Patent Publication Number: US-9429881-B2

Title: Image forming apparatus with movable surface-positioning member

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. application Ser. No. 14/063,663, filed Oct. 25, 2013, which is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2013-064926 filed Mar. 26, 2013, the disclosures of which are incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     Technical Field 
     The present invention relates to an image forming apparatus. 
     SUMMARY 
     According to an aspect of the invention, an image forming apparatus includes an image carrier that forms a color component image using a color component toner and carries the color component image; an intermediate transfer body that faces the image carrier, that is looped over plural span members, that is rotated, and that temporarily carries the color component image formed by the image carrier before transferring the color component image to a recording medium; a first-transfer member that is disposed on a back surface of the intermediate transfer body facing the image carrier, that transfers the color component image carried by the image carrier to the intermediate transfer body by forming a transfer electric field in a first-transfer region between the first-transfer member and the image carrier; a second-transfer member that is disposed so as to be in contact with a front surface of the intermediate transfer body and so as to face one of the span members disposed on the back surface of the intermediate transfer body, that transfers the color component image transferred by the first-transfer member to the intermediate transfer body to the recording medium by forming a transfer electric field in a second-transfer region between the second-transfer member and the span member; a support mechanism that supports the second-transfer member in the second-transfer region so that the second-transfer member is movable toward upstream in a transport direction of the intermediate transfer body; a surface-positioning member that is disposed at upstream of the second-transfer member in the transport direction of the intermediate transfer body, that is in contact with the back surface of the intermediate transfer body, that is movable in a direction that intersects an in-plane direction of the intermediate transfer body; a determination device that determines whether or not the recording medium is of a type having a basis weight or a thickness that is less than or equal to a predetermined value; and a controller that, in a case where the determination device determines that the recording medium is of a type having a basis weight or a thickness that is less than or equal to the predetermined value, controls the support mechanism so as to move the second-transfer member more upstream in the transport direction of the intermediate transfer body than in other cases and controls the position of the surface-positioning member so as to move the surface-positioning member in a direction such that an angle between the intermediate transfer body and the second-transfer member on upstream of the second-transfer member in the transport direction of the intermediate transfer body becomes larger than in other cases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1A  schematically illustrates an image forming apparatus according to an exemplary embodiment of the present invention, and  FIG. 1B  illustrates a part of the image forming apparatus; 
         FIG. 2A  illustrates a second-transfer region and a surrounding area during a second-transfer operation in a case where a recording medium is a thick sheet, and  FIG. 2B  illustrates the second-transfer region and a surrounding region during a second-transfer operation in a case where a recording medium is a thin sheet; 
         FIG. 3  illustrates the overall structure of an image forming apparatus according to a first exemplary embodiment; 
         FIG. 4  illustrates a drive control system of the image forming apparatus according to the first exemplary embodiment; 
         FIG. 5A  illustrates a retraction mechanism for an intermediate transfer body used in the first exemplary embodiment, and  FIG. 5B  illustrates how the retraction mechanism moves; 
         FIG. 6A  illustrates an example of a support structure for supporting one of span rollers for the intermediate transfer body of the image forming apparatus according to the first exemplary embodiment, the span roller being located immediately behind the most downstream image forming unit, and  FIGS. 6B and 6C  illustrate another example of a structure for supporting the span roller illustrated in  FIG. 6A ; 
         FIG. 7A  illustrates an example of the structure of a second-transfer device used in the first exemplary embodiment, and  FIG. 7B  illustrates how a thin sheet passes through a second-transfer region of the second-transfer device; 
         FIG. 8A  illustrates an example of a support mechanism for a second-transfer roller, and  FIG. 8B  illustrates an example of motion of the second-transfer roller; 
         FIG. 9A  illustrates an example of a drive mechanism for a surface-positioning roller,  FIG. 9B  illustrates the surface-positioning roller at a position C (advanced position (in this example, the most advanced position)), and  FIG. 9C  illustrates the surface-positioning roller at a position D (withdrawn position (in this example, the most withdrawn position)); 
         FIG. 10A  illustrates an example of a support structure for supporting a tension adjustment roller, and  FIG. 10B  illustrates an example of motion of the tension adjustment roller; 
         FIG. 11  is a flowchart showing an example of an image forming control process of the image forming apparatus according to the first exemplary embodiment; 
