Patent Publication Number: US-8543038-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT 
     The present invention relates to an image forming apparatus. More specifically, the present invention relates to an image forming apparatus for transferring toner images formed on a plurality of image supporting members to a transfer member to form a color image. 
     In a conventional image forming apparatus of an electro-photography type such as a printer, a facsimile, a copier, a multi function product (MFP), and the likes, there is provided a printing mechanism. In the printing mechanism, after a charging roller charges an image supporting member (a photosensitive drum), an exposure unit exposes the photosensitive drum to form a static latent image thereon. After the static latent image is formed on the photosensitive drum, a developing roller attaches toner or developer to the static latent image with an electrostatic force to form or develop a toner image. Afterward, a transfer roller transfers or prints the toner image to a printing medium. 
     In the conventional image forming apparatus, a plurality of the printing mechanisms is provided for forming toner images in colors. A transportation belt or a transfer belt is provided for transporting the printing medium, and the toner images in colors formed with the printing mechanisms are transferred and overlapped on the printing medium, thereby forming a color image. 
     In the conventional image forming apparatus described above, when a color shift (i.e., a shift in printing positions of the toner images in colors) occurs, printing quality is deteriorated. In order to prevent the color shift, in the conventional image forming apparatus, when the toner images in each color are transferred and overlapped on the transportation belt, it is configured to detect an amount of the shift in the printing positions of the toner images in each color on the transportation belt. Accordingly, an operation of the printing mechanisms is controlled according to the amount of the shift in the printing positions thus detected (refer to Patent Reference). Patent Reference: Japanese Patent Publication No. 2001-134041 
     In the conventional image forming apparatus described above, a gear is generally provided for engaging with another gear provided on the photosensitive drum to drive the photosensitive drum as the image supporting member. The gear is normally formed of a multiple stage (for example, a two-stage gear) for decelerating and transmitting rotational drive from a drive source to the photosensitive drum. The gear for driving the photosensitive drum tends to exhibit a specific characteristic such as a deviation due to a dimensional accuracy of the gears engaging with other or a waggle tolerance of the gear from an axial center thereof. 
     When the gear for driving the photosensitive drum has the specific characteristic or a gear specific characteristic, the gear specific characteristic tends to influence on a rotation of the photosensitive drum, thereby causing a variation in the rotation of the photosensitive drum. The variation in the rotation of the photosensitive drum may be transmitted to the transportation belt. When the variation is transmitted to the transportation belt, a moving speed of the transportation belt may be fluctuated, thereby causing the color shift (i.e., the shift in the printing positions of the toner images in each color). Accordingly, the conventional image forming apparatus has the configuration in which the color shift tends to easily occur due to the influence of the gear specific characteristic. 
     In the conventional image forming apparatus described above, when a pitch distance is set at an optimal level, it is possible to prevent the color shift through a simple control of the printing mechanisms. More specifically, when the pitch distance is set at the optimal level, a transportation distance of the printing medium relative to a cycle of a speed fluctuation of the photosensitive drum becomes a fraction of an integer of the pitch distance. 
     In the conventional image forming apparatus described above, however, it is difficult to set the pitch distance at the optimal level due to a restriction of an apparatus size and a cost. In this case, since the pitch distance is not set at the optimal level, it is difficult to prevent the color shift through a simple control of the printing mechanisms. In other words, in the conventional image forming apparatus described above, when it is difficult to set the pitch distance at the optimal level, it is difficult to prevent the color shift through a simple control of the printing mechanisms. 
     In view of the problems described above, an object of the present invention is to provide an image forming apparatus capable of solving the problems of the conventional image forming apparatus and preventing the color shift through a simple control of printing mechanisms even when it is difficult to set a pitch distance at an optimal level. 
     Further objects and advantages of the invention will be apparent from the following description of the invention. 
     SUMMARY OF THE INVENTION 
     In order to attain the objects described above, according to a first aspect of the present invention, an image forming apparatus includes a first image supporting member for forming a first image; a second image supporting member arranged adjacent to the first image supporting member for forming a second image; a first drive unit for driving the first image supporting member through a first driving force having a first phase; a second drive unit for driving the second image supporting member through a second driving force having a second phase shifted from the first phase; and a transfer unit for transferring the first image and the second image to a printing medium. 
     According to a second aspect of the present invention, an image forming apparatus includes a first image supporting member for forming a first image; a second image supporting member arranged adjacent to the first image supporting member for forming a second image; a first drive unit including a first gear for driving the first image supporting member, said first gear having a first rotational phase; a second drive unit including a second gear for driving the second image supporting member, said second gear having a second rotational phase shifted from the first rotational phase by an angle of about 180 degrees; and a transfer unit including a transportation belt for transferring the first image and the second image to a printing medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view showing an image forming apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a schematic view showing a configuration around a transportation belt of the image forming apparatus according to the first embodiment of the present invention; 
         FIG. 3  is a schematic view showing a configuration of gears for driving an image supporting member of the image forming apparatus according to the first embodiment of the present invention; 
         FIG. 4  is a schematic view showing a rotational phase of the gears of the image forming apparatus according to the first embodiment of the present invention; 
         FIG. 5  is a block diagram showing a configuration of a control system of the image forming apparatus according to the first embodiment of the present invention; 
         FIG. 6  is a graph No.  1  showing a speed fluctuation of a transportation belt of a conventional image forming apparatus; 
         FIG. 7  is a graph No.  2  showing the speed fluctuation of the transportation belt of the conventional image forming apparatus; 
         FIG. 8  is a graph No.  3  showing the speed fluctuation of the transportation belt of a conventional image forming apparatus; 
         FIG. 9  is a graph showing a speed fluctuation of the transportation belt of the image forming apparatus according to the first embodiment of the present invention; 
         FIG. 10  is a schematic view showing a rotational phase of gears of an image forming apparatus according to a second embodiment of the present invention; 
         FIG. 11  is a graph No.  1  showing the speed fluctuation of the transportation belt of the image forming apparatus according to the first embodiment of the present invention; and 
         FIG. 12  is a graph No.  2  showing a speed fluctuation of a transportation belt of the image forming apparatus according to the second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereunder, embodiments of the present invention will be explained with reference to the accompanying drawings. In the following description, the accompanying drawings merely show embodiments of the present invention for describing the present invention. Accordingly, the present invention is not limited to the embodiments shown in the accompanying drawings. Similar components and similar elements are designated with the same reference numerals (or the same reference numerals with an alphabet at an end thereof), and explanations thereof are omitted. 
