Patent Publication Number: US-9897943-B2

Title: Belt unit, transfer unit, and image forming apparatus including a pressing member that presses an edge of a belt

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119(a) to Japanese Patent Application Nos.2014-055161, filed on Mar. 18, 2014, and 2014-153228, filed on Jul. 28, 2014 in the Japan Patent Office, the entire disclosure of each of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     Embodiments of this disclosure relate to a belt unit including a belt, a transfer unit including a belt, and an image forming apparatus including the belt unit or the transfer unit. 
     Description of the Related Art 
     An image forming apparatus includes multiple units having various functions related to image formation within the apparatus body. These units are detachably supported in the apparatus body for the purpose of maintenance, cleaning, exchange of parts, and exchange of units. Such units include a belt unit or a transfer unit including an endless belt that is wound around a plurality of support rollers, which serve as supports and are members used in image transfer and image fixing or in conveyance of paper sheet, which serves as a recording medium. 
     In such a belt unit or a transfer unit, in the case that a belt is used, if a belt made of a multi-layer material including a base material, an elastic layer, and the like is used, the edges of the belt positioned in the belt width direction may become warped due to the influence of heat deformation caused by differences in the thermal expansion coefficient of each layer or temporal degradation. If the belt edges become warped, depending on the level of warping, the attachment/detachment of one unit may interfere with another adjacent unit or the like, and this interference may lead to breakage of the belt or obstruction of the attachment/detachment operation. Hence, a configuration is proposed in which a pressing member is disposed in a direction of warping of the belt edges, and the pressing member is supported by a contact-and-separation assembly that moves the pressing member in the up-down direction as necessary so as to prevent the belt warping from becoming larger. 
     SUMMARY 
     In at least one aspect of this disclosure, there is provided a belt unit detachably attachable to a body of an image forming apparatus. The belt unit includes a belt, a pressing member, and a frame. The belt is wound around a plurality of supports to travel in a belt travel direction. The belt includes a base layer and an elastic layer. The pressing member presses warping on an edge of the belt. The frame supports the plurality of supports. The pressing member is positioned on the frame. 
     In at least one aspect of this disclosure, there is provided an image forming apparatus comprising the belt unit. 
     In at least one aspect of this disclosure, there is provided a belt unit detachably attachable to a body of an image forming apparatus. The belt unit includes a belt, a pressing member, a frame, a rotatable holder, and a contact-and-separation assembly. The belt is wound around a plurality of supports to travel in a belt travel direction. The pressing member presses warping on an edge of the belt. The frame supports the plurality of supports. The pressing member is mounted to the rotatable holder. The contact-and-separation assembly rotates the holder to rotate the pressing member toward and away from the belt. The pressing member is positioned on the frame via the contact-and-separation assembly. 
     In at least one aspect of this disclosure, there is provided a transfer unit includes a belt, a bracket, and a frame. The belt is wound around a plurality of rollers having a surface on which an image is transferred. The bracket has an opposing face opposing the surface of the belt outside a region in which the image is transferred, in a belt width direction. The frame rotatably supports the plurality of rollers. The transfer unit is drawable in the belt width direction from a body of an image forming apparatus. The frame includes a shaft. The bracket includes a hole. The bracket is rotatably supported on the frame with the shaft inserted into the hole. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The aforementioned and other aspects, features, and advantages of the present disclosure would be better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic view of a configuration of an image forming apparatus according to an embodiment of this disclosure; 
         FIGS. 2A and 2B  are enlarged views of characteristics of toner in a developer; 
         FIG. 3  is an enlarged view of a configuration of a transfer unit and a trajectory of a belt during a full color mode according to an embodiment of this disclosure; 
         FIG. 4  is a schematic view of a trajectory of the belt during a black mode of the transfer unit shown in  FIG. 3 ; 
         FIG. 5  is a schematic view of a trajectory of the belt during a lubricant application mode of the transfer unit shown in  FIG. 3 ; 
         FIG. 6  is a schematic view of a trajectory of the belt during an attachment/detachment mode of the transfer unit shown in  FIG. 3 ; 
         FIG. 7A  is a cross-sectional view of a configuration of the belt; 
         FIG. 7B  is an enlarged view of a state in which warping has occurred on the edges of the belt; 
         FIG. 8  is a schematic view of a configuration of a transfer unit according to an embodiment of this disclosure in a plan view; 
         FIG. 9  is a schematic view of a configuration of a pressing member according to an embodiment of this disclosure and its positional relationship with a transfer belt (intermediate transfer belt) in a plan view; 
         FIG. 10  is a schematic view of a configuration of the pressing member disposed at one edge of the transfer belt in a plan view; 
         FIG. 11  is a schematic view of a configuration of the pressing member disposed at the other edge of the transfer belt in a plan view; 
         FIG. 12A  is an enlarged perspective view of a configuration of the pressing member disposed at one edge side of the transfer belt; 
         FIG. 12B  is an enlarged perspective view of a configuration of the pressing member disposed at the other edge side of the transfer belt; 
         FIG. 13A  is an enlarged perspective view of a variation of the pressing member disposed at one edge side of the transfer belt; 
         FIG. 13B  is an enlarged perspective view of a variation of the pressing member disposed at the other edge side of the transfer belt; 
         FIG. 14  is an enlarged view of a configuration of a contact-and-separation assembly of the pressing member and a support roller; 
         FIG. 15A  is an enlarged view of a configuration of the contact-and-separation assembly of the pressing member and a first position of the pressing member; 
         FIG. 15B  is an enlarged view of the operation of the contact-and-separation assembly of the pressing member and a second and third position of the pressing member; 
         FIG. 16A  is an enlarged view of a first position of a support roller on the pressing member side; 
         FIG. 16B  is an enlarged view of a second and third position of the support roller; 
         FIG. 17A  is an enlarged view of the state during image formation of the contact-and-separation assembly of the pressing member and the support roller; 
         FIG. 17B  is an enlarged view of the state during a black mode of the contact-and-separation assembly of the pressing member and the support roller, 
         FIG. 17C  is an enlarged view of the state during an attachment/detachment mode of the contact-and-separation assembly of the pressing member and the support roller; 
         FIG. 18A  is an enlarged view of the configuration and a first position of the contact-and-separation assembly of the support roller on a black side, 
         FIG. 18B  is an enlarged view of the operation and a second position of the contact-and-separation assembly of the support roller on a black side, 
         FIG. 18C  is an enlarged view of the operation and a third position of the contact-and-separation assembly of the support roller on a black side; 
         FIG. 19A  is an enlarged view of a first position of a pressing member according to an embodiment of this disclosure, in which a rotation direction of the pressing member is set to a direction that follows changes in the belt trajectory, 
         FIG. 19B  is an enlarged view of a second position of the pressing member; 
         FIG. 19C  is an enlarged view of a third position of the pressing member; 
         FIG. 20  is an enlarged view of an embodiment in which the pressing member and the support roller are integrated; 
         FIG. 21  is an enlarged view for explaining an interference state due to contact between the edge of the pressing member and the transfer belt and an interference avoidance state; 
         FIG. 22  is an enlarged view of an embodiment in which a seal is provided on the pressing member at an opposing portion that opposes the transfer belt; 
         FIG. 23  is an enlarged view of an embodiment in which a portion of the pressing member that opposes the transfer belt is bent; 
         FIG. 24  is an enlarged view of an embodiment in which a portion of the pressing member that opposes the transfer belt is configured in a roller shape; and 
         FIG. 25  is an enlarged view of another embodiment of the contact-and-separation assembly of the pressing member. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
     DETAILED DESCRIPTION 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve similar results. 
     Although the embodiments are described with technical limitations with reference to the attached drawings, such description is not intended to limit the scope of the disclosure and all of the components or elements described in the embodiments of this disclosure are not necessarily indispensable. 
     Referring now to the drawings, multiple embodiments of the present disclosure will be described sequentially below. In the following embodiments, the same reference codes are allocated to elements (members or components) having the same function or configuration and redundant descriptions thereof are omitted below. The drawings may be partially omitted in order to facilitate the understanding of a partial configuration. The present invention is characterized in that a pressing member that presses the warping of a belt capable of a contact-and-separation operation is provided on a unit that supports the belt, and the pressing member is also capable of contact-and-separation movement in accordance with the contact-and-separation movement of the belt. 
     As shown in  FIG. 1 , an image forming apparatus  1000  according to the present embodiment is a color copier. In  FIG. 1 , the image forming apparatus  1000 , e.g., includes a copier body (copier housing)  100 , which is a body (housing) of an image forming apparatus, a sheet feed table  200 , a scanner  300 , and an automatic document feeder. The copier body  100  is placed on the sheet feed table  200 . The scanner  300  serves as an image reading unit and is mounted on the copier body  100 . The automatic document feeder  400  is mounted on the scanner  300 . In the center of the copier body  100 , a transfer unit  500 , which is a functional unit including a transfer belt (intermediate transfer belt)  10  that serves as an intermediate transfer body and is an endless belt member, is provided. The transfer belt  10  is wound around a plurality of rollers, which serve as a plurality of supports, and can travel by rotating clockwise in  FIG. 1 . The arrow indicated by V shows the travel direction of the transfer belt  10 . On the periphery of the transfer belt  10 , an intermediate transfer body cleaning device  17  that removes residual toner on the transfer belt  10  after image transfer is disposed. Above the transfer unit  500 , four process cartridge units  18 Bk,  18 C,  18 M, and  18 Y of black, cyan, magenta, and yellow are aligned horizontally from a downstream side in the travel direction along the travel direction V. These four process cartridge units  18 Bk,  18 C,  18 M, and  18 Y constitute a tandem image forming unit  20 . An exposure device  21  is disposed above the tandem image forming unit  20 . Each process cartridge unit is a functional unit, and includes a drum-shaped photoconductor  40 Bk,  40 C,  40 M, or  40 Y, which serves as an image bearer. Each process cartridge unit functions to form a toner image using toner, which serves as a developer, of each color on each photoconductor by a functional member of a well-known electrophotographic process, and cleans the surface of the photoconductor after toner image transfer. The process cartridge units are provided adjacent to the transfer unit  500  with a gap therebetween. The process cartridge units and the transfer unit  500  are detachably supported in the copier body  100 . 
