Abstract:
Return roller and carry roller are rotated by one drive source. To the carry roller which nips and holds paper when the paper is held, a driving force of the drive source is transmitted by a clutch only when the return roller carries out the paper toward a return direction. There is controlled paper carry by repeating a step of feeding an n-th sheet of paper on which the image formation on one side is completed to an image formation section, and feeding an n+l-th sheet of paper to a paper side reversal section, and a step of, under the state where the n+1-th sheet of paper is held at the paper side reversal section, ejecting the n-th sheet of paper on which the image formation on both sides is completed, and feeding an n+2-th sheet of paper to the return section.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-351074, filed Nov. 17, 2000, the entire contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to an image forming apparatus which performs both sides printing by re-feeding paper on which image formation on one side thereof is finished by using an image formation mechanism for performing the image formation on one side of paper to the image formation mechanism after reversing the paper at a paper carry route, and by performing the image formation by using the image formation mechanism, and to a controlling method of paper carry at the image forming apparatus.  
           [0004]    2. Description of the Related Art  
           [0005]    Both sides printing by an electrophotographic apparatus and the like is realized by reversing paper on which image formation on one side thereof is finished and by re-feeding the paper to an image formation position by an image formation mechanism.  
           [0006]    In such an image forming apparatus, a paper carry route becomes extremely long. Owing to this, if it is designed so as to perform printing on another sheet of paper after completing the both sides printing on one sheet of paper, a printing speed of the both sides printing becomes extremely slow.  
           [0007]    Here, high-speed both sides printing can be realized by taking a design where plural sheets of paper can individually be carried in the paper carry route by providing plural motors. However, if plural motors are provided in this manner, cost of parts increases while a process burden of a CPU for controlling the motors increases since it becomes necessary to control more motors individually.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    An object of the present invention is to enable high-speed both sides printing with a number of drive sources limited to the minimum.  
           [0009]    According to an aspect of the invention, there is provided an image forming apparatus as below.  
           [0010]    An image forming apparatus comprising;  
           [0011]    an image formation mechanism which forms an image on a paper; a paper carry mechanism which carries the paper at a paper carry route having an image formation section which carries the paper toward one direction so as to make the image formation mechanism perform image formation on the paper, a return section which once takes in the paper carried out from the image formation section and carries out the paper toward a predetermined return direction being different from the direction toward the image formation section while reversing a carry direction, and a paper side reversal section which feeds the paper reversed to the other side after passing the image formation section at the last time to the image formation section at a predetermined timing after temporally stopping and holding the paper carried out from the return section; a drive source which rotates both a return roller provided in the paper carry mechanism in order to take in the paper carried out from the image formation section to the return section and to carry out the paper from the return section to the return direction, and at least one carry roller provided in the paper carry mechanism in order to carry the paper at the paper side reversal section; a clutch which transmits a driving force of the drive source to the carry roller having a possibility to nip and hold the paper when the paper is temporally stopped and held among the carry rollers only when the drive source is operating so that the return roller will carry the paper toward the return direction; and a carry control section which controls the paper carry by repeating a step of feeding the n-th sheet of paper on which the image formation on only one side is completed from the paper side reversal section to the image formation section, and feeding the n+1-th sheet of paper on which the image formation on only one side is completed from the return section to the paper side reversal section, and a step of, under the state where the n+1-th sheet of paper is held at the paper side reversal section, ejecting the n-th sheet of paper on which the image formation on both sides is completed, and feeding the n+2-th sheet of paper from the image formation section to the return section after taking-in the paper to the image formation area and performing the image formation on one side.  
           [0012]    According to another aspect of the invention, there is provided a paper carry controlling method as below.  