     First Embodiment 
     A first embodiment of the present invention will be explained with reference of  FIGS. 1 to 5 . In the following description, it is supposed that an image forming apparatus  1  is a printer of a tandem type using a color electro-photography recording process. 
     First, a configuration of the image forming apparatus  1  according to the first embodiment of the present invention will be explained.  FIG. 1  is a schematic sectional view showing an entire configuration of the image forming apparatus  1  according to the first embodiment of the present invention. 
     As shown in  FIG. 1 , the image forming apparatus  1  includes a sheet supply cassette  2 , so that a printing medium  100  is retained in the sheet supply cassette  2 . A sheet receiver  3  is disposed inside the sheet supply cassette  2  for supporting the printing medium  100 . When a pushup spring (not shown) pushes up the sheet receiver  3 , the printing medium  100  contacts with a sheet supply roller  5  and a pressure roller  6 . 
     In the embodiment, the sheet supply roller  5  is provided for picking up the printing medium  100  from the sheet supply cassette  2 . The pressure roller  6  is disposed at a position facing the sheet supply roller  5 , and rotates following rotations of the sheet supply roller  5 . The sheet supply roller  5  and the pressure roller  6  guide the printing medium  100 , so that the printing medium  100  is transported from the sheet supply cassette  2  toward a transportation path. 
     In the embodiment, after the printing medium  100  is picked up, a register roller  7  and a pressure roller  8  guide the printing medium  100 , so that the printing medium  100  is further transported along the transportation path. The register roller  7  is provided for transporting the printing medium  100 . The pressure roller  8  is disposed at a position facing the register roller  7 , and rotates following rotations of the register roller  7 . As shown in  FIG. 1 , a register roller  7   a  and a pressure roller  8   a  are disposed on an upstream side of the transportation path, and a register roller  7   b  and a pressure roller  8   b  are disposed on a downstream side of the transportation path. 
     In the embodiment, the register roller  7  transports the printing medium  100  through a sheet sensor  60   a  to a transfer belt unit  105 . The sheet sensor  60   a  is provided for detecting a leading edge and a trailing edge of the printing medium  100 . The sheet sensor  60   a  also functions as a writing sensor for defining a writing position of the printing medium  100  (a transfer position of a toner image). 
     In the embodiment, the transfer belt unit  105  functions as a mechanism for moving a transportation belt  12  as a transfer belt. The transfer belt unit  105  includes a belt drive roller  10 ; a belt idle roller  11 ; the transfer belt  12 ; transfer rollers  13 ; a spring  25 ; a cleaning blade  23 ; a collected toner box  24 ; springs  51 ; a belt drive motor  201  (refer to  FIG. 2 ); and the like. 
     In the embodiment, the transportation belt  12  functions as a belt for transporting the printing medium  100 , and also functions as the transfer belt where the toner image is transferred. The transportation belt  12  is formed of a film member without an end portion. Components such as the belt drive roller  10 , the belt idle roller  11 , and the transfer rollers  13  extend the transportation belt  12  between the belt drive roller  10  and the belt idle roller  11 . When the belt drive motor  201  drives and rotates the belt drive roller  10 , the transportation belt  12  moves through a frictional force with the belt drive roller  10 . 
     In the embodiment, the transfer rollers  13  are provided for applying a transfer voltage to a transfer medium (the printing medium  100  and the transportation belt  12 ), so that a developer image (the toner image) is transferred. The transfer rollers  13  are disposed inside the transportation belt  12  without the end portion. The transfer rollers  13  are arranged at a plurality of locations (four locations in the embodiment) corresponding to photosensitive drums  17  in each color. 
     In the embodiment, the spring  25  functions as an urging member for urging the belt idle roller  11 . More specifically, the spring  25  urges the belt idle roller  11  in a direction away from the belt drive roller  10 , so that the transportation belt  12  is extended without being loosen. Accordingly, the spring  25  applies tension to the transportation belt  12 . 
     In the embodiment, the cleaning blade  23  is provided for scraping off developer (toner) transferred to a surface of the transportation belt  12 . The collected toner box  24  is provided for collecting toner scraped off with the cleaning blade  23 . 
     In the embodiment, the springs  51  function as an urging member for urging the transfer rollers  13 . More specifically, the springs  51  urge the transfer rollers  13  toward the photosensitive drums  17  (described later). The belt drive motor  201  (refer to  FIG. 2 ) functions as a drive unit for driving the belt drive roller  10  to rotate, so that the transportation belt  12  moves. 
     In the embodiment, an exposure unit  35  and an image drum unit  101  (referred to as an ID unit  101 ) are arranged above each of the transfer rollers  13 . The exposure unit  35  is provided for exposing the photosensitive drum  17  of the ID unit  101 , so that a static latent image is formed on the photosensitive drum  17 . The ID unit  101  is provided for forming an image on the photosensitive drum  17 . 
     In the embodiment, the ID unit  101  includes the photosensitive drum  17 , a charging roller  30 , a supply roller  26 , a developing roller  21 , and the like. The photosensitive drum  17  functions as the image supporting member on which the static latent image is formed. The charging roller  30  is provided for supplying a specific charge to the photosensitive drum  17 , so that the photosensitive drum  17  is charged. The supply roller  26  is provided for supplying a sufficient amount of toner to the photosensitive drum  17 . The developing roller  21  is provided for charging toner, so that toner is attached to the static latent image formed on the photosensitive drum  17  to form the toner image with a specific layer thickness. 
     In the embodiment, the photosensitive drum  17  is disposed above the transportation belt  12 , so that the photosensitive drum  17  faces the transfer roller  13  with the transportation belt  12  in between. The charging roller  30 , the supply roller  26 , and the developing roller  21  are disposed around the photosensitive drum  17 , so that the charging roller  30 , the supply roller  26 , and the developing roller  21  are pressed against the photosensitive drum  17 . 
