurement outgoing signal transmitted from the display terminal, and calculating a communication delay time (round trip propagation-time=round trip propagation-time determination value) on the basis of a transmission timing of the round-trip-propagation-time-measurement outgoing signal, a reception timing of the round-trip-propagation-time-measurement returning signal, and elapsed time data (receiver elapsed time), included in the round-trip-propagation-time-measurement returning signal, which indicates an elapsed time from a reception timing of the round-trip-propagation-time-measurement outgoing signal in the terminal to a transmission timing of the round-trip-propagation-time-measurement returning signal. The accuracy estimation function is a function of calculating an accuracy estimation value (period-adjusted accuracy estimation value) obtained by estimating an accuracy after adjusting the period (cycle adjustment) of the imaging timing (for example, the imaging synchronization signal), on the basis of a plurality of communication delay times (round trip propagation-times=round trip propagation-time determination values) calculated for a period of time determined in advance and transmission timings of a plurality of outward path signals for round trip propagation-time measurement corresponding to the respective communication delay times (round trip propagation-times=round trip propagation-time determination values). The adjustment execution determination function is a function of determining whether or not a period-adjusted accuracy estimation value has been improved compared with the current accuracy of the imaging timing (for example, the imaging synchronization signal) on the basis of the period-adjusted accuracy estimation value and the current accuracy of the imaging timing (for example, the imaging synchronization signal) and determining whether or not the adjustment of the period (cycle adjustment) of an imaging timing (for example, an imaging synchronization signal) is performed in accordance with a determination result. The cycle adjustment function is a function of adjusting the period (cycle adjustment) of an imaging timing (for example, an imaging synchronization signal) in a case where it is determined that the adjustment of the period (cycle adjustment) of the imaging timing (for example, the imaging synchronization signal) is performed. 
     In addition, according to the embodiments of the present invention, a display terminal (for example, the display terminal  200 ) is a display terminal in an image transfer system (for example, the image transfer system  1 ) that includes an imaging terminal (for example, the imaging terminal  100 ) transmitting captured image data at a period of an imaging timing (a timing signal, for example, an imaging synchronization signal) and the display terminal (for example, the display terminal  200 ) receiving captured image data and displaying the captured image data at a period of a display timing (a timing signal, for example, a display synchronization signal). The display terminal is configured to include a communication delay time calculation assistance function (for example, the function of the round-trip-propagation-time-measurement assistance unit  202 ) of receiving a first signal for measurement (round-trip-propagation-time-measurement outgoing signal) transmitted from the imaging terminal in synchronization with the period of an imaging timing (for example, an imaging synchronization signal), generating a second signal for measurement (round-trip-propagation-time-measurement returning signal) including elapsed time data indicating an elapsed time from a reception timing of the round-trip-propagation-time-measurement outgoing signal to a transmission timing of the second signal for measurement corresponding to the round-trip-propagation-time-measurement outgoing signal, and transmitting the generated second signal for measurement to the imaging terminal. 