           [0013]    A paper carry controlling method in an image forming apparatus comprising;  
           [0014]    an image formation mechanism which forms an image on a paper; a paper carry mechanism which carries the paper at a paper carry route having an image formation section which carries the paper toward one direction so as to make the image formation mechanism perform image formation on the paper, a return section which once takes in the paper carried out from the image formation section and carries out the paper toward a predetermined return direction being different from the direction toward the image formation section while reversing a carry direction, and a paper side reversal section which feeds the paper reversed to the other side after passing the image formation section at the last time to the image formation section at a predetermined timing after temporally stopping and holding the paper carried out from the return section; a drive source which rotates both a return roller provided in the paper carry mechanism in order to take in the paper carried out from the image formation section to the return section and to carry out the paper from the return section to the return direction, and at least one carry roller provided in the paper carry mechanism in order to carry the paper at the paper side reversal section; and a clutch which transmits a driving force of the drive source to the carry roller having a possibility to nip and hold the paper when the paper is temporally stopped and held among the carry rollers only when the drive source is operating so that the return roller will carry the paper toward the return direction, the paper carry controlling method being performed by repeating a step of feeding the n-th sheet of paper on which the image formation on only one side is completed from the paper side reversal section to the image formation section, and feeding the n+1-th sheet of paper on which the image formation on only one side is completed from the return section to the paper side reversal section, and a step of, under the state where the n+1-th sheet of paper is held at the paper side reversal section, ejecting the n-th sheet of paper on which the image formation on both sides is completed, and feeding the n+2-th sheet of paper from the image formation section to the return section after taking-in the paper to the image formation area and performing the image formation on one side.  
           [0015]    Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0016]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.  
         [0017]    [0017]FIG. 1 is a diagram showing a structure of a mechanism portion of an electrophotographic apparatus according to an embodiment of the present invention.  
         [0018]    [0018]FIG. 2 is a block diagram of a control system for paper carry of the electrophotographic apparatus shown in FIG. 1.  
         [0019]    [0019]FIG. 3 is a timing diagram showing timing for the paper carry for a case where both sides printing is performed. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    Hereinafter, an explanation will be given of an embodiment of the present invention with reference to the drawings.  
         [0021]    [0021]FIG. 1 is a diagram showing a structure of a mechanism portion of an electrophotographic apparatus according to this embodiment.  
         [0022]    As shown in FIG. 1, the electrophotographic apparatus in this embodiment has a photosensitive drum  1 , an electrification device  2 , a developing device  3 , a transferring device  4 , a fixing roller  5 , a first feeding roller  6 , a second feeding roller  7 , a third feeding roller  8 , a fourth feeding roller  9 , an anti-side-slide roller (referred to as RGT roller hereinafter)  10 , an ejecting roller  11 , an ADU gate  12 , ADU rollers  13 ,  14 , and  15 , a main motor  16 , a first feeding clutch  17 , a second feeding clutch  18 , a third feeding clutch  19 , a fourth feeding clutch  20 , an RGT clutch  21 , an ADU motor  22 , one-way clutches  23 ,  24 , and  25 , a first carry route sensor (referred to as TR 1  sensor hereinafter)  26 , a second carry route sensor (referred to as TR 2  sensor hereinafter)  27 , an RGT sensor  28 , an ejection sensor  29 , a first jam detection sensor (referred to as first ADU sensor hereinafter)  30 , and a second jam detection sensor (referred to as second ADU sensor hereinafter)  31 .  
         [0023]    The electrification device  2 , the developing device  3 , and the transferring device  4  are arranged along a surface of the photosensitive drum  1  with a predetermined position relation, and perform processes of the respective steps of an electrophotographic process with exposure performed separately at a predetermined position. Owing to this, a toner image is formed for the paper passing through between the photosensitive drum  1  and the transferring device  4 . The toner image is formed on a side of the paper which contacts the photosensitive drum  1 . By pressing and melting the toner image formed on the paper by using the fixing roller  5 , the image is fixed.  
         [0024]    A paper route is indicated by dashed lines in FIG. 1. The paper route is formed with the photosensitive drum  1 , the fixing roller  5 , the first feeding roller  6 , the second feeding roller  7 , the third feeding roller  8 , the fourth feeding roller  9 , the RGT roller  10 , the ejecting roller  11 , the ADU gate  12  and the ADU rollers  13 ,  14 , and  15 , and guide members (not shown).  