     A secondary transfer roller  23 , which serves as a secondary transfer member, is disposed on the opposite side of the tandem image forming unit  20  sandwiching the transfer belt  10  therebetween. The secondary transfer roller  23  is pushed via the transfer belt  10  against a secondary opposing roller  512 , which serves as a secondary transfer opposing member, that supports the transfer belt  10  from the inside, so as to form a secondary transfer portion (nipping portion)  22  a contact portion of the secondary transfer roller  23  and the secondary opposing roller  512 . In the secondary transfer portion (nipping portion)  22 , the application of a transfer bias causes a toner image or composite color image on the transfer belt  10  to be transferred to a paper sheet P, which serves as a sheet-shaped recording medium. A fixing device  25  that fixes an image that has been transferred to the paper sheet P is disposed on a downstream side in a paper conveyance direction from the secondary transfer roller  23 . The fixing device  25  pushes a pressure roller  27 , which serves as a pressure member, against a fixing belt  26 , which is a belt that serves as a fixing member, and the fixing device  25  is detachably supported in the copier body  100 . The fixing device  25  includes a belt, and thus the pressing member capable of contact-and-separation movement of the present invention can be applied to the fixing device  25 . As the secondary transfer opposing member, instead of a roller, an endless belt wound around a plurality of rollers can also be used. In the present embodiment, a contact scheme in which the secondary transfer member is made to contact the transfer belt  10  is adopted, but a non-contact charger can also be disposed as the secondary transfer member. In this case, since it is difficult to provide such a non-contact charger together with a paper conveyance function achieved by rollers and belts, a conveyance unit may be provided separately. In the example of  FIG. 1 , a sheet reverse device  28  that reverses the paper sheet P to which an image is to be recorded on both sides thereof is disposed under the secondary transfer portion (nipping portion)  22  and the fixing device  25  parallel to the above-mentioned tandem image forming unit  20 , and thus the example of  FIG. 1  can be adapted to duplex printing. In the case of only simplex printing, the sheet reverse device  28  can be eliminated. The color copier can also function as a printer that is connectable either via wires or wirelessly to an external terminal such as a computer. The image forming apparatus is not limited to a color copier or printer, and a facsimile machine or a multifunction peripheral equipped with at least two functions such as a copier, a printer, and a facsimile machine can also be used. 
     When using a color copier to obtain a color copy, a color document is set on a document table  30  of the automatic document feeder  400 . Alternatively, the automatic document feeder  400  is opened and a color document is set onto an exposure glass  32  of the scanner  300 , and then the automatic document feeder  400  is closed to press the color document. When a start key is pressed, the scanner  300  is driven after the document is conveyed and moved onto the exposure glass  32  in the case that the document is set in the automatic document feeder  400 , or the scanner  300  is driven immediately in the case that the document is set on the exposure glass  32 , and then a first traveling body  33  and a second traveling body  34  are made to travel. Light is irradiated from a light source with the first traveling body  33  and light that is reflected from the document surface is further reflected toward the second traveling body  34 . The light is reflected by a mirror of the second traveling body  34 , passes through an imaging lens  35 , and enters a reading sensor  36 , and thereby the content of the document is read. 
     When the start key is pressed, the transfer belt  10  rotatably travels in a clockwise direction by a driving motor. Simultaneously, the photoconductors  40 Bk,  40 C,  40 M, and  40 Y of the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y are rotatably driven, and single-color toner images of black, yellow, magenta, and cyan are formed on the respective photoconductors  40 . These single-color toner images are sequentially transferred onto the transfer belt  10  as the transfer belt  10  travels to form a composite color image. 
     Meanwhile, when the start key is pressed, one of a plurality of sheet feed rollers  42  of the sheet feed table  200  is selectively rotated to dispense a paper sheet P from one of a plurality of sheet feeding cassettes  44  which are provided in multiple stages in a paper bank  43 . The paper sheet P that is dispensed is separated sheet-by-sheet by separation rollers  45  and enters a sheet feed path  46 , and then is conveyed by conveyance rollers  47 , guided to a sheet feed path  48  within the copier body  100 , and then hits registration rollers  49  and is stopped. Alternatively, a sheet feed roller  50  is rotated to dispense a paper sheet P on a bypass tray  51 , and then the paper sheet P is separated sheet-by-sheet by separation rollers  52  and enters a bypass feed pathway  53 , and then similarly hits the registration roller  49  and is stopped. The registration roller  49  is rotated to match the timing at which the composite color image on the transfer belt  10  reaches the secondary transfer portion  22 , so as to send the paper sheet P to the secondary transfer portion  22  between the transfer belt  10  and the secondary transfer roller  23 . In the secondary transfer portion  22 , the composite color image is transferred all together onto the paper sheet P. In the case of obtaining a single-color copy, a toner image of a single color is formed and transferred to the transfer belt  10 , and the toner image is then transferred onto the paper sheet P in the secondary transfer portion  22 . 
     The paper sheet P after image transfer is then conveyed by the secondary transfer portion  22  and sent to the fixing device  25 , where the transferred image is fixed by applying heat and pressure in the fixing device  25 . Subsequently, the paper sheet P is switched by a switching pawl  55 , ejected by an ejection roller  56 , and stacked on a discharge tray  57 . Alternatively, the paper sheet P after image transfer is switched by the switching pawl  55  and inserted into the sheet reverse device  28  where the paper sheet P is reversed and then guided again to the secondary transfer portion  22 . After an image is transferred onto the backside of the paper sheet P, it is ejected onto the discharge tray  57  by the ejection roller  56 . Meanwhile, after the image transfer, residual toner that remains on the transfer belt  10  after image transfer is removed by the intermediate transfer body cleaning device  17 , and then the transfer belt  10  is provided for another image formation by the tandem image forming unit  20 . 
     The transfer belt  10  used in the present embodiment is configured in a single layer or multiple layers of PVDF (vinylidene fluoride), ETFE (ethylene-tetrafluoroethylene copolymer), PI (polyimide), PC (polycarbonate), and the like. A conductive material such as carbon black is dispersed in the transfer belt  10 . The volume resistivity of the transfer belt  10  is adjusted to a range of 10 8  to 10 12  Ωcm and the surface resistivity of the transfer belt  10  is adjusted to a range of 10 9  to 10 13  Ωcm. A release layer can be coated onto the surface of the transfer belt  10  as necessary. As a material for the coat, a fluororesin such as ETFE (ethylene-tetrafluoroethylene copolymer), PTFE (polytetrafluoroethylene), PVDF (vinylidene fluoride), PEA (perfluoroalkoxy fluororesin), FEP (tetrafluoroethylene-hexafluoropropylene copolymer), PVF (vinyl fluoride), and the like can be used, but the material for the coat is not limited thereto. As a method for manufacturing the transfer belt  10 , roller coating, centrifugal molding, and the like can be implemented, and the surface of the transfer belt  10  can be polished as necessary. If the volume resistivity of the transfer belt  10  exceeds the above-mentioned range, it is not preferable because the bias necessary for transfer increases and this can lead to increases in the power costs. Also, the charging potential of the transfer belt  10  may increase in the transfer step, the transfer paper separation step, or the like and self electric discharge becomes difficult, and thus a neutralization unit would have to be provided. Further, if the volume resistivity and the surface resistivity fall below the above-mentioned ranges, damping of the charging potential becomes faster and this is advantageous in terms of neutralization by self electric discharge, but the current during transfer flows in a surface direction and thus toner scattering may occur. Therefore, the volume resistivity and the surface resistivity of the transfer belt  10  in the present invention are preferably within the above-mentioned ranges. 
     An elastic belt having a rubber layer can also be used as the transfer belt  10 . By using an elastic belt, the transfer belt  10  is compressed at the secondary transfer portion (nipping portion)  22  such that any gaps with the paper sheet P having asperities or the like are filled, and thus the transferability is improved. With only a rubber layer, the stretch of the belt increases, and thus in the transfer belt  10 , a resin layer such as a polyimide layer (PI layer) can be provided on a base layer. A layer having a low friction coefficient can also be provided on a surface layer of the transfer belt  10 . The volume resistivity and the surface resistivity were measured by connecting an HRS probe (inside electrode diameter of 5.9 mm, ring electrode inner diameter of 11 mm) to a high resistance resistivity meter (made by Mitsubishi Chemical Corporation: HIRESTA-IP) and applying a voltage of 100V (a surface resistivity of 500 V) to the top and bottom of the transfer belt  10 . The measured value after 10 seconds was used as the value of the volume resistivity and surface resistivity. 
     A shape factor SF-1 of the toner, which is a developer, used in image formation in the present embodiment is preferably in the range of 100 to 180, and a shape factor SF-2 thereof is preferably in the range of 100 to 180.  FIG. 2  schematically represents the shape of the toner in order to explain the shape factor SF-1 and the shape factor SF-2. The shape factor SF-1 indicates a roundness ratio of the toner shape, and is represented by formula (1) below. The shape factor SF-1 is a value obtained by dividing the square of a maximum length MXLNG of a shape achieved by projecting the toner on a two-dimensional plane by a graphic form areas AREA, and then multiplying the result by 100π/4.
 
SF-1={(MXLNG) 2 /AREA}×(100π/4)   formula (1)
 
If the value of SF-1 is 100, the toner shape is spherical, and the toner shape becomes more irregular as the value of SF-1 increases. The shape factor SF-2 indicates an unevenness ratio of the toner shape, and is represented by formula (2) below. The shape factor SF-2 is a value obtained by dividing the square of a peripheral length PERI of a graphic form achieved by projecting the toner on a two-dimensional plane by the graphic form area AREA, and then multiplying the result by 100/4π.