         FIG. 12A  illustrates the positional relationship between each photoconductor and the intermediate transfer body in a case where the image forming mode is a full color mode,  FIG. 12B  illustrates the positional relationship between each photoconductor and the intermediate transfer body in a case where the image forming mode is a monochrome mode,  FIG. 12C  schematically illustrates how a tension is applied to the intermediate transfer body when the distance between the most downstream image forming unit and a span roller immediately behind the image forming unit is small, and  FIG. 12D  schematically illustrates how a tension is applied to the intermediate transfer body when the distance between the most downstream image forming unit and the span roller immediately behind the image forming unit is large; 
         FIG. 13  illustrates how the way the intermediate transfer body is looped over rollers changes while the image forming apparatus according to the first exemplary embodiment is forming an image; 
         FIG. 14A  illustrates a first modification of a support structure for supporting a tension adjustment roller used in the first exemplary embodiment, and  FIG. 14B  illustrates a second modification of the support structure; 
         FIG. 15  illustrates a part of an image forming apparatus according to a second exemplary embodiment; 
         FIG. 16  is a flowchart showing an example of an image forming control process of the image forming apparatus according to the second exemplary embodiment; 
         FIG. 17  illustrates a part of an image forming apparatus according to a third exemplary embodiment; 
         FIG. 18  illustrates a part of an image forming apparatus according to a fourth exemplary embodiment; 
         FIG. 19  illustrates a modification of the image forming apparatus according to the fourth exemplary embodiment; 
         FIG. 20  illustrates a part of an image forming apparatus according to a fifth exemplary embodiment; 
         FIG. 21  is a table showing an example of drive control of the image forming apparatus according to the fifth exemplary embodiment; 
         FIG. 22  is a table showing the results of evaluating the influence of transfer conditions and the position of the second-transfer roller on the sheet passing performance of the image forming apparatus of Example 1 for various types of sheets; and 
         FIG. 23  is a table showing the results of evaluating the influence of transfer conditions, the position of the second-transfer roller, the position of the surface-positioning roller, and the position of the tension adjustment roller on the sheet-passing performance of the image forming apparatus of Example 2 for various types of sheets. 
     
    
    
     DETAILED DESCRIPTION 
     Overview of Exemplary Embodiments 
       FIG. 1A  schematically illustrates an image forming apparatus according to an exemplary embodiment of the present invention.  FIG. 1B  illustrates a region in the image forming apparatus near a second-transfer region. 
     Referring to  FIGS. 1A and 1B , the image forming apparatus includes one or more image carriers  1  (in this example,  1   a  to  1   d ), an intermediate transfer body  2 , first-transfer members  4 , plural span members  3  (in this example,  3   a  to  3   c ), a second-transfer member  5 , a support mechanism  6 , a surface-positioning member  7 , a determination device  11 , and a controller  12 . The image carriers  1  each form a color component image using a color component toner and carry the color component image. The intermediate transfer body  2  has a small thickness, is disposed so as to face the image carriers  1 , is looped over the span members  3 , and is rotated. The intermediate transfer body  2  temporarily carries color component images formed by the image carriers  1  before transferring the images to a recording medium S. The first-transfer members  4  are disposed on the back surface of the intermediate transfer body  2  facing a corresponding one the image carriers  1 , and each transfer a color component image carried by the image carrier  1  to the intermediate transfer body  2  by forming a transfer electric field in a first-transfer region between the first-transfer member  4  and the image carrier  1 . The second-transfer member  5  is disposed so as to face the span member  3  (in this example,  3   c ) on the back side of the intermediate transfer body  2  and so as to be in contact with a front surface of the intermediate transfer body  2 . The second-transfer member  5  transfers the color component images, which have been transferred to the intermediate transfer body  2  by the first-transfer members  4 , to the recording medium S by forming a transfer electric field in a second-transfer region between the second-transfer member  5  and the span member  3   c . The support mechanism  6  supports the second-transfer member  5  in such a way that the second-transfer member  5  is movable upstream in the transport direction of the intermediate transfer body  2 . The surface-positioning member  7  is disposed upstream of the second-transfer member  5  in the transport direction of the intermediate transfer body  2  so as to be in contact with the back surface of the intermediate transfer body  2 . The surface-positioning member  7  is movable forward and backward in a direction that intersects the in-plane direction of the intermediate transfer body  2  and forms a transport path surface of the intermediate transfer body  2  extending to the second-transfer region. The determination device  11  determines whether or not the recording medium S is of a type having a basis weight or a thickness that is less than or equal to a predetermined value. In a case where the determination device  11  determines that the recording medium S is of a type having a basis weight or a thickness that is less than or equal to a predetermined value, the controller  12  controls the support mechanism  6  so as to move the second-transfer member  5  more upstream in the transport direction of the intermediate transfer body  2  than in other cases and controls the position of the surface-positioning member  7  so as to move the surface-positioning member  7  in a direction such that the angle between the intermediate transfer body  2  the second-transfer member  5  becomes larger than in other cases. 