         [0025]    The photosensitive drum  1 , the fixing roller  5 , the first feeding roller  6 , the second feeding roller  7 , the third feeding roller  8 , the fourth feeding roller  9 , and the RGT roller  10  all rotate in an arrowed direction shown in FIG. 1 by receiving a driving force from the main motor  16 . Moreover, the ejecting roller  11  and the ADU rollers  13 ,  14 , and  15  all rotates in the arrowed direction shown in FIG. 1 by receiving the driving force from the ADU motor  22 .  
         [0026]    The first feeding roller  6  feeds paper from a paper cassette PC 1 , the second feeding roller  7  feeds paper from a paper cassette PC 2 , and the third feeding roller  8  and the fourth feeding roller  9  feed paper from a paper cassette PC 3  respectively to the RGT roller  10 .  
         [0027]    The RGT roller  10  feeds the paper to the fixing roller  5  through between the photosensitive drum  1  and the transferring device  4 .  
         [0028]    The ejecting roller  11  can rotate in a normal direction and a reverse direction in accordance with a rotation direction of the ADU motor  22 . Under a reverse rotation state, the ejecting roller  11  pulls in the paper to be carried by the fixing roller  5 , and ejects the paper to a stacker ST as need arises. Moreover, under a normal rotation state, the ejecting roller  11  feeds the paper which is nipped and held to the ADU roller  13 .  
         [0029]    When the paper is fed to the ejecting roller  11  by the fixing roller  5 , the ADU gate  12  is pushed-up by the paper as shown in FIG. 1 so that it does not interfere with the paper carry. Moreover, the ADU gate  12  falls with its own weight so as to block the route from the fixing roller  5  to the ejecting roller  11  after a back edge from a carry direction of the paper to be fed to the ejecting roller  11  by the fixing roller  5  passes through, and it forms the route reaching the ADU roller  13  from the ejecting roller  11 .  
         [0030]    The ADU rollers  13 ,  14 , and  15  carry the paper to be carried from the ejecting roller  11  in order, and feed the paper to the RGT roller  10  in the same carry direction.  
         [0031]    The main motor  16  generates the driving force for rotating the photosensitive drum  1 , the fixing roller  5 , the first feeding roller  6 , the second feeding roller  7 , the third feeding roller  8 , the fourth feeding roller  9 , and the RGT roller  10 , respectively.  
         [0032]    The first feeding clutch  17 , the second feeding clutch  18 , the third feeding clutch  19 , and the fourth feeding clutch  20  transmit/intercept driving from the main motor  16  to the first feeding roller  6 , the second feeding roller  7 , the third feeding roller  8 , and the fourth feeding roller  9 , respectively.  
         [0033]    The ADU motor  22  generates the driving force for rotating the ejecting roller  11 , and the ADU rollers  13 ,  14 , and  15 , respectively. The ADU motor  22  can perform both the normal rotation and the reverse rotation, and generates the driving force for normally-rotating the ejecting roller  11  during the normal rotation and the driving force for reversely-rotating the ejecting roller  11  during the reverse rotation, respectively.  
         [0034]    The one-way clutches  23 ,  24 , and  25  have a well-known structure which transmits only the driving force in one direction. The one-way clutches  23 ,  24 , and  25  have a mesh direction being the normal-rotation direction of the ADU motor  22 , and rotate the ADU rollers  13 ,  14 , and  15  in the arrowed direction shown in FIG. 1 when the ADU motor  22  is in the normal rotation. Moreover, concerning the one-way clutches  23 ,  24 , and  25 , the reverse-rotation direction of the ADU motor  22  is an idle rotation direction.  
         [0035]    The TR 1  sensor  26  is arranged at a position through which the paper to be carried by the second feeding roller  7  and the paper to be carried by the third feeding roller  8  pass, and detects the existence of the paper at the position.  
         [0036]    The TR 2  sensor  27  is arranged at a position through which the paper carried by the third feeding roller  8  passes, and detects the existence of the paper at the position.  
         [0037]    The RGT sensor  28  is arranged near the RGT roller  10  at a position through which the paper to be fed to the RGT roller  10  by the first feeding roller  6 , the second feeding roller  7 , the third feeding roller  8 , and the ADU roller  15  passes, and detects the existence of the paper at the position.  