 
SF-2={(PERI) 2 /AREA}×(100/4π)   formula (2)
 
If the value of SF-2 is 100, the toner surface has no unevenness, and the unevenness on the toner surface becomes more prominent as the value of SF-2 increases. These shape factors were specifically measured by capturing a toner image with a scanning electron microscope (S-800: from Hitachi, Ltd.), introducing the toner image into an image analyzer (LUSEX3: from Nireco Corporation), and then analyzing the toner image to calculate the shape factors.
 
     If the toner shape approaches a sphere, the contact state between toner particles or between a toner particle and the photoconductors becomes point contact. Thus, the adsorption power between toner particles becomes weak and the liquidity increases. The adsorption power between a toner particle and the photoconductors also becomes weak and the transfer ratio increases. If one of the shape factors SF-1 and SF-2 exceeds 180, it is not preferable because the transfer ratio decreases and the cleaning performance when the toner adheres to the transfer member also deteriorates. Further, the toner particle diameter is preferably in a range of 4 to 10 μm in terms of volume-weighted average particle diameter. If the toner particle diameter is smaller than this range, background fog may occur during development, and the liquidity becomes worse. In addition, the toner easily agglomerates and thus dropout readily occurs. Conversely, if the toner particle diameter is larger than the above-mentioned range, a high definition image cannot be obtained due to toner scattering and poor resolution. In the present embodiment, a toner having a volume-weighted average particle diameter of 6.5 μm was used. 
     Next, the transfer unit  500  will be explained in further detail.  FIG. 3  is a schematic view of the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y and the transfer unit  500  when viewed from the front side of the copier body. In  FIG. 3 , the transfer unit  500  includes first to tenth driven rollers  501  to  510 , which serve as a plurality of supports, a driving motor  511 , which serves as a support, the secondary opposing roller  512 , and the transfer belt  10  that is wound around the rollers  501  to  512 . The driving motor  511  and the driven roller  508  are disposed at the right end side and left end side of the copier body  100 . The transfer belt  10  opposes the photoconductors  40 Bk,  40 C,  40 M, and  40 Y of the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y on an upper side over which the transfer belt  10  is wound between the driving motor  511  and the driven roller  508 . A tension roller  15  that compresses the transfer belt  10  from the outside toward the inside is positioned between the driven roller  506  and the driven roller  507 . The driving motor  511  is rotatably driven in a clockwise direction in  FIG. 3  by a driving motor M 3 , which serves as a driving source. 
     On the upper side of the transfer belt  10 , the driven rollers  509 ,  510 ,  501 , and  502  are disposed with intervals therebetween from the upstream side toward the downstream side in the belt travel direction. Inside the transfer belt  10 , primary transfer rollers  14 Bk,  14 C,  14 M, and  14 Y as a primary transfer member are disposed at areas respectively opposing the photoconductors  40 Bk,  40 C,  40 M, and  40 Y. The primary transfer rollers  14 Bk,  14 C,  14 M, and  14 Y are provided such that they are movable by a contact-and-separation assembly between a contact position, which is a first position, at which a top surface  10   a  of the transfer belt  10  contacts the photoconductors  40 Bk,  40 C,  40 M, and  40 Y, and a separated position at which the top surface  10   a  of the transfer belt  10  is separated from the photoconductors  40 Bk,  40 C,  40 M, and  40 Y. The separated position includes a second position at which the primary transfer rollers  14 Bk,  14 C,  14 M, and  14 Y are located when a lubricant is applied to the transfer belt  10  and a third position at which the primary transfer rollers  14 Bk,  14 C,  14 M, and  14 Y are located when the transfer unit  500  is attached/detached to the copier body  100 . The primary transfer rollers  14 Bk,  14 C,  14 M, and  14 Y are configured in a well-known manner in which they are respectively rotatably supported by support arms  141 Bk,  141 C,  141 M, and  141 Y that are swung by an electric driving source such as a driving motor which constitutes the contact-and-separation assembly, and held in the first to third positions by adjusting the angle of the support arms  141 Bk,  141 C,  141 M, and  141 Y with the driving motor. In other words, the transfer belt  10  is displaceable such that it can take multiple trajectories that are not parallel to each other. 
     The driven rollers  501 ,  502 , and  509  which are parallel to the primary transfer rollers  14 Bk,  14 C,  14 M, and  14 Y are also provided to be movable between a contact position (first position) and a separated position (second and third positions). The movement of the driven rollers  501 ,  502 , and  509  will be explained in more detail below. 
     In the present embodiment, the transfer unit  500  is configured such that the transfer belt  10  occupies four states (hereinafter referred to as “four modes”). The four modes are as follows: a full color mode in which the four photoconductors  40 Bk,  40 C,  40 M, and  40 Y of black, cyan, magenta, and yellow are used, a black mode in which only the black photoconductor  40 Bk is used, a lubricant application mode in which lubricant is applied onto the transfer belt  10  in a stand-by state, and an attachment/detachment mode in which the transfer unit  500  is attached/detached to/from the copier body  100 . 
       FIG. 3  illustrates the positional relationship of the transfer belt  10 , the primary transfer rollers  14 Bk,  14 C,  14 M, and  14 Y, and the driven rollers  501 ,  502 , and  510  during the full color mode. In the present embodiment, in the full color mode, the primary transfer rollers  14 Bk,  14 C,  14 M, and  14 Y and the driven rollers  501 ,  502 , and  510  occupy the first position, and thereby the top surface  10   a  of the transfer belt  10  is in a contact state with all of the photoconductors  40 Bk,  40 C,  40 M, and  40 Y. 
       FIG. 4  illustrates the positional relationship of the transfer belt  10 , the primary transfer rollers  14 Bk,  14 C,  14 M, and  14 Y, and the driven rollers  501 ,  502 , and  510  during the black mode. In the present embodiment, in the black mode, the primary transfer roller  14 Bk and the driven rollers  501  and  502  disposed in front of and behind the primary transfer roller  14 Bk occupy the first position, and thereby the top surface  10   a  of the transfer belt  10  is in a contact state with the photoconductor  40 Bk. The primary transfer rollers  14 C,  14 M, and  14 Y and the driven roller  510  occupy the second position, which is the separated position, and thereby the top surface  10   a  of the transfer belt  10  is in a separated state from the photoconductors  40 C,  40 M, and  40 Y. The transfer belt  10  at this time is in an inclined state in which the primary transfer roller  14 Y side is lower based on the transfer portion formed between the transfer belt  10  and the primary transfer roller  14 Bk. In other words, in the present embodiment, the trajectory of the transfer belt  10  in the mode in which the belt is separated is not parallel to the trajectory of the transfer belt  10  in the mode in which the belt is in contact. 
       FIG. 5  illustrates the positional relationship of the transfer belt  10 , the primary transfer rollers  14 Bk,  14 C,  14 M, and  14 Y, and the driven rollers  501 ,  502 , and  510  during the lubricant application mode. In the present embodiment, in the lubricant application mode, the primary transfer rollers  14 C,  14 M, and  14 Y and the driven roller  510  are held in the second position (separated position), and the primary transfer roller  14 Bk and the driven rollers  501  and  502  occupy the second position (separated position). Therefore, the primary transfer rollers  14 Bk,  14 C,  14 M, and  14 Y and the driven rollers  501 ,  502 , and  510  all occupy the second position, and thereby the top surface  10   a  of the transfer belt  10  is in a separated state from all of the photoconductors  40 Bk,  40 C,  40 M, and  40 Y. 
       FIG. 6  illustrates the positional relationship of the transfer belt  10 , the primary transfer rollers  14 Bk,  14 C,  14 M, and  14 Y, and the driven rollers  501 ,  502 , and  510  during the attachment/detachment mode. In the present embodiment, in the attachment/detachment mode, the primary transfer rollers  14 Bk,  14 C,  14 M, and  14 Y and the driven rollers  501 ,  502 , and  510  occupy the third position (separated position) at which they are separated further downwards from the photoconductors than in the application mode, and thereby the top surface  10   a  of the transfer belt  10  is in a further separated state from all of the photoconductors  40 Bk,  40 C,  40 M, and  40 Y than in the lubricant application mode. Herein, the second and third positions are provided as separated positions, but the third position can also be set as the second position so as to eliminate the second position. 
     As shown in  FIGS. 3 to 6 , a gap X is provided between the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y and the transfer unit  500  in order to prevent interference between the units. In the present embodiment, the gap X is 6 mm. In the present embodiment, the separated state between the transfer belt  10  and the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y is larger when the transfer belt  10  is in the separated position than in the contact position. 
     In the present embodiment, as shown in  FIG. 7A , the transfer belt  10  is made of an elastic belt having a base layer  10 A and an elastic layer  10 B. Thus, upwards warping (in an orientation in which the elastic layer  10 B contracts) occurs easily at belt edges  10   b  and  10   c  located in a belt width direction Y, which is the axial direction of the photoconductors, as shown in  FIG. 7B  due to differences in the thermal expansion coefficient of the materials of the base layer  10 A and the elastic layer  10 B. If the warping at the belt edges  10   b  and  10   c  becomes larger than the gap X, the transfer belt  10  and the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y may interfere with each other even if the transfer belt  10  is in the separated position when attaching/detaching the transfer unit  500  or the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y to/from the copier body  100 . In the case of the transfer belt  10  used in the present embodiment, warping of 14 to 19 millimeters occurred. If the gap X is increased, the above-described interference can be prevented. However, on the other hand, increasing the gap size may lead to an increase in the size of the copier body, or it may be difficult to increase the size of the gap X if the copier body is small. 