     In addition, according to the embodiments of the present invention, an adjustment method is a method of adjusting periods of an imaging timing (for example, an imaging synchronization signal) and a display timing (for example, a display synchronization signal) so as to match each other (cycle adjustment) in an image transfer system (for example, the image transfer system  1 ) that includes an imaging terminal (for example, the imaging terminal  100 ) transmitting captured image data at a period of an imaging timing (a timing signal, for example, an imaging synchronization signal) and a display terminal (for example, the display terminal  200 ) receiving captured image data and displaying the captured image data at a period of a display timing (a timing signal, for example, a display synchronization signal). The adjustment method includes a process of generating a first signal for measurement (round-trip-propagation-time-measurement outgoing signal) in synchronization with the period of an imaging timing (for example, an imaging synchronization signal) and transmitting the generated first signal for measurement from any one terminal out of the imaging terminal and the display terminal to the other terminal, a process of receiving a round-trip-propagation-time-measurement outgoing signal in the other terminal, a process of generating a round-trip-propagation-time-measurement returning signal including elapsed time data (receiver elapsed time) indicating an elapsed time from a reception timing of the round-trip-propagation-time-measurement outgoing signal in the other terminal to a transmission timing of a second signal for measurement (round-trip-propagation-time-measurement returning signal) in the other terminal and transmitting the generated round-trip-propagation-time-measurement returning signal to one terminal, a process of receiving a round-trip-propagation-time-measurement returning signal in one terminal and calculating a communication delay time (round trip propagation-time=round trip propagation-time determination value) on the basis of a transmission timing of a round-trip-propagation-time-measurement outgoing signal, a reception timing of a round-trip-propagation-time-measurement returning signal, and a receiver elapsed time, a process of setting any one of an imaging timing (for example, an imaging synchronization signal) and a display timing (for example, a display synchronization signal) to be an adjustment target timing (for example, an imaging synchronization signal) in any one terminal out of the imaging terminal and the display terminal and calculating an accuracy estimation value (period-adjusted accuracy estimation value) obtained by estimating an accuracy after adjusting the period (cycle adjustment) of the adjustment target timing (for example, the imaging synchronization signal), on the basis of a plurality of communication delay times (round trip propagation-times=round trip propagation-time determination values) calculated for a period of time determined in advance and transmission timings of a plurality of outward path signals for round trip propagation-time measurement corresponding to the respective communication delay times (round trip propagation-times=round trip propagation-time determination values), a process of determining whether or not a period-adjusted accuracy estimation value has been improved compared with the current accuracy of the adjustment target timing (for example, an imaging synchronization signal) on the basis of the period-adjusted accuracy estimation value and the current accuracy of the adjustment target timing (for example, the imaging synchronization signal) in any one terminal out of the imaging terminal and the display terminal and determining whether or not the adjustment of the period (cycle adjustment) of an adjustment target timing (for example, an imaging synchronization signal) is performed in accordance with a determination result, and a process of adjusting the period (cycle adjustment) of an adjustment target timing (for example, an imaging synchronization signal) in any one terminal out of the imaging terminal and the display terminal in a case where it is determined that the adjustment of the period (cycle adjustment) of the adjustment target timing (for example, the imaging synchronization signal) is performed. 
     In addition, according to the embodiments of the present invention, an adjustment method is a method of adjusting periods of an imaging timing (for example, an imaging synchronization signal) and a display timing (for example, a display synchronization signal) so as to match each other (cycle adjustment) in an imaging terminal of an image transfer system (for example, the image transfer system  1 ) that includes the imaging terminal (for example, the imaging terminal  100 ) transmitting captured image data at a period of an imaging timing (a timing signal, for example, an imaging synchronization signal) and a display terminal (for example, the display terminal  200 ) receiving captured image data and displaying the captured image data at a period of a display timing (a timing signal, for example, a display synchronization signal). The adjustment method includes a process of generating a first signal for measurement (round-trip-propagation-time-measurement outgoing signal) in synchronization with the period of an imaging timing (for example, an imaging synchronization signal) and transmitting the generated first signal for measurement to the display terminal, a process of receiving a round-trip-propagation-time-measurement returning signal including elapsed time data (receiver elapsed time) indicating an elapsed time, transmitted from the display terminal, from a reception timing of a round-trip-propagation-time-measurement outgoing signal to a transmission timing of a second signal for measurement (round-trip-propagation-time-measurement returning signal) corresponding to the round-trip-propagation-time-measurement outgoing signal, a process of calculating a communication delay time (round trip propagation-time=round trip propagation-time determination value) on the basis of a transmission timing of a round-trip-propagation-time-measurement outgoing signal, a reception timing of a round-trip-propagation-time-measurement returning signal, and a receiver elapsed time, a process of calculating an accuracy estimation value (period-adjusted accuracy estimation value) obtained by estimating an accuracy after adjusting the period (cycle adjustment) of an imaging timing (for example, an imaging synchronization signal) on the basis of a plurality of communication delay times (round trip propagation-times=round trip propagation-time determination values) calculated for a period of time determined in advance and transmission timings of a plurality of outward path signals for round trip propagation-time measurement corresponding to the respective communication delay times (round trip propagation-times=round trip propagation-time determination values), a process of determining whether or not a period-adjusted accuracy estimation value has been improved compared with the current accuracy of the imaging timing (for example, the imaging synchronization signal) on the basis of the period-adjusted accuracy estimation value and the current accuracy of the adjustment target timing (for example, the imaging synchronization signal) and determining whether or not the adjustment of the period (cycle adjustment) of an imaging timing (for example, an imaging synchronization signal) is performed in accordance with a determination result, and a process of adjusting the period (cycle adjustment) of an imaging timing (for example, an imaging synchronization signal) in a case where it is determined that the adjustment of the period (cycle adjustment) of the imaging timing (for example, the imaging synchronization signal) is performed. 