         [0038]    The ejection sensor  29  is arranged near the ADU gate  12  at a position through which the paper fed from the fixing roller  5  to the ejecting roller  11  passes, and detects the existence of the paper at the position.  
         [0039]    The first ADU sensor  30  and the second ADU sensor  31  are respectively arranged at a position through which the paper to be fed from the ADU roller  13  to the ADU roller  14  passes and at a position through which the paper to be fed from the ADU roller  14  to the ADU roller  15  passes, and detect the existence of the paper at the positions, respectively.  
         [0040]    The carry route of the paper formed with the arrangement as described above can roughly be classified into a feed section  101 , an image formation section  102 , a ejection/return section  103 , and a paper side reversal section  104  as shown by circling with broken lines in FIG. 1.  
         [0041]    The feeding section  101  is a section for feeding a new sheet of paper to the image formation section  102 . The image formation section  102  is a section for performing an image formation operation to the paper. The paper ejection/return section  103  is a section for ejecting the paper on which the necessary image formation is completed to the stacker ST. Moreover, the ejection/return section  103  is a section for returning the paper on which the image formation on only one side is completed to the paper side reversal section  104 . The paper side reversal section  104  is a section for reversing the paper when it passes the image formation section  102 .  
         [0042]    [0042]FIG. 2 is a block diagram showing a control system for the paper carry of the electrophotographic apparatus in this embodiment. Besides, the parts being the same as those in FIG. 1 are given the same numerals respectively, and the detailed explanations thereof are omitted.  
         [0043]    As shown in FIG. 2, there are comprised a CPU  32 , a ROM  33 , a RAM  34 , an interface portion (referred to as IF portion hereinafter)  35 , and an input/output portion (referred to as I/O portion hereinafter)  36  as the control system, and they are connected to each other via a bus  37 .  
         [0044]    The CPU  32  controls the main motor  16  and the ADU motor  22 , or the first feeding clutch  17 , the second feeding clutch  18 , the third feeding clutch  19 , the fourth feeding clutch  20 , and the RGT clutch  21  while referring to the respective detected results of the TR 1  sensor  26 , the TR 2  sensor  27 , the RGT sensor  28 , the ejection sensor  29 , the first ADU sensor  30 , and the second ADU sensor  31 . The CPU  32  realizes the control by a software process based on an operation program stored in the ROM  33 .  
         [0045]    The ROM  33  stores data and the like that are necessary for the operation program used by the CPU  32  or the CPU  32  to perform various sorts of processes.  
         [0046]    The RAM  34  is used as a work area and the like for the CPU  32  to perform the various sorts of processes.  
         [0047]    The IF portion  35  performs an interface process for a main CPU (not shown) and the like for totally-controlling the operation of the whole electrophotographic apparatus in this embodiment and the CPU  32  to give/receive the various sorts of data.  
         [0048]    To the I/O portion  36 , there are connected the main motor  16 , the ADU motor  22 , the first feeding clutch  17 , the second feeding clutch  18 , the third feeding clutch  19 , the fourth feeding clutch  20 , the RGT clutch  21 , the TR 1  sensor  26 , the TR 2  sensor  27 , the RGT sensor  28 , the ejection sensor  29 , the first ADU sensor  30 , and the second ADU sensor  31 , respectively. The I/O portion  36  performs the input/output process of a signal relating to each of these portions.  
         [0049]    Next, an explanation will be given of the operation of the electrophotographic apparatus composed as above. Besides, the operation itself for the image formation on the paper is similar to that of the conventional electrophotographic apparatus so that an explanation thereof is omitted. Here, an explanation will mainly be given of the operation of the paper carry for the both sides printing.  
         [0050]    [0050]FIG. 3 is a timing drawing showing the timing of the paper carry of the case where the both sides printing is performed by using the paper stored in the paper cassette PC 3  in FIG. 1.  
         [0051]    An explanation will be given of the operation of each portion at the time with reference to FIG. 3.  