     Thus, in the present embodiment, as shown in  FIG. 8 , a pressing member  530  and a pressing member  531  that respectively oppose and press the belt edge  10   b  and the belt edge  10   c  are provided to the transfer unit  500 . In the present embodiment, a plurality (three) of the pressing members  530  are disposed with intervals therebetween in the belt travel direction V on the belt edge  10   b  side, and a plurality (three) of the pressing members  531  are disposed with intervals therebetween in the belt travel direction V on the belt edge  10   c  side. The pressing members  530  and  531  are disposed to be positioned within the gap X on the belt edges  10   b  and  10   c  at positions outside of an image transfer region G of the transfer belt  10  indicated by diagonal lines. For the sake of convenience, the pressing members  530  and  531  positioned on the upstream side in the belt travel direction and disposed near the support roller  510  may also be referred to as the first pressing members  530 ( 1 ) and  531 ( 1 ), the pressing members  530  and  531  positioned on the downstream side in the belt travel direction and disposed near the support roller  501  may also be referred to as the third pressing members  530 ( 3 ) and  531 ( 3 ), and the pressing members  530  and  531  positioned between the first pressing members  530  and  531  and the third pressing members  530  and  531  may also be referred to as the second pressing members  530 ( 2 ) and  531 ( 2 ). The configurations of the first to third pressing members  530  and  531  are identical other than the positions at which they are disposed, and thus when explaining their common configuration, the reference codes ( 1 ) to ( 3 ) will be appropriately omitted. In the present embodiment, the pressing members  530  and  531  are positioned on frame boards  520  and  521 . 
     In this way, if the pressing members  530  and  531  are respectively positioned on the frame boards  520  and  521  of the transfer unit  500  such that they are positioned within the gap X on the belt edges  10   b  and  10   c  at positions outside of the image transfer region G, even if the belt edges  10   b  and  10   c  of the transfer belt  10  warp, the belt edges  10   b  and  10   c  cannot warp beyond the pressing members  530  and  531  within the gap X due to the existence of the pressing members  530  and  531 . Therefore, the warping of the belt edges  10   b  and  10   c  is pressed down, and thus there is no contact between the transfer belt  10  and the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y even when attaching/detaching the transfer unit  500  or the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y to/from the copier body  100 . Accordingly, breakage of the transfer belt  10  can be prevented and the attachment/detachment operation of the units can be smoothly carried out, and this also contributes to improving the operability. Also, by providing the pressing members  530  and  531  to the transfer unit  500  including the transfer belt  10 , any variability that may occur during assembly can be reduced compared to providing the pressing members  530  and  531  to another unit, and warping of the belt edges  10   b  and  10   c  can be accurately pressed down. 
     In the present embodiment, the pressing members  530  and  531  are disposed such that there are three on each side in the belt travel direction V. However, the pressing members  530  and  531  can also be disposed such that there are two on each side, and should be disposed such that there is at least one on each side. If there is one on each side or two on each side, the length of each pressing member  530  and  531  in the belt travel direction V is preferably longer than in the case of three on each side so that the range over which they can press down the warped belt edges  10   b  and  10   c  is widened. Further, if one each of the pressing members  530  and  531  is disposed in the belt width direction Y, they are preferably disposed centered on an area at which the warping of the belt edges  10   b  and  10   c  is the largest. 
     As shown in  FIG. 9 , overlaps a 1  and a 2 , which are the overlapping widths of the belt edges  10   b  and  10   c  and the pressing members  530  and  531  in the belt width direction Y, are preferably secured in an amount such that the belt edges  10   b  and  10   c  do not come out from the pressing member  530  and the pressing member  531  even if the transfer belt  10  is traveling at an incline in the belt width direction Y. 
     In the above-described embodiment, the pressing member  530  is disposed on the belt edge  10   b  side, and the pressing member  531  is disposed on the belt edge  10   c  side. However, in the arrangement of the pressing members, the pressing members do not have to be disposed on both edges ( 10   b  and  10   c ) of the transfer belt  10 , and the pressing members can be arranged such that the pressing members  530  are disposed on only the belt edge  10   b  side, which is the back side of the copier body, as shown in  FIG. 10 , or the pressing members  531  are disposed on only the belt edge  10   c  side, which is the front side (attachment/detachment side) of the copier body, as shown in  FIG. 11 . 
     As shown in  FIG. 10 , in the case that the pressing members  530  are disposed on only the belt edge  10   b  side, an effect is achieved during detachment of the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y and during attachment/detachment of the transfer unit  500 . If there were parts of the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y on the belt edge  10   b  side that could easily catch on the transfer belt  10 , since the belt edge  10   b  is pressed down by the pressing members  530 , any interference between the parts of the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y that could easily catch on the transfer belt  10  and the transfer belt  10  would be eliminated. Therefore, breakage of the transfer belt  10  during detachment of the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y could be prevented. Also, when detaching the transfer unit  500 , since warping of the belt edge  10   b  at the back side of the copier body is pressed down by the pressing members  530 , any interference between the transfer belt  10  and the process cartridges would be eliminated during detachment in which the transfer unit  500  is moved from the back side of the copier body  100  to the front side or during attachment in which the transfer unit is moved from the front side to the back side, and thus breakage of the transfer belt  10  could be prevented. 
     As shown in  FIG. 11 , in the case that the pressing members  531  are disposed on only the belt edge  10   c  side, which is the front side (attachment/detachment side) of the copier body, an effect is achieved during detachment of the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y and during detachment of the transfer unit  500 . If there were parts of the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y on the belt edge  10   c  side of the transfer belt  10  that could easily catch during attachment/detachment, as long as these parts that could easily catch are installed at the front side (attachment/detachment side) of the copier body, even if warping occurred on the belt edge  10   c  at the front side, the warping would be pressed down by the pressing members  531 . Thus, the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y would be prevented from catching on the belt edge  10   c , and breakage of the transfer belt  10  could be prevented. Also, when detaching the transfer belt  10 , even if warping occurred on the belt edge  10   c , which is at the front side during detachment, the warping would be pressed down by the pressing members  531 . Thus, the transfer belt  10  would be prevented from catching on the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y, and breakage of the transfer belt  10  could be prevented. 
     Next, the specific configuration of the pressing members  530  and  531  and the support roller  510  and a contact-and-separation assembly  600  thereof, as well as a contact-and-separation assembly  700  of the support rollers  501  and  502  will be explained. The contact-and-separation assembly  600  rotates the pressing members  530  and  531  and the support roller  510  toward and away from the transfer belt  10 . The contact-and-separation assembly  700  rotates the support rollers  501  and  502  toward and away from the transfer belt  10 . As shown in  FIG. 12A , each pressing member  530  includes a mount  5301  and a pressing part  5302  that opposes the belt edge, and as shown in  FIG. 12B , each pressing member  531  includes a mount  5311  and a pressing part  5312 . In the pressing members  530  and  531 , the mounts  5301  and  5311  are respectively detachably mounted by fasteners  560  and  561  such as bolts or screws to holders  550  and  551 . The pressing members  530  and  531  are preferably configured to be detachably attachable, because by doing so the pressing members  530  and  531  can be exchanged with another type of pressing member according to the type of belt that warps or the arrangement/condition thereof or replaced with a new pressing member in the case of breakage of the pressing members  530  and  531 . Further, when detaching the transfer unit  500  from the copier body  100  to exchange the transfer belt  10 , removing at least one of the pressing members  530  or the pressing members  531  from the holders  550  or  551  can facilitate the exchange of the transfer belt  10 . 
     As shown in  FIGS. 12A and 12B , the pressing parts  5302  and  5312  have a panel shape that extends in the belt travel direction V, and at least one edge  5302   a ,  5312   a  that approaches the transfer belt  10  during rotation of the pressing members  530  and  531  is formed as an inclined surface that is inclined in a direction away from the top surface  10   a  of the transfer belt  10 . Underside surfaces  5302   c  and  5312   c  of the pressing parts  5302  and  5312 , which are the faces that oppose the transfer belt  10 , are disposed so as to overlap above the belt edge  10   b  and the belt edge  10   c  opposing the belt edge  10   b  and the belt edge  10   c  of the transfer belt  10 . The other edges  5302   b  and  5312   b  of the pressing parts  5302  and  5312  located on the opposite side of the edges  5302   a  and  5312   b  can be flat surfaces that are not inclined as shown in  FIGS. 12A and 12B , or they can be formed as inclined surfaces that are inclined in a direction away from the top surface  10   a  of the transfer belt  10  as shown in  FIGS. 13A and 13B . Also, the underside surface  5302   c ,  5312   c  sides that contact the transfer belt  10  and the corners/edges of the edges  5302   a  and  5312   a  are preferably worked to be beveled or curved so that they will not easily scratch the transfer belt  10  when contacting the belt edges  10   b  and  10   c.    
     As shown in  FIG. 8 , the transfer unit  500  includes a pair of frame boards  520  and  521  that oppose each other in the belt width direction Y. On the frame board  520 , a slider  522  that constitutes the contact-and-separation assembly  600 , which causes the pressing member  530  and the support roller  510  to perform the contact-and-separation operation, and a slider  524  that constitutes the contact-and-separation assembly  700 , which causes the support rollers  501  and  502  to perform the contact-and-separation operation, are aligned in parallel in the belt travel direction V. On the frame board  521 , a slider  523  that constitutes the contact-and-separation assembly  600 , which causes the pressing member  531  and the support roller  510  to perform the contact-and-separation operation, and a slider  525  that constitutes the contact-and-separation assembly  700 , which causes the support rollers  501  and  502  to perform the contact-and-separation operation, are aligned in parallel in the belt travel direction V. The slider  522  and the slider  523  are disposed opposing each other and constitute sliders for color, and the slider  524  and the slider  525  are disposed opposing each other and constitute sliders for black. The color sliders  522  and  523  and the black sliders  524  and  525  are respectively supported by the frame boards  520  and  521  such that they can move parallel to each other in the belt travel direction V. The configuration related to the color sliders  522  and  523  will be explained below as a color-side configuration, and the configuration related to the black sliders  524  and  525  will be explained below as a black-side configuration. 