     In addition, according to the embodiments of the present invention, an adjustment assistance method is a method of assisting adjustment (cycle adjustment) for matching periods of an imaging timing (for example, an imaging synchronization signal) and a display timing (for example, a display synchronization signal) in a display terminal of an image transfer system (for example, the image transfer system  1 ) that includes an imaging terminal (for example, the imaging terminal  100 ) transmitting captured image data at a period of an imaging timing (a timing signal, for example, an imaging synchronization signal) and the display terminal (for example, the display terminal  200 ) receiving captured image data and displaying the captured image data at a period of a display timing (a timing signal, for example, a display synchronization signal). The adjustment assistance method includes a process of receiving a first signal for measurement (round-trip-propagation-time-measurement outgoing signal) transmitted from the imaging terminal in synchronization with the period of an imaging timing (for example, an imaging synchronization signal), generating a round-trip-propagation-time-measurement returning signal including elapsed time data (receiver elapsed time) indicating an elapsed time from a reception timing of a round-trip-propagation-time-measurement outgoing signal to a transmission timing of a second signal for measurement (round-trip-propagation-time-measurement returning signal) corresponding to the round-trip-propagation-time-measurement outgoing signal, and transmitting the generated round-trip-propagation-time-measurement returning signal to the imaging terminal. 
     In addition, according to the embodiments of the present invention, an adjustment program is a program causing a computer to execute an adjustment method, an adjustment method is an adjustment method of adjusting periods of an imaging timing (for example, an imaging synchronization signal) and a display timing (for example, a display synchronization signal) so as to match each other (cycle adjustment) in an imaging terminal of an image transfer system (for example, the image transfer system  1 ) that includes the imaging terminal (for example, the imaging terminal  100 ) transmitting captured image data at a period of an imaging timing (a timing signal, for example, an imaging synchronization signal) and a display terminal (for example, the display terminal  200 ) receiving captured image data and displaying the captured image data at a period of a display timing (a timing signal, for example, a display synchronization signal). The adjustment program causes the computer to execute a process of generating a first signal for measurement (round-trip-propagation-time-measurement outgoing signal) in synchronization with the period of an imaging timing (for example, an imaging synchronization signal) and transmitting the generated first signal for measurement to the display terminal, a process of receiving a round-trip-propagation-time-measurement returning signal including elapsed time data (receiver elapsed time) indicating an elapsed time, transmitted from the display terminal, from a reception timing of a round-trip-propagation-time-measurement outgoing signal to a transmission timing of a second signal for measurement (round-trip-propagation-time-measurement returning signal) corresponding to the round-trip-propagation-time-measurement outgoing signal, a process of calculating a communication delay time (round trip propagation-time=round trip propagation-time determination value) on the basis of a transmission timing of a round-trip-propagation-time-measurement outgoing signal, a reception timing of a round-trip-propagation-time-measurement returning signal, and a receiver elapsed time, a process of calculating an accuracy estimation value (period-adjusted accuracy estimation value) obtained by estimating an accuracy after adjusting the period (cycle adjustment) of an imaging timing (for example, an imaging synchronization signal) on the basis of a plurality of communication delay times (round trip propagation-times=round trip propagation-time determination values) calculated for a period of time determined in advance and transmission timings of a plurality of outward path signals for round trip propagation-time measurement corresponding to the respective communication delay times (round trip propagation-times=round trip propagation-time determination values), a process of determining whether or not a period-adjusted accuracy estimation value has been improved compared with the current accuracy of the imaging timing (for example, the imaging synchronization signal) on the basis of the period-adjusted accuracy estimation value and the current accuracy of the imaging timing (for example, the imaging synchronization signal) and determining whether or not the adjustment of the period (cycle adjustment) of an imaging timing (for example, an imaging synchronization signal) is performed in accordance with a determination result, and a process of adjusting the period (cycle adjustment) of an imaging timing (for example, an imaging synchronization signal) in a case where it is determined that the adjustment of the period (cycle adjustment) of the imaging timing (for example, the imaging synchronization signal) is performed. 