     In the embodiment, the image forming apparatus  1  includes four ID units  101  corresponding to colors of black (B), yellow (Y), magenta (M), and cyan (C). Accordingly, the ID units  101  constitute an image forming portion  110  for forming a color image. The ID units  101  have an identical configuration except retaining toner in a different color. 
     In the following description, it is supposed that the ID units  101  are produced using identical components through a same manufacturing process, so that the ID units  101  have an identical operational characteristic. When it is necessary to differentiate a component corresponding to each color, the component is designated with a reference numeral attached with an alphabetical letter B, Y, M, or C such as the ID unit  101 B,  101 Y,  101 M, or  101 C. 
     In the image forming apparatus  1 , when the printing medium  100  is transported to the transfer belt unit  105 , the belt drive motor  201  (refer to  FIG. 2 ) is driven to move the transportation belt  12 . Accordingly, the printing medium  100  passes through under the ID units  101 B,  101 Y,  101 M, and  101 C. At this moment, in the image forming apparatus  1 , after the charging rollers  30  charge the photosensitive drums  17  as the image supporting members, the exposure units  35  expose the photosensitive drums  17  to form the static latent images thereon. Then, the developing rollers  21  attach toner to the static latent images through an electro-static force to form the toner images, so that the transfer rollers  13  transfer (print) the toner images to the printing medium  100 . Through the process described above, the color image is formed on the printing medium  100 . 
     After the color image is formed on the printing medium  100 , the transfer belt unit  105  transports the printing medium  100  to a fixing unit  106 . The fixing unit  106  includes a fixing roller  39 , a pressing roller  40 , a halogen lamp  41 , a fixing unit drive motor  204  (refer to  FIG. 2 ), and the like. The fixing roller  39  is provided for fixing the toner images to the printing medium  100 . The pressing roller  40  is arranged to face the fixing roller  39 , and rotates following a rotation of the fixing roller  39 . The halogen lamp  41  is disposed inside the fixing roller  39  as a heating source for heating the printing medium  100 . The fixing unit drive motor  204  is a drive unit for driving the fixing roller  39  to rotate. 
     In the image forming apparatus  1 , the halogen lamp  41  heats the printing medium  100 , and the fixing roller  39  and the pressing roller  40  press and transport the printing medium  100 , so that the toner images transferred to the printing medium  100  are melted and fixed to the printing medium  100 . 
     After the toner images are fixed to the printing medium  100 , the fixing unit  106  transports the printing medium  100  to a discharge roller  43  and a discharge roller  44 . The discharge roller  43  transports and discharges the printing medium  100  to a facedown stacker  49 . The discharge roller  44  is arranged to face the discharge roller  43 , and rotates following a rotation of the discharge roller  43 . More specifically, a discharge roller  43   a  and a discharge roller  44   a  are disposed on an upstream side of the transportation path, and a discharge roller  43   b  and a discharge roller  44   b  are disposed on a downstream side of the transportation path. Accordingly, the discharge roller  43  transports and discharges the printing medium  100  to the facedown stacker  49 . 
     A configuration around the transportation belt  12  of the image forming apparatus  1  will be explained next with reference to  FIG. 2 .  FIG. 2  is a schematic view showing the configuration around the transportation belt  12  of the image forming apparatus  1  according to the first embodiment of the present invention. 
     As shown in  FIG. 2 , in the image forming apparatus  1 , the transportation belt  12  is extended between the belt drive roller  10  and the belt idle roller  11 . A drive of the belt drive motor  201  is transmitted to the belt drive roller  10  through a drive transmission mechanism (not shown). Accordingly, the belt drive roller  10  rotates, so that the transportation belt  12  moves through friction between the transportation belt  12  and the belt drive roller  10 . 
     As described above, in the image forming apparatus  1 , the transfer rollers  13  corresponding to colors of black (B), yellow (Y), magenta (M), and cyan (C) are disposed inside the loop of the transportation belt  12  without the end portion. Further, the photosensitive drums  17 B,  17 Y,  17 M, and  17 C of the ID units  101  (refer to  FIG. 1 ) are disposed at an upper portion of the image forming apparatus  1  to face the transfer rollers  13 , respectively. 
     In the embodiment, the photosensitive drums  17 B,  17 Y,  17 M, and  17 C of the ID units  101  have an outer circumferential length Lc. Further, the ID units  101  are arranged with a pitch distance Lp (an distance between axes of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C). A drive of an ID unit drive motor  202  is transmitted to the photosensitive drums  17 B,  17 Y,  17 M, and  17 C through a drive transmission mechanism (not shown). 
     In the embodiment, the ID units  101  are positioned at a specific location in a lateral direction with a side plate (not shown). The exposure unit  35  is situated above each of the ID units  101 . The exposure units  35  are positioned at specific locations in a lateral direction with the side plate (not shown) of the ID units  101 . 
     A configuration of gears for driving the image supporting members (the photosensitive drums  17 ) of the image forming apparatus  1  will be explained next with reference to  FIGS. 3 and 4 . 
       FIG. 3  is a schematic view showing the configuration of the gears for driving the image supporting members  17  of the image forming apparatus  1  according to the first embodiment of the present invention. More specifically,  FIG. 3  is a schematic perspective view showing an example of a drive unit for driving the photosensitive drums  17 . 
       FIG. 4  is a schematic view showing a rotational phase of the gears of the image forming apparatus  1  according to the first embodiment of the present invention. More specifically,  FIG. 4  is a schematic view showing rotational phases of two gears for driving the photosensitive drums  17 . 
     As shown in  FIG. 3 , the image forming apparatus  1  includes the ID unit drive motor  202  and four deceleration idle gears  65  (or deceleration gears  65 ) corresponding to the photosensitive drums  17  in each color. Each of the deceleration idle gears  65  functions as a transmission member for decelerating and transmitting the drive of the ID unit drive motor  202  to the photosensitive drum  17 . Each of the deceleration idle gears  65  includes a large gear  66  and a small gear  67 . 