         [0052]    At first, the CPU  32  starts up the main motor  16  when a start of printing operation is requested by the main CPU (not shown). However, the CPU  32  keeps the first to the fourth feeding clutches  17  to  20  and the RGT clutch  21  under an intercepted state at the time. Owing to this, the photosensitive drum  1  and the fixing roller  5  start the rotation here (timing T 1 ).  
         [0053]    After this, when performing the image formation becomes possible, the CPU  32  rotates the third feeding roller  8  and the fourth feeding roller  9  by making the third feeding clutch  19  and the fourth feeding clutch  20  into a transmission state, and feeds the first sheet of paper to the RGT roller  10  (period PA).  
         [0054]    Now, when the paper is carried by the third feeding roller  8  and the fourth feeding roller  9 , the TR 2  sensor  27 , the TR 1  sensor  26 , and the RGT sensor  28  are turned-on in order (timings T 2 , T 3 , and T 4 ). Then, the CUP  32  makes the RGT clutch  21  into the transmission state and starts the rotation of the RGT roller  10  responding to the fact that the RGT sensor  28  is turned-on (timing T 5 ). In this manner, the first sheet of paper is taken-in to the image formation section  102  by the rotation of the RGT roller  10 , and the image formation on the obverse side of the paper (the side facing to the photosensitive drum  1  at the time) is performed. Then, if the back edge of the first sheet of paper passes through the RGT sensor  28  and the RGT sensor  28  is turned-off (timing T 6 ), the CPU  32  makes the RGT clutch  21  into the intercepted state and stops the rotation of the RGT roller  10  at about the timing at which the back edge of the first sheet of paper passes thorough the RGT roller  10  (timing T 7 ). At the time, the first sheet of paper is already nipped and held by the fixing roller  5 , and is carried toward the ejecting roller  11  by the fixing roller  5 .  
         [0055]    When the first sheet of paper arrives at the feed sensor  29  and the ejection sensor  29  is turned-on, the CPU  32  reverses the ADU motor  22  during the period where the ejection sensor  29  continues to be on (period PB). In this manner, during the period PB, the first sheet of paper is taken-in to the ejection/return section  103 , and the ejecting roller is suspended under the state where the back edge of the first sheet of paper is nipped and held by the ejecting roller  11 .  
         [0056]    Although the first sheet of paper is carried as above, when the back edge of the first sheet of paper passes through the TR 1  sensor  26  and the TR 1  sensor  26  is turned-off, the CPU  32  rotates the third feeding roller  8  and the fourth feeding roller  9  by making the third feeding clutch  19  and the fourth feeding clutch  20  into the transmission state, and feeds the second sheet of paper to the RGT roller  10  (period PC). That is to say, the second sheet of paper is fed to the RGT roller  10  at the same time as carrying the first sheet of paper from the image formation section  102  to the ejection/return section  103 .  
         [0057]    Now, if the period PB during which the ejection sensor  29  is turned-on by the first sheet of paper ends, the CPU  32  then starts the rotation of the ADU motor  22  from the timing at which some time has passed (timing T 8 ). Owing to this, the first sheet of paper nipped and held by the ejecting roller  11  is fed from the ejection/return section  103  to the paper side reversal section  104  by the ejecting roller  11 . The mash direction of the one-way clutches  23 ,  24 , and  25  is the normal rotation so that the ADU rollers  13 ,  14 , and  15  also rotate at the time, and the first sheet of paper fed by the ejecting roller  11  is pulled-in to the paper side reversal section  104  by these ADU rollers  13 ,  14 , and  15 .  
         [0058]    On the other hand, the CPU  32  starts the rotation of the RGT roller  10  responding to the fact that a fixed time t (for example, 0.5 second) has passed from the timing T 8  at which the normal rotation of the ADU motor  22  starts (timing T 9 ). Owing to this, the second sheet of paper carried to the RGT roller  10  is taken-in to the image formation section  102 , and the image formation on the obverse side is performed. Besides, although the timing at which the rotation of the RGT roller  10  is started is determined on the basis of the timing besides an ON-timing of the RGT sensor  28 , it is only for the case where the second sheet of paper is fed. After this, the CPU  32  performs a rotation control of the RGT roller  10  in accordance with the state of the RGT sensor  28  at the timing that is similar to relations between the above-mentioned a timings T 4  and T 5 , and between the above-mentioned timings T 6  and T 7 .  