     In addition, according to the embodiments of the present invention, an adjustment support program is a program causing a computer to execute an adjustment assistance method of assisting adjustment (cycle adjustment) for matching periods of an imaging timing (for example, an imaging synchronization signal) and a display timing (for example, a display synchronization signal) in a display terminal of an image transfer system (for example, the image transfer system  1 ) that includes an imaging terminal (for example, the imaging terminal  100 ) transmitting captured image data at a period of an imaging timing (a timing signal, for example, an imaging synchronization signal) and the display terminal (for example, the display terminal  200 ) receiving captured image data and displaying the captured image data at a period of a display timing (a timing signal, for example, a display synchronization signal). The adjustment support program causes the computer to execute a process of receiving a first signal for measurement (round-trip-propagation-time-measurement outgoing signal) transmitted from the imaging terminal in synchronization with the period of an imaging timing (for example, an imaging synchronization signal), generating a round-trip-propagation-time-measurement returning signal including elapsed time data (receiver elapsed time) indicating an elapsed time from a reception timing of a round-trip-propagation-time-measurement outgoing signal to a transmission timing of a second signal for measurement (round-trip-propagation-time-measurement returning signal) corresponding to the round-trip-propagation-time-measurement outgoing signal, and transmitting the generated round-trip-propagation-time-measurement returning signal to the imaging terminal. 
     As described above, according to the embodiments of the present invention, in an image transfer system in which captured image data is wirelessly transferred between an imaging terminal and a display terminal, a round-trip-propagation-time-measurement outgoing signal and a round-trip-propagation-time-measurement returning signal are transmitted and received between the imaging terminal and the display terminal. Further, in the embodiments of the present invention, one terminal transmitting a round-trip-propagation-time-measurement outgoing signal and receiving a round-trip-propagation-time-measurement returning signal out of the imaging terminal and the display terminal calculates a round trip propagation-time required for transmission and reception of a signal in wireless transfer between the imaging terminal and the display terminal on the basis of a transmission time of the round-trip-propagation-time-measurement outgoing signal and a reception time of the round-trip-propagation-time-measurement returning signal corresponding to the transmitted round-trip-propagation-time-measurement outgoing signal. Thereafter, in the embodiments of the present invention, the accuracy of a timing signal in a case where the period of the timing signal (in the embodiments, an imaging synchronization signal) generated by the imaging terminal is temporarily adjusted is estimated on the basis of the calculated round trip propagation-time whenever a period of time determined in advance elapses. Thereafter, in the embodiments of the present invention, it is determined whether or not cycle adjustment for a timing signal generated by the imaging terminal is performed by comparing the estimated accuracy of the timing signal with the current accuracy of the timing signal, and the period of the timing signal is adjusted in a case where it is determined that cycle adjustment is performed. Thereby, in the embodiments of the present invention, even when the period of each of a timing signal generated by the imaging terminal and a timing signal (in the embodiments, a display synchronization signal) generated by the display terminal is shifted with the elapse of time due to an error of a phase or a period in a reference clock signal of each of the imaging terminal and the display terminal, the period of the timing signal generated by the imaging terminal can be matched to the period of the timing signal generated by the display terminal. Thus, in the embodiments of the present invention, even when a variation in a transmission time or a significant delay occurs in wireless transfer between the imaging terminal and the display terminal, the wireless transfer can be performed in a state where a delay in wireless transmission exceeding a predetermined range is excluded. Thereby, in the embodiments of the present invention, it is possible to wirelessly transfer captured image data from the imaging terminal to the display terminal without exceeding a validity period of a display image and to stably display a display image corresponding to the captured image data transmitted from the imaging terminal in the display terminal. 