     In the embodiment, the large gear  66  engages a motor gear  203  for transmitting the drive of the ID unit drive motor  202  and an idle gear  70  or a connecting gear  70  for connecting the deceleration idle gears  65 . The connecting gears  70  are disposed at three locations along the transportation direction of the printing medium  100 . The connecting gear  70  disposed at a first row of the three locations connects the photosensitive drum  17 B at a first row and the photosensitive drum  17 Y at a second row. The connecting gear  70  disposed at a second row of the three locations connects the photosensitive drum  17 Y at the second row and the photosensitive drum  17 M at a third row. The connecting gear  70  disposed at a third row of the three locations connects the photosensitive drum  17 M at the third row and the photosensitive drum  17 C at a fourth row. 
     In the following description, the connecting gears  70  are designated as the connecting gears  70   a ,  70   b , and  70   c  when it is necessary to be differentiated for each color. The connecting gears  70   a ,  70   b , and  70   c  have an identical diameter. 
     In the embodiment, the small gear  67  engages a gear  18  or a photosensitive drum gear  18  disposed along an outer circumference of the photosensitive drums  17  at an end portion thereof. The photosensitive drum gears  18  of the photosensitive drums  17  have an identical diameter. 
     In the embodiment, the image forming apparatus  1  includes four drive units. The first drive unit is formed of a photosensitive drum gear  18 B for driving the photosensitive drum  17 B, a deceleration gear  65 B as a first gear, and the motor gear  202 . The second drive unit is formed of a photosensitive drum gear  18 Y for driving the photosensitive drum  17 Y, a deceleration gear  65 Y as a second gear, and the connecting gear  70   a.    
     Similarly, the third drive unit is formed of a photosensitive drum gear  18 M for driving the photosensitive drum  17 M, a deceleration gear  65 M as a third gear, and the connecting gear  70   b . The fourth drive unit is formed of a photosensitive drum gear  18 C for driving the photosensitive drum  17 C, a deceleration gear  65 C as a fourth gear, and the connecting gear  70   c.    
     In the embodiment, each of the deceleration idle gears  65  has a mark  120  (refer to  FIG. 4 ) as a standard for aligning a position of the rotational phase thereof. Accordingly, it is possible to visually confirm a shift of rotational phases of adjacent gears. More specifically, marks  120 B,  120 Y,  120 M, and  120 C are disposed at a same position (a same teeth position) when the deceleration gears  65  are produced. Each of the deceleration idle gears  65  is produced using a mold metal having a same dimension. 
     As shown in  FIG. 3 , in the image forming apparatus  1 , the ID unit drive motor  202  is connected to the large gear  66 B of the deceleration gear  65 B at the first row through a motor gear  203 . Accordingly, in the image forming apparatus  1 , the rotational drive of the ID unit drive motor  202  is transmitted to the large gear  66 B of the deceleration gear  65 B at the first row through the motor gear  203 . 
     When the rotational drive of the ID unit drive motor  202  is transmitted to the large gear  66 B of the deceleration gear  65 B at the first row, the rotational drive is decelerated according to a gear ratio between the small gear  67 B and the photosensitive drum gear  18 B, and is transmitted to the photosensitive drum  17 B. Accordingly, the photosensitive drum  17 B at the first row rotates. 
     In the embodiment, the rotational drive of the ID unit drive motor  202  transmitted to the large gear  66 B of the deceleration gear  65 B at the first row is transmitted to the large gear  66 Y of the deceleration gear  66 Y at the second row through the connecting gear  70   a  at the first row. 
     When the rotational drive is transmitted of the ID unit drive motor  202  to the large gear  66 Y of the deceleration gear  66 Y at the second row, the rotational drive is decelerated according to a gear ratio between the small gear  67 Y and the photosensitive drum gear  18 Y, and is transmitted to the photosensitive drum  17 Y. Accordingly, the photosensitive drum  17 Y at the second row rotates. 
     Similarly, in the image forming apparatus  1 , the rotational drive of the ID unit drive motor  202  is transmitted to the photosensitive drum  17 M at the third row, so that the photosensitive drum  17 M at the third row rotates. Further, the rotational drive of the ID unit drive motor  202  is transmitted to the photosensitive drum  17 C at the fourth row, so that the photosensitive drum  17 C at the fourth row rotates. 
     In the embodiment, the image forming apparatus  1  is configured such that the gears for driving the image supporting members or the photosensitive drums  17  have the rotational phases as shown in  FIG. 4 . 
     More specifically, in the image forming apparatus  1 , the photosensitive drums  17 B and  17 Y form one pair, and the photosensitive drums  17 M and  17 C form another pair. In the one pair, the photosensitive drums  17 B and  17 Y have the deceleration gears  65  or the gears for driving the photosensitive drums  17 B and  17 Y having the rotational phases shifted by about 180 degrees (180 degrees plus minus 20 degrees) with each other. Accordingly, in the image forming apparatus  1 , the photosensitive drums  17 B and  17 Y forming the one pair have inverted phases of variation cycles in the rotational speeds thereof with each other at abutting positions with respect to the transportation belt  12 . 
     A configuration of a control system of the image forming apparatus  1  will be explained next with reference to  FIG. 5 .  FIG. 5  is a block diagram showing the configuration of the control system of the image forming apparatus  1  according to the first embodiment of the present invention. 
     As shown in  FIG. 5 , as functional units of the control system, the image forming apparatus  1  includes a control unit  300 , a sheet supply transportation control unit  302 , an image forming control unit  303 , a belt drive control unit  304 , a fixing unit drive control unit  306 , and the likes. 
     In the embodiment, the control unit  300  is a functional unit provided for controlling an entire operation of the image forming apparatus  1 . The sheet supply transportation control unit  302  is a functional unit provided for controlling an operation of a transportation unit ranging from the sheet supply cassette  2  to the transfer belt unit  105 . As the transportation unit, the image forming apparatus  1  includes a sheet supply roller drive motor  115  for driving the sheet supply roller  5  to rotate, and register roller drive motors  117   a  and  117   b  for driving the register rollers  7   a  and  7   b  to rotate, respectively. 