         [0059]    In this manner, when the second sheet of paper is carried, and it arrives at and turns on the ejection sensor  29 , the CPU  32  makes the ADU motor  22  start the reversal rotation (timing T 10 ). Owing to this, the normal rotation of the ADU motor  22  is finished, and the carry of the first sheet of paper from the ejection/return section  103  to the paper side reversal section  104  is performed during the period from the timing T 8  to the timing T 10 , during which the ADU motor  22  is normally-rotating (period PD). Besides, the fixed period t is a waiting time for adjusting the period PD to the length in which the entire first sheet of paper can surely be carried to the paper side reversal section  104 , and it is set as it is considered appropriate by considering a paper carry speed or a dimension condition among the respective rollers.  
         [0060]    Then, the CPU  32  reverses the ADU motor  22  while there continues the state where the ejection sensor  29  is turned-on by the second sheet of paper (period PE). In this manner, the ejecting roller  11  is suspended under the state where the second sheet of paper is taken-in to the ejection/return section  103 , and the back edge of the second sheet of paper is nipped and held by the ejecting roller  11  during the period PE. At the time, although the first sheet of paper exists in the paper side reversal section  104  and is nipped and held by at least any one of the ADU rollers  13 ,  14 , and  15 , the first sheet of paper stays at the paper side reversal section  104  since these ADU rollers  13 ,  14 , and  15  are stopped due to the fact that the reverse rotation of the ADU motor  22  is in the idle-rotation direction of the one-way clutches  23 ,  24 , and  15 .  
         [0061]    If the period PE during which the ejection sensor  29  is turned-on by the second sheet of paper ends, the CPU  32  then starts the normal rotation of the ADU motor  22  from the timing at which some time has passed (timing T 11 ). Owing to this, the second sheet of paper nipped and held by the ejecting roller  11  is fed from the ejection/return section  103  to the paper side reversal section  104  by the ejecting roller  11 . Since the ADU rollers  13 ,  14 , and  15  also rotate at the time, the second sheet of paper that is fed by the ejecting roller  11  is pulled-in to the paper side reversal section  104  by these ADU rollers  13 ,  14 , and  15 .  
         [0062]    Moreover, the ADU rollers  13 ,  14 , and  15  rotate in this manner so that the first sheet of paper stacked at the paper side reversal section  104  is delivered from the paper surface reversing section  104  by the ADU rollers  13 ,  14 , and  15 . Then, responding to the fact that the first sheet of paper arrives at the RGT sensor  28 , there are performed taking-in the paper to the image formation section  102 , the image formation on the paper, and delivery of the paper from the image formation section  102  in a similar manner to above. At the time, the side of the paper facing the photo-sensitive drum  1  is a reverse side of the side that passed through the image formation section  102  at the last time, and the image formation on the reverse side is performed. Then, until when the first sheet of paper arrives at the ejection sensor  29  and the ejection sensor  29  is turned-on (timing T 12 ), there is continued the state where pulling-in the second sheet of paper to the paper side reversal section  104  as above and the delivery of the first sheet of paper from the paper side reversal section  104  are performed (period PF).  
         [0063]    Now, when the back edge of the first sheet of paper fed to the image formation section  102  for the image formation on the reverse side passes through the RGT sensor  28  and the RGT sensor  28  is turned-off (timing T 13 ), the CPU  32  rotates the third feeding roller  8  and the fourth feeding roller  9  responding to it, and feeds the third sheet of paper to the RGT roller  10  (period PG). In this manner, at the same time as performing the image formation on the reverse side of the first sheet of paper and the delivery of the first sheet of paper from the image formation section  102 , there are performed the feed of the third sheet of paper, taking-in the third sheet of paper to the image formation section  102 , and the image formation on the obverse side of the third sheet of paper.  