     A transport member  8  shown in  FIGS. 1A and 1B  transports the recording medium S toward the second-transfer region. 
     The image forming apparatus according to the present exemplary embodiment is an intermediate-transfer-type image forming apparatus. Here, the image forming apparatus may have only one image carrier  1  or plural image carriers  1 . An image forming apparatus having plural image carriers is called a tandem-type. 
     For example, in a case where the image forming apparatus is a tandem-type apparatus having plural image carriers  1 , the image carriers  1  may be constantly in contact with the intermediate transfer body  2  during an image forming operation. Alternatively, the image forming apparatus may further include a contact/separation mechanism for making the intermediate transfer body  2  be in contact with or separated from some the image carriers  1  used in an image forming operation. 
     The intermediate transfer body  2 , which a small thickness, may be an intermediate transfer belt or may be an intermediate transfer drum having a thin wall. 
     Each of the first-transfer members  4  may be a transfer member (for example, a transfer roller) that is in contact with the back surface of the intermediate transfer body  2  or may be a non-contact corotron or the like, as long as the first-transfer member  4  is capable of forming a transfer electric field in the first-transfer region between the first-transfer member  4  and the image carrier  1 . 
     The second-transfer member  5  may be any member that is capable of forming a transfer electric field in the second-transfer region between the second-transfer member  5  and an opposing member and that is disposed so as to be in contact with the front surface of the intermediate transfer body  2 . Typically, the second-transfer member  5  is a roller. 
     The support mechanism  6  may be any mechanism, such as a mechanism having a pressing lever, as long as the support mechanism  6  is capable of moving the second-transfer member  5  upstream in the transport direction of the intermediate transfer body  2  while pressing the intermediate transfer body  2  against the opposing member. 
     The surface-positioning member  7  may be moved forward and backward by using, for example, a cam. The displacement amount of the surface-positioning member  7  may be appropriately determined in accordance with the movement amount of the second-transfer member  5  upstream in the transport direction of the intermediate transfer body  2 . (The movement amount is an offset amount corresponding to an angle between a reference line connecting the center position of an opposing member to the center position of the second-transfer member before the second-transfer member is moved and a reference line connecting the center position of an opposing member to the center position of the second-transfer member after the second-transfer member is moved.) 
     The determination device  11  may be any device that is capable of determining whether or not a recording medium of a type having a basis weight or a thickness that is at or below a predetermined threshold (so-called thin sheet). For example, the determination device  11  may any device that performs such determination on the basis of information about the selected position of a recording medium selector or information obtained by a detector that detects the type of a recording medium. 
     The controller  12  may be any device that is capable of performing the following control operations when the recording medium S has a basis weight or a thickness that is less than or equal to a predetermined value: causing the second-transfer member  5  to be displaced upstream in the transport direction of the intermediate transfer body  2  by a predetermined offset amount, causing the position of the surface-positioning member  7  to be moved in a direction such that the angle between the intermediate transfer body  2  and the second-transfer member  5  is increased, and causing the path of the front surface of the intermediate transfer body  2  to be moved in a direction away from the recording medium S. 