     (Color-Side Configuration) 
     As shown in  FIGS. 3 and 14 , in the sliders  522  and  523 , a pair of opposing long holes  526  and a pair of opposing long holes  527  that extend in the longitudinal direction are formed with intervals therebetween in the longitudinal direction. A pair of shaft-shaped pins  528  and  529  that protrude from opposing faces of the frame boards  520  and  521  are inserted into the long holes  526  and  527  to support the sliders  522  and  523  so that they can slidably move in the longitudinal direction on the frame boards  520  and  521  and to position the sliders  522  and  523  in the up-down direction. Instead of inserting pins or shafts into the long holes  526  and  527 , rollers that are rotatably supported on the frame boards  520  and  521  can also be inserted. Sheet metal is used to make the sliders  522  and  523  in order to secure the rigidity thereof. However, in consideration of the friction during sliding with the pins  528  and  529 , the long holes  526  and  527  themselves can be formed using a resin material with good slidability, or a resin material with good slidability can be disposed on the inside of the long holes  526  and  527  so that the pins  528  and  529  are held by the resin material so that they can slide. Alternatively, the pins  528  and  529  can be made of metal and then the outer periphery thereof can be covered or coated with a resin material with good slidability, or a lubricant such as grease can be applied to the outer periphery of the pins  528  and  529 . 
     A pair of ball bearings  540 , which serve as cam followers, are rotatably supported on the sliders  522  and  523 . Outer peripheral surfaces  541   a  of a pair of contact-and-separation cams  541 , which serve as contact-and-separation members, respectively contact outer peripheral surfaces  540   a  of the ball bearings  540 , and thereby the sliders  522  and  523  are positioned in the longitudinal direction. The contact-and-separation cams  541  are eccentrically fixed so that they are both in the same phase on a single cam shaft  542  that is rotatably provided on the frame boards  520  and  521 , and thus the contact-and-separation cams  541  rotate integrally on the same axis when the cam shaft  542  is rotated. The cam shaft  542  is driven to rotate by a driving motor M 1 , which serves as a driving source. A pair of tension coil springs  543 , which serve as return units, are mounted at one end to the sliders  522  and  523  and the other end to the frame boards  520  and  521  to bias the sliders  522  and  523  toward the right direction (return direction) in  FIG. 14 . 
     Cam receivers  546  are formed on ends  522   a  and  523   a , which are return-side ends, of the sliders  522  and  523 . Manual cams  547  are rotatably supported by a cam shaft  548  on the frame boards  520  and  521  that oppose the cam receivers  546 . A manual lever  549  for manually rotatably operating the cam shaft  548  is fixed to an end of the cam shaft  548 . The manual lever  549  is disposed at the front side of the copier body. 
     As shown in  FIGS. 3, 14, 15A, and 15B , the holders  550  and  551  that hold the pressing members  530  and  531  are rotatably supported by metal rotary support shafts  533  on the frame boards  520  and  521  of the transfer unit  500 . The support portions of the holders  550  and  551  by the rotary support shafts  533  are upstream in the belt travel direction V, and the pressing parts  5302  and  5312  are disposed downstream from the support portions in the belt travel direction V. Therefore, in the pressing members  530  and  531 , the pressing parts  5302  and  5312  located downstream from the rotary support shafts  533  in the belt travel direction V are rotatably supported centered on the rotary support shafts  533 . The rotation direction is a direction in which the top surface  10   a  of the transfer belt  10  is made to contact or separate from the photoconductors  40 Y,  40 M, and  40 C. Also, the direction indicated by arrow A in  FIG. 15A  indicates a predetermined direction, and arrow B indicates a reverse direction of the predetermined direction. Further, the transfer unit  500  is drawable from the copier body  100  in the belt width direction Y in a state in which brackets are rotated in the predetermined direction (arrow A direction) and the reverse direction (arrow B direction). 
     In the present embodiment, brackets  800  that are rotated by the contact-and-separation assembly  600  are formed by mounting the pressing members  530  and  531  to the holders  550  and  551 . The brackets  800 , which include the pressing members  530  and  531  and the holders  550  and  551 , are supported on the frame boards  520  and  521 , and include an opposing face (underside surface)  5302   c  and  5312   c  that opposes the top surface  10   a  of the transfer belt  10  on the outside in the belt width direction of the region G in which the image is transferred. As shown in  FIGS. 12 and 13 , the brackets  800  are constituted by the pressing members  530  and  531 , which serve as first members and include the opposing faces (underside surfaces)  5302   c  and  5312   c , and the holders  550  and  551 , which serve as second members that hold the pressing members  530  and  531  and include holes  801  and  801  formed in the belt width direction Y. The brackets  800  are preferably configured as arms that are rotatable centered on the rotary support shafts  533  by inserting the rotary support shafts  533  shown in  FIGS. 14 and 15  into the holes  801  and  801 . The brackets  800  can also be configured as levers that rotate centered on the rotary support shafts  533  upon receiving a force from pins  558  and  578 . 
     In the present embodiment, the pressing members  530  and  531  and the holders  550  and  551  are fastened by the fasteners  560  and  561  to constitute the brackets  800 . However, the brackets can also be constituted by integrally molding the pressing members  530  and  531  and the holders  550  and  551 . In the present embodiment, as shown in  FIGS. 12 and 13 , a hole  801  is provided in each bracket  800  and the rotary support shafts  533  are provided to the frame boards  520  and  521 . However, instead of this configuration, the rotary support shafts  553  can be provided to the brackets  800  and the holes  801  can be provided to the frame boards  520  and  521  so as to rotatably support the brackets  800  on the frame boards  520  and  521 . 
     In the present embodiment, openings  557  (contact faces  557 A) are formed in the brackets  800  and pins  558  are provided to the sliders  522  and  523 . However, instead of this configuration, the pins  558  can be provided to the bracket  800  and the openings  557  (contact faces  557 A) can be formed in the sliders  522  and  523 . Also, in the present embodiment, the present invention is applied to an image forming apparatus in which a toner image formed on the photoconductors  40 Bk,  40 C,  40 M, and  40 Y is transferred to a transfer belt  10 , which serves as a belt. However, the constitution of the apparatus is not limited thereto. For example, the present invention can also be applied to an apparatus in which a paper sheet P is conveyed between the photoconductors  40 Bk,  40 C,  40 M, and  40 Y and a conveyance belt, which serves as a belt, and then the toner image formed on the photoconductors  40 Bk,  40 C,  40 M, and  40 Y is transferred to the paper sheet P. Further, the present invention can be applied to a monochrome image forming apparatus instead of a color image forming apparatus. 
     In the present embodiment, the pressing members  530  and  531  are positioned on the frame boards  520  and  521 . In this positioning, in the state shown in  FIG. 15A , the position of the rotation center of the bracket  800  relative to the frame board  520  can be accurately determined by inserting the rotary support shaft  533  provided to the frame board  520  into the hole  801  formed in the bracket  800 . The accuracy can be improved to the extent that no other parts are interposed between the frame board  520  and the bracket  800 . In other words, the rotary support shaft  533  constitutes a main reference of the bracket  800  relative to the frame board  520 . Also, since a stopper  555  provided to the frame board  520  and a contact part  556  formed on the bracket contact each other, the rotation angle (the amount of rotation of the pressing members  530  and  531  centered on the rotary support shafts  533 ) of the bracket  800  relative to the frame board  520  can be accurately determined The accuracy can be improved to the extent that no other parts are interposed between the frame board  520  and the bracket  800 . In other words, the stopper  555  constitutes a sub reference of the bracket  800  relative to the frame board  520 . In the state shown in  FIG. 15B , the position of the rotation center of the bracket  800  relative to the frame board  520  can be accurately determined by inserting the rotary support shaft  533  provided to the frame board  520  into the hole  801  formed in the bracket  800 . In other words, in the present embodiment, in both the state shown in  FIG. 15A  and the state shown in  FIG. 15B , the bracket  800  can be accurately positioned relative to the frame board  520  by at least using the rotary support shaft  533  as a main reference of the bracket  800  relative to the frame board  520 . 
     Tension coil springs  553  are mounted at one end to the holders  550  and  551  and at the other end to the frame boards  520  and  521  to bias the holders  550  and  551  in the counter clockwise direction in  FIG. 15A . As shown in  FIG. 15A , contact parts  556  that contact the stoppers  555  provided on the frame boards  520  and  521  at an arbitrary position are formed on the holders  550  and  551 . The stoppers  555  provided on the frame boards  520  and  521  are formed by bending the frame boards  520  and  521 . Instead of by bending, the stoppers  555  can also be constituted by providing studs or the like onto the frame boards  520  and  521 . The pressing members  530  and  531  are restricted from rotating in the counterclockwise direction when the contact parts  556  of the holders  550  and  551  contact the stoppers  555 , and thus the pressing members  530  and  531  are held in the first position. In other words, in the positioning of the pressing members  530  and  531 , the belt pressing position of the pressing members  530  and  531  can be accurately set with other members within the transfer unit  500  by contacting the contact parts  556  of the holders  550  and  551  to the stoppers  555  of the frame boards  520  and  521 . The arbitrary position is the first position. In other words, in the present embodiment, the brackets  800  are positioned by contacting the contact parts  556  of the holders  550  and  551  to the stoppers  555 . 
     The openings  557  are formed in the holders  550  and  551 , and the pins  558 , which are provided to the sliders  522  and  523  and serve as transmitting parts, are inserted into the openings  557 . The size of the openings  557  is larger than the diameter of the pins  558 , and the size is set such that the rotating operation is not obstructed when the holders  550  and  551  rotate. If the sliders  522  and  523  move from an initial position shown in  FIGS. 14 and 15A  toward the left direction in  FIGS. 14 and 17A  which is a separation direction, the pins  558  contact side surfaces  557 A of the openings  557  positioned in the movement direction so as to push the holders  550  and  551 . The holders  550  and  551  are rotated by the pushing operation in the clockwise direction centered on the rotary support shafts  533  to move to the second position, which is a separated position. 