         [0064]    At the time, the CPU  32  reverses the ADU motor  22  sometime between the timing T 12  at which the first sheet of paper arrives at the ejection sensor  29  and the timing (timing T 14 ) at which the ejection sensor  29  is turned-off at the second time since the timing T 12 . That is to say, the CPU  32  reverses the ADU motor  22  sometime between the time at which the first sheet of paper arrives at the ejection sensor  29  and the time at which the back edge of the third sheet of paper passes through the ejection sensor  29 . As a result, even after the back edge of the first sheet of paper reaches to the ejecting roller  11 , the ejecting roller  11  continues the reverse rotation so that the first sheet of paper on which the image formation on the reverse side is completed, that is to say, the first sheet of paper on which the both sides printing is completed is ejected to the stacker ST by the ejecting roller  11  (period PH). Then, the third sheet of paper that reaches to the ejecting roller  11  following the first sheet of paper is taken-in to the eject/return section  103  by the ejecting roller  11  and is stopped under the state where the back edge thereof is nipped and held by the ejecting roller  11  (period PI).  
         [0065]    Although the second sheet of paper is taken-in to the paper side reversal section  104  at the start timing T 12  during the period PH, the paper stays at the paper side reversal section  104  during the period PH and the period PI since the ADU rollers  13 ,  14 , and  15  do not rotate.  
         [0066]    Hereinafter, at similar timings to the relation of the above-mentioned carry timings of the first, second, and third sheets of paper, there are repeatedly performed the image formation on the reverse side of an n-th sheet of paper and ejection thereof, taking-in an n+1-th sheet of paper to the paper side reversal section  104 , the feed of an n+2-th sheet of paper, the image formation on the obverse side of the n+2-th sheet of paper, and taking-in the n+2-th sheet of paper to the ejection/return section  103 .  
         [0067]    In this manner, according to this embodiment, the image formation operation by circulating two sheets of paper can be performed all the time at the image formation section  102 , the ejection/return section  103 , and the paper side reversal section  104  at the time of performing the both sides printing, and the high-speed both sides printing is possible.  
         [0068]    In addition, since this embodiment is designed so that the drive of the ADU rollers  13 ,  14 , and  15  for performing the paper carry at the paper side reversal section  104  will be performed by the ADU motor  22  for driving the ejecting roller  11 , a structure thereof is simpler and the process burden of the CPU  32  is less than the case where the drive of the ejecting roller  11  and the drive of the ADU rollers  13 ,  14 , and  15  are performed by individual motors.  
         [0069]    Moreover, in this embodiment, clutch control is unnecessary since the one-way clutches  23 ,  24 , and  25  are used in order to make the rotation of the ADU rollers  13 ,  14 , and  15  only in one direction so that the burden to the CPU  32  is less.  
         [0070]    Besides, the present invention is not limited by the above-mentioned embodiment. For example, although driving force transmission to the ADU rollers  13 ,  14 , and  15  is performed via the one-way clutches  23 ,  24 , and  25  in the above-mentioned embodiment, other sorts of clutches may also be used. However only, in that case, it is necessary to additionally perform a new clutch control by using the CPU  32 .  
         [0071]    Moreover, although it is assumed that three ADU rollers  13 ,  14 , and  15  are provided to the paper side reversal section  104 , and the one-way clutches  23 ,  24 , and  25  are provided to all of these three ADU rollers  13 ,  14 , and  15  in the above-mentioned embodiment, a number of the rollers provided to the paper side reversal section  104  may be an optional, and moreover, it is sufficient that the one-way clutches are provided only to the rollers having a possibility to nip and hold the paper that is kept at the paper side reversal section  104 .  
         [0072]    Moreover, factors such as the position relations of various sorts of roller or various sorts of sensor, or a shape of the paper carry route can optionally be changed as long as it does not mar the original function, and the concrete timings for the paper carry are to be changed in accordance with these functional structures.  
         [0073]    Moreover, although there is shown an example in which the image forming apparatus according to the present invention is applied to the electrophotographic apparatus in the above-mentioned embodiment, a method of the image formation may be optional, and the present invention is not limited to the application to the electrophotographic apparatus.  
         [0074]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.