     Meanwhile, in the embodiments of the present invention, an image transfer system configured such that the period of an imaging synchronization signal generated by an imaging terminal is adjusted so as to match the period of a display synchronization signal generated by a display terminal has been described. That is, in the embodiments of the present invention, a case where a timing signal to be subjected to cycle adjustment is an imaging synchronization signal generated by the imaging terminal has been described. However, the timing signal to be subjected to cycle adjustment is not limited to the timing signals generated by the imaging terminals described in the embodiments of the present invention. For example, the timing signal to be subjected to cycle adjustment may be a timing signal generated by the display terminal. That is, the image transfer system may be configured such that the period of a display synchronization signal generated by the display terminal is adjusted so as to match the period of an imaging synchronization signal generated by the imaging terminal. Even in this case, it is possible to easily configure an image transfer system configured to adjust a period using a timing signal generated by the display terminal as a target timing signal by applying the concept of the present invention. 
     Meanwhile, the above-described various processes according to the image transfer system  1  of the present embodiment, the round-trip-propagation-time measurement unit  106  included in the imaging terminal  100 , the after-cycle-adjustment accuracy estimation unit  105 , the cycle adjustment determination unit  104 , the cycle adjustment unit  101 , and the round-trip-propagation-time-measurement assistance unit  202  included in the display terminal  200  may be performed by recording programs for realizing the processes performed by, for example, the image transfer system  1  shown in  FIG. 2  or a portion thereof, for example, the round-trip-propagation-time measurement unit  106  included in the imaging terminal  100 , the after-cycle-adjustment accuracy estimation unit  105 , the cycle adjustment determination unit  104 , the cycle adjustment unit  101 , and the round-trip-propagation-time-measurement assistance unit  202  included in the display terminal  200  in a computer-readable recording medium and causing a computer system to read the programs recorded on the recording medium to execute the programs. Meanwhile, the “computer system” as mentioned herein may be a computer system including hardware such as OS and peripheral devices. In addition, it is assumed that the “computer system” also includes a homepage providing environment (or a display environment) as long as a WWW system is used. In addition, the “computer-readable recording medium” refers to a writable non-volatile memory such as a flexible disc, a magneto-optical disc, a ROM, or a flash memory, a portable medium such as a CD-ROM, or a storage device such as a hard disk built into the computer system. 
     Further, it is assumed that the “computer-readable recording medium” also includes a computer-readable recording medium that stores programs for a fixed time like a non-volatile memory (for example, a dynamic random access memory (DRAM)) inside the computer system serving as a server or a client in a case where a program is transmitted through a network such as the Internet or a communication line such as a telephone line. In addition, the above-described program may be transmitted from the computer system in which the program is stored in a storage device or the like to another computer system through a transmission medium or by transmitted waves in the transmission medium. Here, the “transmission medium” transmitting the program refers to a medium having a function of transmitting information like a network (communication network) such as the Internet or a communication line (communication wire) such as a telephone line. In addition, the above-described program may be a program for realizing a portion of the above-described function. Further, the above-described program may be a so-called differential file (differential program) capable of realizing the above-described function in combination with a program which is recorded in advance in the computer system. 
     The embodiments of the invention have been described above with reference to the drawings, but specific structures of the invention are not limited to the embodiments and may include various modifications without departing from the scope of the invention. The invention is not limited to the above-mentioned embodiments and is limited only by the accompanying claims.