     As shown in  FIGS. 14 and 16A , the support roller  510  is rotatably supported by a shaft  571  on a pair of holders  570  that have the same function as the holders  550  and  551 . The holders  570  are rotatably supported by metal support shafts  572  on the frame boards  520  and  521  of the transfer unit  500 . Support portions of the holders  570  on the rotary support shafts  572  are disposed on the upstream side in the belt travel direction from the shaft  571  that supports the support roller  510 . Therefore, the support roller  510  is rotatably supported centered on the rotary support shafts  572  on the downstream side in the belt travel direction of the rotary support shafts  572 . 
     Tension coil springs  573  are mounted at one end to the holders  570  and at the other end to the frame boards  520  and  521  to bias the holders  570  in the counter clockwise direction in  FIG. 16A . Contact parts  576  that contact stoppers  575  formed on the frame boards  520  and  521  are formed on the holders  570  that are biased in the counterclockwise direction in  FIG. 16A . The support roller  510  is restricted from rotating in the counterclockwise direction when the contact parts  576  contact the stoppers  575 , and thus the support roller  510  is held in the first position. Openings  577  are formed in the holders  570 , and the pins  578 , which are provided to the sliders  522  and  523  and serve as transmitting parts, are inserted into the openings  577 . The size of the openings  577  is larger than the diameter of the pins  578 , and the size is set such that the rotating operation is not obstructed when the holders  570  rotate. If the sliders  522  and  523  move from an initial position shown in  FIGS. 14 and 16A  toward the left direction in  FIGS. 14 and 16A  which is a separation direction, the pins  578  contact a side surface  577 A of the openings  577  positioned in the movement direction so as to push the holders  570 . The holders  570  are rotated by the pushing operation in the clockwise direction centered on the rotary support shafts  572  to move to the second position, which is a separated position. In the present embodiment, rotary movement of the pressing members  530  and  531  and the support roller  510  in the counterclockwise direction in  FIG. 16  is rotation in a contact direction (the predetermined direction indicated by arrow A), and rotary movement in the clockwise direction in  FIG. 16  (the predetermined direction indicated by arrow B) is rotation in a separation direction. 
     (Black-Side Configuration) 
     As shown in  FIGS. 3, 14, and 18A , in the sliders  524  and  525  that constitute the contact-and-separation assembly  700 , a pair of opposing long holes  586  and a pair of opposing long holes  587  that extend in the longitudinal direction are formed with intervals therebetween in the longitudinal direction. A pair of pins  588  and  589  that protrude from opposing faces of the frame boards  520  and  521  are inserted into the long holes  586  and  587  to support the sliders  524  and  525  so that they can slidably move in the longitudinal direction on the frame boards  520  and  521  and to position the sliders  524  and  525  in the up-down direction. Instead of inserting pins or shafts into the long holes  586  and  587 , rollers that are rotatably supported on the frame boards  520  and  521  can also be inserted. Sheet metal is used to make the sliders  524  and  525  in order to secure the rigidity thereof. However, in consideration of the friction during sliding with the pins  588  and  589 , the long holes  586  and  587  themselves can be formed using a resin material with good slidability, or a resin material with good slidability can be disposed on the inside of the long holes  586  and  587  so as to receive the pins  588  and  589  with the resin material. Alternatively, the pins  588  and  589  can be made of metal and then the outer periphery thereof can be covered or coated with a resin material with good slidability, or a lubricant such as grease can be applied to the outer periphery of the pins  588  and  589 . 
     A pair of ball bearings  590 , which serve as cam followers, are rotatably supported on the sliders  524  and  525 . Outer peripheral surfaces  591   a  of a pair of contact-and-separation cams  591 , which serve as contact-and-separation members, respectively contact outer peripheral surfaces  590   a  of the ball bearings  590 , and thereby the sliders  524  and  525  are positioned in the longitudinal direction. The contact-and-separation cams  591  are eccentrically fixed so that they are both in the same phase on a single cam shaft  592  that is rotatably provided on the frame boards  520  and  521 , and thus the contact-and-separation cams  591  rotatably move when the cam shaft  592  rotates. The cam shaft  592  is driven to rotate by a driving motor M 2 , which serves as a driving source. A pair of tension coil springs  593 , which serve as return units, are mounted at one end to the sliders  524  and  525  and the other end to the frame boards  520  and  521  to bias the sliders  524  and  525  toward the right direction (return direction) in  FIG. 18 . 
     As shown in  FIG. 18A , the support rollers  501  and  502  are respectively rotatably supported by shafts  596  and  597  on pairs of holders  594  and  595  which constitute the contact-and-separation assembly  700 . The holders  594  and  595  are rotatably supported by metal rotary support shafts  598  and  599  on the frame boards  520  and  521  of the transfer unit  500 . Support portions of the holders  594  and  595  on the rotary support shafts  598  and  599  are disposed on the upstream side in the belt travel direction V from the shafts  596  and  597  that support the support rollers  501  and  502 . Therefore, the support rollers  501  and  502  are rotatably supported centered on the rotary support shafts  598  and  599  on the downstream side in the belt travel direction from the rotary support shafts  598  and  599 . 
     Tension coil springs  602  and  603  are mounted at one end to the holders  594  and  595  and at the other end to the frame boards  520  and  521  to bias the holders  594  and  595  in the counter clockwise direction in  FIG. 18A . Contact parts  607  and  608  that respectively contact stoppers  605  and  606  formed on the frame boards  520  and  521  are formed on the holders  594  and  595  that are biased in the counterclockwise direction in  FIG. 18A . The support rollers  501  and  502  are restricted from rotating in the counterclockwise direction when the contact parts  607  and  608  contact the stoppers  605  and  606 , and thus the support rollers  501  and  502  are held in the first position. Openings  609  and  610  are respectively formed in the holders  594  and  595 , and pins  611  and  612 , which are provided to the sliders  524  and  525  and serve as transmitting parts, are inserted into the openings  609  and  610 . The size of the openings  609  and  610  is larger than the diameter of the pins  611  and  612 , and the size is set such that the rotating operation is not obstructed when the holders  594  and  595  rotate. If the sliders  524  and  525  move from an initial position shown in  FIG. 18A  toward the left direction which is a separation direction shown in  FIG. 18B , the pins  611  and  612  contact side surfaces  609 A and  610 A of the openings  609  and  610  positioned in the movement direction so as to push the holders  594  and  595 . The holders  594  and  595  are rotated by the pushing operation in the clockwise direction centered on the rotary support shafts  598  and  599  to move to the second position, which is a separated position. 
     The sliding operation of the sliders  522  and  523  and the sliders  524  and  525  will now be explained. 
     (Operation During Full Color Mode) 
     In the full color mode, as shown in  FIG. 17A , the sliders  522  and  523  are pushed toward the right side in  FIG. 17A  by the action of the tension springs  543 . At this time, the contact-and-separation cams  541 , which are eccentric cams, contact the ball bearings  540  at a portion where the eccentric distance between the cam shaft  542  and the outer peripheral surfaces  541   a  is the shortest. Also, the manual cams  547  are in a state in which they are separated from the cam receivers  546 . Thus, in the full color mode, the support roller  510  and the three groups of the first to third pressing members  530  and  531  (the brackets  800 ) are in the first position shown in  FIG. 3 . Of course, the primary transfer rollers  14 Y,  14 M, and  14 C are also in the first position. Further, as shown in  FIG. 18A , the sliders  524  and  525  are pushed toward the right side in  FIG. 18A  by the action of the tension springs  593 . At this time, the contact-and-separation cams  591 , which are eccentric cams, contact the ball bearings  590  at a portion where the eccentric distance between the cam shaft  592  and the outer peripheral surfaces  591   a  is the shortest. Therefore, in the full color mode, the support rollers  501  and  502  are in the first position shown in  FIG. 3 . Of course, the primary transfer roller  14 Bk is also in the first position. At this time, the trajectory of the transfer belt  10  is held in an approximately horizontal trajectory. 
     (Operation During Black Mode) 
     In the black mode, the driving motor M 1  drives the contact-and-separation cams  541  to rotate 180 degrees, and then the driving motor M 1  stops. Thereby, the contact-and-separation cams  541  contact the ball bearings  540  at a portion where the eccentric distance between the cam shaft  542  and the outer peripheral surfaces  541   a  is the longest. Therefore, the sliders  522  and  523  slidingly move from the state shown in  FIG. 17A  toward the left side to enter the state shown in  FIG. 17B . When the sliders  522  and  523  move, as shown in  FIGS. 15B and 16B , the pins  558  and  578  respectively contact the side surfaces  557 A and  577 A of the holder openings  557  and  577  to push toward the left side. Thereby, the pressing members  530 ,  531 , and  570  are pushed to rotate in the clockwise direction, and thus the pressing members  530  and  531  and the support roller  510  move to the second position, which is a separated position that is dropped below the first position. Further, in the black mode, the primary transfer rollers  14 C,  14 M, and  14 Y also move from the first position to the second position shown in  FIG. 4 . Therefore, the trajectory of the transfer belt  10  becomes a descending trajectory as shown in  FIG. 4  in which the trajectory is inclined downwards and to the left from an area of contact between the primary transfer roller  14 Bk and the photoconductor  40 Bk, and thus the transfer belt  10  is in the second position in which it is separated from the photoconductors  40 C,  40 M, and  40 Y. 
     (Lubricant Application Mode) 
     In the lubricant application mode, the sliders  522  and  523  hold the pressing members  530  and  531  and the support roller  510  in the second position, and the driving motor M 2  is driven. When the driving motor M 2  is driven, the contact-and-separation cams  591  rotate 180 degrees as shown in  FIG. 18A , and then the driving motor M 2  stops. Thereby, the contact-and-separation cams  591  contact the ball bearings  590  at a portion where the eccentric distance between the cam shaft  592  and the outer peripheral surfaces  590   a  is the longest. Therefore, the sliders  524  and  525  slidingly move from the state shown in  FIG. 18A  toward the left side to enter the state shown in  FIG. 18B . When the sliders  524  and  525  move, the pins  611  and  612  contact the side surfaces  609 A and  610 A of the holder openings  609  and  610  to push toward the left side. Thereby, the holders  594  and  595  are pushed to rotate in the clockwise direction, and thus the support rollers  501  and  502  move to the second position, which is a separated position that is dropped below the first position. Further, in the lubricant application mode, not only the color primary transfer rollers  14 C,  14 M, and  14 Y but also the black primary transfer roller  14 Bk also move from the first position to the second position shown in  FIG. 5 . Therefore, the trajectory of the transfer belt  10  becomes positioned lower than the belt trajectory in the full color mode, and the transfer belt  10  is in the second position in which it is separated from all of the photoconductors  40 Bk,  40 C,  40 M, and  40 Y. 