     In the embodiment, the image forming control unit  303  is a functional unit provided for controlling an operation of the image forming portion  110 . The belt drive control unit  304  is a functional unit provided for controlling an operation of the transfer belt unit  105 . The fixing unit drive control unit  306  is a functional unit provided for controlling an operation of the fixing unit  106 . Each of the functional units is formed of a storage unit such as an ROM and an RAM (not shown) and a CPU. Each of the functional units may be integrated with other functional unit. 
     In the embodiment, the control unit  300  includes a calculation unit  301 . The calculation unit  301  is provided for controlling the sheet supply transportation control unit  302 , the image forming control unit  303 , the belt drive control unit  304 , and the fixing unit drive control unit  306 . 
     In the embodiment, when the calculation unit  301  receives a print instruction from a host device, the calculation unit  301  instructs the sheet supply transportation control unit  302  to perform a sheet supply process and a transportation process. Accordingly, the sheet supply transportation control unit  302  drives the sheet supply roller  5  (refer to  FIG. 1 ) to rotate, thereby performing the sheet supply process. In the sheet supply process, the printing medium  100  placed on the sheet receiver  3  is picked up toward the transportation path and transported to the register roller  7   a.    
     In the next step, the sheet supply transportation control unit  302  performs the transportation process. In the transportation process, the sheet supply transportation control unit  302  drives the register rollers  7   a  and  7   b  to rotate, so that the printing medium  100  is transported toward the downstream side of the transportation path. 
     In the embodiment, in the image forming apparatus  1 , when the printing medium  100  passes through the sheet sensor  60   a , the sheet sensor  60   a  notifies the control unit  300  of a passing timing. Afterward, the control unit  300  notifies the passing timing to the image forming control unit  303  and the belt drive control unit  304 . 
     In the embodiment, the image forming control unit  303  controls an operation of the image forming apparatus  1 , so that the following processes are performed. First, the image forming control unit  303  controls the exposure units  35 B,  35 Y,  35 M, and  35 C to form the static latent images on the photosensitive drums  17 B,  17 Y,  17 M, and  17 C at timings corresponding to installed positions of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C of the ID units  101 . Then, the image forming control unit  303  controls the developing rollers  21 B,  21 Y,  21 M, and  21 C to attach toner to the static latent images through an electrostatic force, thereby forming (developing) the toner images. 
     In the embodiment, the belt drive control unit  304  controls a rotational drive of the belt drive motor  201 . More specifically, the rotational drive of the belt drive motor  201  is transmitted to the belt drive roller  10  through a drive transmission unit (not shown), so that the belt drive roller  10  drives the transportation belt  12  to move through frictional resistance therebetween. The printing medium  100  is attached to the transportation belt  12  through an electrostatic force. Accordingly, when the transportation belt  12  moves, the printing medium  100  is transported toward the downstream side, so that the toner images developed on the photosensitive drums  17 B,  17 Y,  17 M, and  17 C are overlapped and transferred to the printing medium  100 . 
     In the embodiment, after the toner images are transferred to the printing medium  100 , the transportation belt  12  transports the printing medium  100  to the fixing unit  106 . At this moment, the calculation unit  301  instructs the fixing unit drive control unit  306  to perform a fixing process. Accordingly, the fixing unit drive control unit  306  controls the operation of the fixing unit  106  to perform the fixing process as follows. First, the fixing unit drive control unit  306  controls the halogen lamp  41  as the heating source of the fixing unit  106  to turn on to obtain a temperature and a speed instructed with the control unit  300 . 
     In the next step, the fixing unit drive control unit  306  controls the fixing unit drive motor  204  to rotate, so that the fixing roller  39  rotates. When the fixing roller  39  rotates, the fixing roller  39  transports the printing medium  100  to the discharge roller  43   a  and the discharge roller  44   a . Accordingly, the toner images transferred to the printing medium  100  are melted and fixed to the printing medium  100 . Afterward, the discharge roller  43   a , the discharge roller  44   a , the transportation roller  43   b , the transportation roller  44   b , a discharge roller  43   c , and a discharge roller  44   c  sequentially transport the printing medium  100  to the facedown stacker  49 , so that the printing medium  100  is discharged on the facedown stacker  49 . 
     An operation of the photosensitive drums  17  and the transportation belt  12  will be explained next with reference to  FIGS. 2 and 3 . As shown in  FIG. 3 , the ID unit drive motor  202  drives each of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C through each of the deceleration gears  65 B,  65 Y,  65 M, and  65 C and each of the connecting gears  70   a ,  70   b , and  70   c . At this moment, the surfaces of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C cause speed fluctuations or speed variances due to specific characteristic (for example, a deviation due to a dimensional accuracy of the gears engaging with other or a waggle tolerance of the gear from an axial center thereof). 
     When the surfaces of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C cause the speed fluctuations, the speed fluctuations are transmitted to a speed of the transportation belt  12  and a speed of the printing medium  100  through a frictional force therebetween. As a result, a surface of a specific photosensitive drum (the photosensitive drum  17 B at the first row in  FIG. 2 ) has a speed V 1 , while a surface of another photosensitive drum (the photosensitive drum  17 Y at the second row in  FIG. 2 ) has a speed V 2 . At this moment, the transportation belt  12  has a speed VB. 
     A relationship between the speed fluctuations of the surface of the photosensitive drums  17  (referred to as a speed fluctuation of the photosensitive drum  17 ) and the speed fluctuation of the transportation belt  12  (referred to as a speed fluctuation of the transportation belt  12 ) in the image forming apparatus  1  will be explained next with reference to  FIGS. 6 to 9 . 
     As described above, the ID units  101  are produced using the identical components through the same manufacturing process, so that the ID units  101  have the identical operational characteristic. Accordingly, the deceleration gears  65 B,  65 Y,  65 M, and  65 C for driving the photosensitive drums  17 B,  17 Y,  17 M, and  17 C have an identical characteristic. Further, rotations of the deceleration gears  65 B,  65 Y,  65 M, and  65 C are directly transmitted to the photosensitive drums  17 B,  17 Y,  17 M, and  17 C, respectively. Accordingly, when the deceleration gears  65 B,  65 Y,  65 M, and  65 C drive the photosensitive drums  17 B,  17 Y,  17 M, and  17 C, speeds of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C are fluctuated in a similar manner. 