     (Separation Mode) 
     In a separation mode, the manual lever  549  shown in  FIG. 14  is operated to rotate counterclockwise, which causes the cam shaft  548  and the integrated manual cams  547  to rotate as well. Thereby, the cam receivers  546  of the sliders  522  and  523  that were pushed toward the left side by the contact-and-separation cams  541  as shown in  FIG. 17B  are pushed toward the left side by the manual cams  547  as shown in  FIG. 17C . Therefore, as shown in  FIGS. 15B and 16B , the pins  558  and  578  push the side surfaces  557 A and  577 A of the holder openings  557  and  577  further toward the left side so that the pressing members  530 ,  531 , and  570  rotate further in the clockwise direction. Thereby, the pressing members  530  and  531  and the support roller  510  move from the second position indicated with solid lines to the third position, which is a separated position indicated by dot-dot-dash lines that is dropped below the second position. In other words, in the present embodiment, the trajectory of the transfer belt  10  in a mode during belt separation is not parallel to the trajectory of the transfer belt  10  during belt contact. The sliders  524  and  525  are also moved by cams further toward the left side as shown in  FIG. 18C  than in the position toward the left side to which they were moved by the contact-and-separation cams  591 , and thus the pins  611  and  612  push the side surfaces  609 A and  610 A of the openings  609  and  610  further toward the left side. Thereby, the holders  594  and  595  are pushed to rotate in the clockwise direction, and thus the support rollers  501  and  502  move to the third position, which is a separated position that is dropped below the second position. Therefore, the transfer belt  10  enters a trajectory in which it is further separated from the photoconductors  40 Bk,  40 C,  40 M, and  40 Y than in the trajectory of the second position. 
     In this way, if the pressing members  530  and  531  that press the belt edges  10   b  and  10   c  from the belt top surface  10   a  side are provided on the frame boards  520  and  521  of the transfer unit  500 , the positional accuracy between the transfer belt  10  and the pressing members  530  and  531  can be improved, and warping on the belt edges  10   b  and  10   c  can be accurately pressed down. Also, since the contact and separation of the transfer belt  10  and the pressing members  530  and  531  can be accurately managed, wear of the transfer belt  10  can be easily adjusted and the durability of the transfer belt  10  can be improved. It is also conceivable to hold the pressing members  530  and  531  with, for example, the primary transfer rollers as in the prior art. In this case, the positional accuracy with the primary transfer rollers is retained, but the most important positional relationship of the pressing members  530  and  531  is that with the belt edges  10   b  and  10   c  of the transfer belt  10 . The transfer belt  10  is supported by a plurality of support rollers at portions other than the primary transfer portion, and thus if the transfer belt  10  is held on the primary transfer rollers, there may be significant accumulation of positional accuracy errors. However, by positioning the pressing members  530  and  531  on the frame boards  520  and  521  that hold the support rollers  501  to  510  that support the transfer belt  10  as in the present embodiment, the positional accuracy between the transfer belt  10  and the belt edges  10   b  and  10   c  can be improved. 
     If the pressing members  530  and  531  are rotatably provided on the frame boards  520  and  521  as the brackets  800  by mounting them on the holders  550  and  551 , the gap X can be decreased compared to, for example, a case in which the pressing members  530  and  531  move up and down parallel to each other in the up-down direction on the frame boards  520  and  521 . Also, if the pressing members  530  and  531  are supported on the primary transfer rollers, a transfer bias is applied to the primary transfer rollers, and thus a resin must be used for the support portions in order to prevent leaks. Therefore, it is necessary to set the gap X in consideration of the thermal expansion coefficient of the resin, and as a result the gap X tends to increase. If the gap X is large, the transfer unit  500  or the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y may tilt during an attachment/detachment operation, leading to an increase in the frictional forces caused by contact with the transfer belt  10  or the surrounding members (units). However, if the pressing members  530  and  531  are configured to be able to rotate as in the present embodiment, at least the rotary support shafts  533  can be made of metal, and thus the thermal expansion can be reduced and the gap X can be set to be smaller than when using a resin. Accordingly, increases in the frictional forces caused by contact with the transfer belt  10  or the surrounding units due to unnecessary tilting during attachment/detachment of the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y can be suppressed, and thus the units can be moved smoothly. In the present embodiment, when providing the pressing members  530  and  531  on the frame boards  520  and  521 , concerns regarding bias leak are alleviated and the range of materials that can be selected is broadened, and thus the costs can also be reduced. 
     In the present embodiment, since a plurality of the belt pressing members  530  and  531  are disposed in the belt travel direction V, they can handle warping over a wide range of the belt edges  10   b  and  10   c . Thus, interference between the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y and the belt edges  10   b  and  10   c  that approach each other can be more reliably avoided, and breakage of the transfer belt  10  can be reduced, and this also contributes to improving the operability. Further, by disposing a plurality of the pressing members  530  and  531  in the belt travel direction V, the position of each belt pressing member  530  and  531  can be arbitrarily adjusted according to the level of warping of the belt edges  10   b  and  10   c . 
     In the present embodiment, the trajectory of the transfer belt  10  during a contact state in which the transfer belt  10  is in contact with the photoconductors  40 Bk,  40 C,  40 M, and  40 Y differs from the trajectory of the transfer belt  10  during a separated state in which the transfer belt  10  is separated from the photoconductors  40 Bk,  40 C,  40 M, and  40 Y. However, if the pressing members  530  and  531  that press the belt edges  10   b  and  10   c  from the belt top surface  10   a  side are rotatably provided on the frame boards  520  and  521 , the rotation direction of the pressing members  530  and  531  can be set to a direction that follows the changes in the belt trajectory. In other words, as shown in  FIGS. 19A to 19C , if the rotation direction is set such that the angle of the pressing members  530  and  531  and the belt trajectory does not change during contact and separation, contact between the flat underside surfaces  5302   c  and  5312   c  of the pressing parts  5302  and  5312  of the pressing members  530  and  531  and the belt edges  10   b  and  10   c  can be avoided, and thus breakage of the transfer belt  10  can be better prevented. 
     In the above-described embodiment, the support roller  510  and the pressing members  530  and  531  are supported by separate holders. However, for example, as shown in  FIG. 20 , the support roller  510  can be integrally provided by rotatably mounting the support roller  510  with the shaft  571  on the holders  550  and  551  that hold the pressing members  530  and  531 . In this case, since the support roller  510 , which influences the trajectory of the transfer belt  10 , and the pressing members  530  and  531  are supported by the same member, the positional accuracy between the pressing members  530  and  531  and the belt edges  10   b  and  10   c  can be improved, and thus breakage of the transfer belt  10  can be better prevented. 
     In the above-described embodiment, the contact parts  556  provided on the holders  550  and  551  that hold the pressing members  530  and  531  are made to contact the stoppers  555  formed on the frame boards  520  and  521  to hold the pressing members  530  and  531  in the first position. However, the members on which the stoppers  555  are provided are not limited to the frame boards  520  and  521 . For example, in the case that either the process cartridge units or the photoconductors are configured as units, the stoppers  555  can be provided on a base of such photoconductor units. This configuration is preferable because the pressing members  530  and  531  can be accurately arranged with members within the photoconductor units. 
     In the above-described embodiment, the edges  5302   a  and  5312   a  positioned on the rotation side of the pressing parts  5302  and  5312  of the pressing members  530  and  531  are inclined surfaces that are inclined in a direction away from the top surface  10   a  of the transfer belt  10 . In other words, the pressing parts  5302  and  5312  of the pressing members  530  and  531  are tapered. 
       FIG. 21  compares a case in which the edges  5302   a  and  5312   a  are inclined surfaces and a case in which they are not inclined surfaces. In  FIG. 21 , the solid lines indicate the first position of the pressing members  530  and  531  and the position of the transfer belt  10  in the full color mode. The dot-dot-dash lines indicate the third position of the pressing members  530  and  531  and the position of the transfer belt  10  in the detachment mode. If the pressing members  530  and  531  are provided such that they can rotate as in the present embodiment, the movement distance increases toward the edges  5302   a  and  5312   a  of the pressing parts  5302  and  5312  which are at an area that is spaced apart from the rotation support. If the edges  5302   a  and  5312   a  are not formed as inclined surfaces, when the pressing parts  5302  and  5312  move downwards in the detachment mode, the corners of the edges  5302   a  and  5312   a  would contact the top surface  10   a  on the belt edge side as shown by the dashed lines in  FIG. 21 . In the detachment mode, the transfer belt  10  does not travel, and thus there would be no wear even in the state in which the corners of the edges  5302   a  and  5312   a  contact the top surface  10   a  at the belt edges. However, depending on the position of the top surface  10   a  at the belt edges and the rotation angle of the edges  5302   a  and  5312   a , the contact state may strengthen and thus the top surface  10   a  may become recessed or a fold mark may be generated on the top surface  10   a . Therefore, it is preferable to provide a gap to the extent possible between the pressing parts  5302  and  5312  and the transfer belt  10  (belt edges  10   b  and  10   c ), or ensure that the pressing parts  5302  and  5312  do not contact the top surface  10   a  at the corners thereof in the case that there is contact between the two. Accordingly, if the edges  5302   a  and  5312   a  of the pressing parts  5302  and  5312  are configured as inclined surfaces that are inclined in a direction away from the top surface  10   a  of the transfer belt  10 , contact between the edges  5302   a  and  5312   a  and the top surface  10   a  of the transfer belt  10  can be avoided, and thus breakage of the transfer belt  10  can be reduced and the belt durability can be improved. 
     The underside surfaces  5302   c  and  5312   c  of the pressing parts  5302  and  5312  may contact the belt edges  10   b  and  10   c  depending on the position of the transfer belt  10  and the level of warping of the belt edges  10   b  and  10   c . Thus, a sheet  650  with good slidability is preferably adhered as a friction reducing part to the underside surfaces  5302   c  and  5312   c  that can contact the belt edges  10   b  and  10   c . As the sheet  650 , a polyethylene terephthalate sheet (PET sheet), a polyurethane sheet (PUR sheet), and the like can be used. As the friction reducing part, instead of the sheet  650 , a brush-like sheet  651  on which short fibers are implanted can be adhered to the underside surfaces  5302   c  and  5312   c  as shown in  FIG. 22  in order to increase the slidability between the underside surfaces  5302   c  and  5312   c  and the belt edges  10   b  and  10   c . This configuration is preferable compared to just the sheet  650  with good slidability because the contact area is reduced and thus the frictional resistance is also reduced, and because it can be anticipated that scattered toner and paper dust that is adhered to the belt edges  10   b  and  10   c  can be easily cleaned by the short fibers. 
     When removing transfer belt  10  from the transfer unit  500  that has been detached from the copier body  100  to replace it with a new transfer belt and then newly resetting the new transfer belt on the support rollers  501  to  510 , it is necessary to adjust the position of the belt in the depth direction. When adjusting the position of the belt in the depth direction at the back side of the copier body, the position adjustment is conducted at the belt edge  10   b  side. Thus, for example, as shown in  FIGS. 12A and 13A , markings  690 , which serve as a position reference for positioning the belt edge  10   b  when mounting the transfer belt  10  on the plurality of support rollers, can be provided on the pressing part  5302  of the pressing member  530 . This configuration is preferable because the belt position can be easily adjusted by mounting and setting the transfer belt  10  on the support rollers such that the belt edge  10   b  is matched to the markings  690 . 
     In the example shown in  FIGS. 12A and 13A , the markings  690  are formed as two parallel lines extending in the belt travel direction on a top surface  5302   d  that is on the opposite site of the underside surface  5302   c  of the pressing part  5302 . When mounting and setting the transfer belt  10 , the transfer belt  10  can be easily positioned in the depth direction by setting the transfer belt  10  so that the end on the belt edge  10   b  is positioned between the two lines. Instead of the pressing part  5302 , the markings  690  for positioning in the depth direction can also be formed on the pressing part  5312  of the pressing member  531  that presses the belt edge  10   c  positioned on the front side of the copier body as shown in  FIGS. 12B and 13B . In this case as well, two parallel lines extending in the belt travel direction are formed on a top surface  5312   d  that is on the opposite site of the underside surface  5312   c  of the pressing part  5312 . The markings  690  are not limited to two parallel lines, and can be configured as one line extending in the belt travel direction, so that the transfer belt  10  is mounted and set by matching the belt edge  10   b  or the belt edge  10   c  to this line. 
     The belt edges  10   b  and  10   c  of the transfer belt  10  must be disposed under the pressing parts  5302  and  5312  of the pressing members  530  and  531 . From another perspective, the pressing part  5302  and the pressing part  5312  will not function if they are not disposed above the belt edge  10   b  and the belt edge  10   c . When mounting and setting the transfer belt  10 , an operation in which the belt edges  10   b  and  10   c  are slipped under the pressing members  530  and  531  must be performed. Thus, providing the markings  690  on the top surfaces  5302   d  and  5312   d  of the pressing parts  5302  and  5312  enables visual confirmation of this operation by an operator, and therefore contributes to preventing incorrect setting of the transfer belt  10 . 
     The positional relationship between the belt edges  10   b  and  10   c  of the transfer belt  10  and the pressing members  530  and  531  will now be explained. In the above-described embodiment, the transfer belt  10  travels in the belt travel direction V when the transfer belt  10  is in the full color mode, the black mode, and the lubricant application mode. Therefore, at least in the modes in which the transfer belt  10  is in a traveling state, the underside surfaces  5302   c  and  5312   c  of the pressing members  530  and  531  and the belt edges  10   b  and  10   c  are preferably in a non-contact state. In the detachment mode, the transfer belt  10  is not in a traveling state, and thus the underside surfaces  5302   c  and  5312   c  of the pressing members  530  and  531  and the belt edges  10   b  and  10   c  are in a contact state. Therein, by pushing the warped belt edges  10   b  and  10   c  downwards, contact between the transfer belt  10  and the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y can be avoided during an attachment/detachment operation of the transfer unit  500  or the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y, and this also contributes to improving the durability of the transfer belt  10 . 
     In the above-described embodiment, in the trajectory of the transfer belt  10  in the black mode, the transfer belt  10  approaches the photoconductor  40 Bk as it travels from the photoconductor  40 Y toward the photoconductor  40 Bk side. Therefore, if the third pressing members  530 ( 3 ) and  531 ( 3 ) shown in  FIG. 4  are moved, they might bite into the belt edges  10   b  and  10   c . Thus, the pressing members  530 ( 3 ) and  531 ( 3 ) can be held in the first position, or if the pressing members  530 ( 3 ) and  531 ( 3 ) are moved from the first position to the second position, they can be moved into a separated state (the second position) to a position where the pressing parts  5302  and  5312  of the pressing members  530 ( 3 ) and  531 ( 3 ) do not bite into the belt edges  10   b  and  10   c . 
     In the above-described embodiment, in the black mode, the first pressing members  530 ( 1 ) and  531 ( 1 ) and the second pressing members  530 ( 2 ) and  531 ( 2 ) are also moved from the first position to the second position so that they are in a separated state from the belt edges  10   b  and  10   c . However, since contact between the warped belt edges  10   b  and  10   c  and the process cartridge units  18 Bk,  18 C,  18 M, and  18 Y must ultimately be avoided in the detachment mode, in other modes in which the belt edges are positioned above their position in the detachment mode, the first pressing members  530 ( 1 ) and  531 ( 1 ) and the second pressing members  530 ( 2 ) and  531 ( 2 ) do not necessarily have to be positioned in a separated position. In the above-described embodiment, the reason that the first pressing members  530 ( 1 ) and  531 ( 1 ) and the second pressing members  530 ( 2 ) and  531 ( 2 ) are moved from the first position to the second position so that they are in a separated position in the black mode is as follows. If the amount of one movement in the contact-and-separation operation is increased, a problem arises in that the diameter of the contact-and-separation cams  541  increases or the contact-and-separation torque increases. Thus, by moving the first pressing members  530 ( 1 ) and  531 ( 1 ) and the second pressing members  530 ( 2 ) and  531 ( 2 ) in advance before transitioning to the detachment mode, the movement amount to the third position decreases, and as a result increases in the diameter of the contact-and-separation cams  541  or increases in the size of the driving motor M 1  can be prevented. 
     In order to reduce the frictional resistance between the warped belt edges  10   b  and  10   c  and the pressing members  530  and  531 , the underside surfaces  5302   c  and  5312   c  of the pressing parts  5302  and  5312  can be formed as curved surfaces that have a bend as shown in  FIG. 23  instead of flat surfaces. In other words, in  FIG. 23 , the curved surfaces function as friction reducing parts. Alternatively, instead of configuring the pressing parts  5302  and  5312  in a panel shape, rotator  652  that have curved circumferential surfaces and can rotate such as a roller or a roller can be provided as pressing parts on the mounts  5301  and  5311  as shown in  FIG. 24 . In other words, rotator  652  that can rotate can be provided on the pressing members  530  and  531  at areas opposing the belt edges  10   b  and  10   c  of the transfer belt  10 . 
     The pressing members  530  and  531  and the support roller  510  as well as the configuration of the contact-and-separation assembly  600  thereof are not limited to an embodiment for preventing warping of the transfer belt  10  of the transfer unit  500 . As applicable functional units, the above members can be applied to all units that can be detachably attachable to the copier body  100 , include a traveling belt, and have other units disposed adjacent thereto. For example, the above members can be applied to a direct transfer system transfer unit in which a toner image is directly transferred to a paper sheet while the paper sheet P is being conveyed without first transferring the toner image to a transfer belt, a conveyance unit in which a belt that conveys a paper sheet P travels to convey the paper sheet, a secondary transfer unit in which a secondary transfer member is constituted by a belt, and the like, and therein the same operational effects as the present embodiment can be obtained. 
     In the above-described embodiment, metal rotary support shafts  533  are provided on the frame boards  520  and  521 , holes  801  and  801  are formed in the holders  550  and  551 , and the rotary support shafts  533  are inserted into the holes  801  and  801 . Thereby, the holders  550  and  551  are configured as arms that are rotatable centered on the rotary support shafts  533 . However, the above-described embodiment is not limited to this configuration. For example, as shown in  FIGS. 25A and 25B , the holders  550  and  551  can be configured as arms that are rotatable centered on the rotary support shafts  533  by forming the holes  801  and  801  in the frame boards  520  and  521 , providing the metal rotary support shafts  533  at areas of the holders  550  and  551  that oppose the holes  801  and  801 , and inserting the rotary support shafts  533 ,  533  of the holders  550  and  551  into the holes  801  and  801  of the frames. In this disclosure, examples of embodiments have been explained above, but the present invention is not limited to the specific embodiments, and various deformations and variations are possible within the scope of the present invention as recited in the claims unless otherwise specifically limited in the above explanations. The above-described effects are merely examples of the optimal effects that are achieved by at least one embodiment of this disclosure, and the effects achieved by the present invention are not limited to the above-described effects. 
     According to at least one aspect of this disclosure, the pressing member that presses warping on the edges of the belt is provided on the frame that supports the plurality of supports around which the belt is wound. Therefore, variations in the positional accuracy with the belt can be reduced, and the durability of the belt can be improved without enlarging the gap between adjacent units.