     In the following description, it is supposed that the speed fluctuations of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C and the speed fluctuation of the transportation belt  12  have a cyclic wave shape (a cosign curve in an example shown in  FIG. 9 ) with a cycle T. Further, it is supposed that two of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C constitute one pair. With respect to the two of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C (referred to as a first photosensitive drum and a second photosensitive drum), the relationship between the speed fluctuations of the photosensitive drums  17  and the speed fluctuation of the transportation belt  12  will be explained. 
     In the following description, the photosensitive drums  17 B and  17 Y will be explained as the first photosensitive drum and the second photosensitive drum, respectively. Note that the first photosensitive drum and the second photosensitive drum are not limited to the photosensitive drums  17 B and  17 Y, and may be other photosensitive drums such as the photosensitive drums  17 Y and  17 M. 
       FIG. 6  is a graph No.  1  showing a speed fluctuation of a transportation belt of a conventional image forming apparatus according to a conventional technology.  FIG. 7  is a graph No.  2  showing the speed fluctuation of the transportation belt of the conventional image forming apparatus.  FIG. 8  is a graph No.  3  showing the speed fluctuation of the transportation belt of a conventional image forming apparatus.  FIG. 9  is a graph showing the speed fluctuation of the transportation belt  12  of the image forming apparatus  1  according to the first embodiment of the present invention. 
     More specifically,  FIG. 6  is the graph No.  1  showing an actual measurement result of the speed fluctuation of the transportation belt of the conventional image forming apparatus according to the conventional technology. 
       FIG. 7  is the graph No.  2  showing a schematic wave shape of the speed fluctuation of the transportation belt of the conventional image forming apparatus when the pitch distance Lp is set at an optimal level. When the pitch distance Lp is set at the optimal level, a transportation distance of a printing medium (i.e., a moving distance of the transportation belt) relative to the cycle T of the speed fluctuation of the photosensitive drum becomes a fraction of an integer of the pitch distance.  FIG. 8  is the graph No.  3  showing a schematic wave shape of the speed fluctuation of the transportation belt of the conventional image forming apparatus when the pitch distance Lp is not set at the optimal level. 
     As shown in  FIG. 6 , the speed fluctuation of the transportation belt of the conventional image forming apparatus has a wave shape LB. As shown in  FIGS. 7 and 8 , the speed fluctuation of the first photosensitive drum has a wave shape L 1 , and the speed fluctuation of the second photosensitive drum has a wave shape L 2 . In  FIGS. 7 and 8 , the toner image formed on the first photosensitive drum is transferred at a first transfer position Tr 1 , and the toner image formed on the second photosensitive drum is transferred at a second transfer position Tr 2 . 
       FIG. 9  is the graph showing the speed fluctuation of the transportation belt  12  of the image forming apparatus  1  according to the first embodiment of the present invention as compared with the speed fluctuation of the transportation belt of the conventional image forming apparatus. As shown in  FIG. 9 , the speed fluctuation of the transportation belt  12  of the image forming apparatus  1  has a wave shape LA 1 . 
     As shown in the example shown in  FIG. 7 , the speed fluctuation of the first photosensitive drum and the speed fluctuation of the second photosensitive drum have the constant cycle T (second). Further, the speed fluctuation of the first photosensitive drum and the speed fluctuation of the second photosensitive drum are transmitted to the transportation belt. Accordingly, the speed of the transportation belt is fluctuated at a transmission efficiency (W) corresponding to 5 to 20% of a combined wave shape of the speed fluctuation of the first photosensitive drum and the speed fluctuation of the second photosensitive drum. 
     As a result, as shown in  FIG. 7 , the speed of the transportation belt is fluctuated based on a cosign curve with the cycle T and a vibration amplitude Vo×W, in which Vo is an average speed. More specifically, the speed of the transportation belt is fluctuated based on the cosign curve represented with the following equation (1). Note that  FIG. 7  is the graph No.  2  showing the schematic wave shape of the speed fluctuation of the transportation belt of the conventional image forming apparatus when the pitch distance Lp is set at the optimal level. 
     
       
         
           
             
               
                 
                   
                     V 
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       V 
                       0 
                     
                     ⁢ 
                     W 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       sin 
                       ⁡ 
                       
                         ( 
                         
                           
                             2 
                             ⁢ 
                             π 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             t 
                           
                           T 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In the following description, it is supposed that after the toner image in one color formed on the first photosensitive drum is transferred to the printing medium, the toner image in another color formed on the second photosensitive drum is transferred to the printing medium. At this moment, due to the speed fluctuation of the transportation belt, the two toner images are transferred to positions shifted with each other (that is, the color shift). 
     An amount of the shifted positions of the two toner images (that is, an amount of the color shift) Δx(t) is represented with the following equation (2): 
                     Δ   ⁢           ⁢     x   ⁡     (   t   )         =       ∫     t   ⁢           ⁢   0       t   ⁢           ⁢   1       ⁢       ⅆ     v   ⁡     (   t   )           ⅆ   t                 (   2   )               
where ΔV(t) is the speed fluctuation of the transportation belt; t 0  is a time when the first transfer position Tr 1  of the first photosensitive drum passes through a specific position of the surface of the transportation belt (a specific position of the printing medium transported with the transportation belt); and t 1  is a time when the second transfer position Tr 2  of the second photosensitive drum passes through the specific position of the surface of the transportation belt.
 
     Next, as shown in  FIG. 7 , it is supposed that the fluctuation cycle T of the average speed Vo of the transportation belt becomes a difference between t 1  and t 0  (T=t 1 −t 0 ) or an n times of the fluctuation cycle T becomes the difference (n×T=(t 1 −t 0 )), in which n is a natural number. 
     In this case, the amount of the shifted positions of the two toner images (that is, the amount of the color shift) Δx(t) is represented with the following equation (3): 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             Δ 
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                           = 
                           
                             
                               V 
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                                 sin 
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                   ⁢ 
                   
                     ( 
                     
                       
                         ∵ 
                         
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                               t 
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     In this case, the amount of the shifted positions of the two toner images (that is, the amount of the color shift) Δx(t) becomes zero (Δx(t)=0). Accordingly, the color shift does not occur on the surface of the transportation belt and the surface of the printing medium transported with the transportation belt. 
     In the conventional image forming apparatus, it is difficult to set the pitch distance at the optimal level due to a restriction of an apparatus size and a cost. More specifically, it is difficult to set the transportation distance of the printing medium (the moving distance of the transportation belt) relative to the cycle T of the speed fluctuation of the photosensitive drum to a fraction of an integer of the pitch distance Lp. 
       FIG. 8  is the graph No.  3  showing the schematic wave shape of the speed fluctuation of the transportation belt of the conventional image forming apparatus when the pitch distance Lp is not set at the optimal level. 
     As shown in  FIG. 8 , the fluctuation cycle T of the average speed Vo of the transportation belt is not equal to the difference between t 1  and t 0  (T=t 1 −t 0 ) or the n times of the fluctuation cycle T is not equal to the difference (n×T=(t 1 −t 0 )), in which n is a natural number. In this case, the pitch distance Lp is not equal to the cycle T (Lp≠T), so that the amount of the color shift is not equal to zero. Accordingly, the color shift by the amount Δx(t) occurs on the surface of the transportation belt and the surface of the printing medium transported with the transportation belt. 
     In the embodiment, as shown in  FIG. 9 , the image forming apparatus  1  is configured such that the cycles T of the speed fluctuations of the first photosensitive drum  17 B and the second photosensitive drum  17 Y have inverted phases at a same position on the surface of the transportation belt  12 . 
     More specifically, in the image forming apparatus  1 , the deceleration gear  65 B at the first row for driving the first photosensitive drum  17 B is arranged to have the rotational phase shifted by about 180 degrees with respect to that of the deceleration gear  65 Y at the second row for driving the second photosensitive drum  17 Y. Accordingly, the mark  120 B is in an inverted phase with respect to the mark  120 Y as shown in  FIG. 4 . As a result, as shown in  FIG. 9 , in the image forming apparatus  1 , it is possible to minimize the speed fluctuation of the transportation belt  12  at a minimum level. 
     In the embodiment, similarly, the deceleration gear  65 M at the third row for driving the photosensitive drum  17 M is arranged to have the rotational phase shifted by about 180 degrees with respect to that of the deceleration gear  65 C at the fourth row for driving the photosensitive drum  17 C. Accordingly, the mark  120 M is in an inverted phase with respect to the mark  120 C as shown in  FIG. 4 . As a result, as shown in  FIG. 9 , in the image forming apparatus  1 , it is possible to further minimize the speed fluctuation of the transportation belt  12  at a minimum level. 
     As described above, in the image forming apparatus  1  in the first embodiment, the deceleration gears  65 , which cause the speed fluctuations of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C, have the rotational phases shifted by about 180 degrees with each other. Accordingly, even when it is difficult to set the pitch distance Lp at the optimal level, that is, to set the transportation distance of the printing medium  100  (the moving distance of the transportation belt  12 ) relative to the cycle T of the speed fluctuations of the photosensitive drums  17  to a fraction of an integer of the pitch distance Lp, it is possible to minimize the speed fluctuation of the transportation belt  12  at a minimum level. As a result, it is possible to minimize a reduction in an accuracy of the color shift (an accuracy of overlap printing in each color) to a minimum level. Further, it is possible to prevent the color shift through a simple control of the printing mechanisms. 
     Further, in the image forming apparatus  1  in the first embodiment, it is possible to freely set the rotational phases of one pair of the photosensitive drums relative to another pair (or more than one pair) of the photosensitive drums. Accordingly, for example, even when one pair of the photosensitive drums have a drive gear row separated from that of another pair (or more than one pair) of the photosensitive drums, it is possible to arrange the drive gear row regardless of the phases thereof. 
     Second Embodiment 
     A second embodiment of the present invention will be explained next. A configuration of an image forming apparatus  1   a  will be explained with reference to  FIG. 10 .  FIG. 10  is a schematic view showing a rotational phase of gears for driving the image supporting members  17  of the image forming apparatus la according to the second embodiment of the present invention. 
     In the first embodiment, in the image forming apparatus  1  (refer to  FIG. 4 ), two of the deceleration gears  65 B,  65 Y,  65 M, and  65 C for driving two of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C form one pair, and the two of the deceleration gears  65 B,  65 Y,  65 M, and  65 C have the rotational phases shifted by about 180 degrees with each other. 
     In the second embodiment, in the image forming apparatus  1   a , two of the deceleration gears  65 B,  65 Y,  65 M, and  65 C for driving two of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C form one pair, so that two pairs are formed. Further, one of the two pairs of the deceleration gears  65 B,  65 Y,  65 M, and  65 C have the rotational phases shifted by about 180 degrees with each other, and the other of the two pairs of the deceleration gears  65 B,  65 Y,  65 M, and  65 C have the rotational phases shifted by about 90 degrees relative to those of the one of the two pairs. 
     When the rotational phases of the deceleration gears  65 B,  65 Y,  65 M, and  65 C are shifted by about 90 degrees, a variance in a degree of the shift may occur due to the engagement or accuracy of the gears of the gear rows. In this case, when the shifted angle is within 70 degrees and 110 degrees (90 degrees plus minus 20 degrees), it is possible to obtain a similar effect. 
     A relationship between the speed fluctuations of the photosensitive drums  17  and the speed fluctuation of the transportation belt  12  in the image forming apparatus  1   a  will be explained next with reference to  FIGS. 11 and 12 .  FIGS. 11 and 12  are graphs showing the speed fluctuation of the transportation belt  12 . 
     More specifically,  FIG. 11  is a graph No.  1  showing the speed fluctuation of the transportation belt  12  of the image forming apparatus  1  according to the first embodiment of the present invention as a comparison. As shown in  FIG. 11 , the speed fluctuation of the first photosensitive drum  17 B and the speed fluctuation of the second photosensitive drum  17 Y have the wave shapes with a harmonic component, and the harmonic component is laterally asymmetric. 
       FIG. 12  is a graph No.  2  showing the speed fluctuation of the transportation belt  12  of the image forming apparatus  1   a  according to the second embodiment of the present invention. As shown in  FIG. 12 , the speed fluctuations of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C have laterally asymmetric wave shapes. Note that, in  FIG. 12 , the photosensitive drums  17 B,  17 Y,  17 M, and  17 C are represented as the first photosensitive drum  17 B, the second photosensitive drum  17 Y, the third photosensitive drum  17 M, and the fourth photosensitive drum  17 C, respectively. The first photosensitive drum  17 B, the second photosensitive drum  17 Y, the third photosensitive drum  17 M, and the fourth photosensitive drum  17 C exhibit the speed fluctuations having wave shapes L 1 , L 2 , L 3 , and L 4 , respectively. Further, the transportation belt  12  of the image forming apparatus la exhibits the speed fluctuation having a wave shape LA 2 . 
     As described above, in the first embodiment, in the image forming apparatus  1  (refer to  FIG. 4 ), two of the deceleration gears  65 B,  65 Y,  65 M, and  65 C for driving two of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C form one pair, and the two of the deceleration gears  65 B,  65 Y,  65 M, and  65 C have the rotational phases shifted by about 180 degrees with each other. In other words, the image forming apparatus  1 , among the deceleration gears  65 B,  65 Y,  65 M, and  65 C for driving the photosensitive drums  17 B,  17 Y,  17 M, and  17 C, one pair of the two of the deceleration gears  65 B,  65 Y,  65 M, and  65 C have the rotational phases shifted by about 180 degrees. 
     In this case, as shown in  FIG. 4 , the deceleration gears  65 Y,  65 M, and  65 C are shifted by an equal interval of about 180 degrees as a specific angle with the deceleration gear  65 B as a standard. When the rotational phases of the deceleration gears  65 B,  65 Y,  65 M, and  65 C are shifted by about 180 degrees, a variance in a degree of the shift may occur due to the engagement or accuracy of the gears of the gear rows. In this case, when the shifted angle is within 160 degrees and 200 degrees (180 degrees plus minus 20 degrees), it is possible to obtain a similar effect. 
     Accordingly, in the first embodiment, it is possible to minimize the speed fluctuation of the transportation belt  12  to a minimum level when the speed fluctuations of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C have the wave shapes similar to a cosign curve or an approximate cosign curve. 
     However, in the image forming apparatus  1 , the speed fluctuations of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C may not have the wave shapes similar to a cosign curve or an approximate cosign curve. In this case, as shown in  FIG. 11 , the transportation belt  12  of the image forming apparatus  1  exhibits the speed fluctuation having the wave shape LA 1 . More specifically, the speed fluctuation does not have a flat wave shape when only two of the deceleration gears  65 B,  65 Y,  65 M, and  65 C for driving two of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C have the rotational phases shifted by about 180 degrees with each other. 
     On the other hand, in the image forming apparatus  1   a  in the second embodiment as described above, two of the deceleration gears  65 B,  65 Y,  65 M, and  65 C for driving two of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C form one pair, so that two pairs are formed. Further, one of the two pairs of the deceleration gears  65 B,  65 Y,  65 M, and  65 C have the rotational phases shifted by about 180 degrees with each other, and the other of the two pairs of the deceleration gears  65 B,  65 Y,  65 M, and  65 C have the rotational phases shifted by about 90 degrees relative to those of the one of the two pairs. In other words, in the image forming apparatus  1   a , the deceleration gears  65 B,  65 Y,  65 M, and  65 C for driving the photosensitive drums  17 B,  17 Y,  17 M, and  17 C have the rotational phases shifted by about 90 degrees with each other. 
     Accordingly, even when the speed fluctuations of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C have the laterally asymmetric wave shapes deviated from a cosign curve, the transportation belt  12  of the image forming apparatus  1   a  exhibits the speed fluctuation having the wave shape LA 2  as shown in  FIG. 12 . 
     More specifically, the two pairs of the deceleration gears  65 B,  65 Y,  65 M, and  65 C (the deceleration gears  65 B and  65 Y and the deceleration gears  65 M and  65 C in the example shown in  FIG. 10 ) for driving the photosensitive drums  17 B,  17 Y,  17 M, and  17 C (the photosensitive drums  17 B and  17 Y and the photosensitive drums  17 M and  17 C in the example shown in  FIG. 10 ) have the rotational phases inverted with each other. Accordingly, it is possible to efficiently suppress the speed fluctuation of the transportation belt  12 . 
     As described above, in the image forming apparatus la in the second embodiment, similar to the image forming apparatus  1  in the first embodiment, even when it is difficult to set the pitch distance Lp at the optimal level, it is possible to minimize the speed fluctuation of the transportation belt  12  at a minimum level. As a result, it is possible to minimize a reduction in an accuracy of the color shift (an accuracy of overlap printing in each color) to a minimum level. Further, it is possible to prevent the color shift through a simple control of the printing mechanisms. 
     Further, in the image forming apparatus  1   a  in the second embodiment, as compared with the image forming apparatus  1  in the first embodiment, even when the speed fluctuations of the photosensitive drums  17 B,  17 Y,  17 M, and  17 C have the laterally asymmetric wave shapes deviated from a cosign curve, it is possible to minimize the speed fluctuation of the transportation belt  12  at a minimum level. As a result, without increasing cost, it is possible to improve an accuracy of the color shift (an accuracy of overlap printing in each color) due to the speed fluctuation of the transportation belt  12 . 
     In the embodiments described above, the color printer of the electro-photography type is explained as the image forming apparatus. The present invention is applicable to a monochrome printer, a copier, a facsimile, a multi-function product, and the likes. 
     Further, it is possible to change the order of the colors to be printed (that is, the arrangement of the ID units  101 ). Further, it is possible to change toner in black, yellow, magenta, and cyan retained in the image forming portion  110  to toner in other colors. Further, it is possible to change the image forming portion  110  to form an image in other colors in addition to the images in black, yellow, magenta, and cyan. 
     The disclosure of Japanese Patent Application No. 2009-118226, filed on May 15, 2009, is incorporated in the application. 
     While the invention has been explained with reference to the specific embodiments of the invention, the explanation is illustrative and the invention is limited only by the appended claims.