     When the second-transfer member  5  is displaced so as to be offset, the direction in which recording medium S is output from the second-transfer region shifts in a direction away from the intermediate transfer body  2 . As a result, a thin recording medium S is prevented from adhering to the intermediate transfer body  2 . 
     As the second-transfer member  5  is displaced so as to be offset, the distance between the intermediate transfer body  2  and the second-transfer member  5  is reduced. Accordingly, the distance between the intermediate transfer body  2  and the recording medium S is reduced. In this example, it is possible to separate the intermediate transfer body  2  from an approaching recording medium S by moving the surface-positioning member  7 . Therefore, discharge due to a transfer electric field near the entrance of the second-transfer region, which may occur if the distance between the second-transfer member  5  and the intermediate transfer body  2  is too small, is effectively prevented, and thereby disturbance of an image on the intermediate transfer body  2  before the image is transferred is effectively prevented. 
     In this example, when the recording medium S is a so-called thick sheet S 1 , which has a basis weight or a thickness that is greater than a predetermined value, the second-transfer member  5  and the surface-positioning member  7  are respectively located at predetermined positions (a position A and a position C) as illustrated in  FIG. 2A . The thick sheet S 1 , which is relatively rigid, passes through the second-transfer region while being subjected to a transfer electric field in the second-transfer region. Then, the thick sheet S 1  is output along a reference line L 1 , which is substantially perpendicular to a central reference line O 1  connecting the centers of the second-transfer member  5  and the opposing member  3   c.    
     On the other hand, when the recording medium S is a so-called thin sheet S 2 , which has a basis weight or a thickness that is less than or equal to the predetermined value, as illustrated in  FIG. 2B , the second-transfer member  5  moves to a position B that is offset from the position A by a predetermined amount in the transport direction of the intermediate transfer body  2 , and the surface-positioning member  7  moves from the position C to a position D so as to increase the angle between the intermediate transfer body  2  and the second-transfer member  5 . 
     In this state, a central reference line O 2 , which connects the centers of the second-transfer member  5  and the opposing member  3   c , is inclined rightward in  FIG. 2B  by an angle β with respect to the central reference line O 1 . Therefore, a reference line L 2 , which is substantially perpendicular to the central reference line O 2 , is inclined so as to be separated from the intermediate transfer body  2  as compared with the reference line L 1 . The thin sheet S 2 , which is relatively flexible, passes through the second-transfer region while being subjected to a transfer electric field, and is output along the reference line L 2 . The thin sheet S 2  is output while maintaining a sufficient distance from the intermediate transfer body  2  so that the thin sheet S 2  may not adhere the intermediate transfer body  2 . 
     Because the surface-positioning member  7  moves in a direction such that the angle between the intermediate transfer body  2  and the second-transfer member  5  is increased, the angle between a part of the intermediate transfer body  2  in front of the entrance of the second-transfer region and the second-transfer member  5  does not become excessively small. As a result, it is not likely that discharge due to a transfer electric field occurs at the entrance of the second-transfer region and it is not likely that disturbance of an image on the intermediate transfer body  2  occurs. 
     The image forming apparatus according to the present exemplary embodiment may be configured as described below. 
     First, the controller  12  may determine an appropriate movement amount of the surface-positioning member  7  as follows. That is, when the determination device  11  determines that the recording medium is of a type having a basis weight or a thickness that is less than or equal to a predetermined value, the controller  12  may set the angle between the intermediate transfer body  2  and the tangential direction of the second-transfer member  5  on the entrance side of the second-transfer region be substantially the same as the angle formed before the second-transfer member  5  and the surface-positioning member  7  are moved. 
     In this case, when the basis weight or the thickness of the recording medium S is less than or equal to a predetermined value, as the second-transfer member  5  becomes displaced so as to be offset upstream in the transport direction of the intermediate transfer body  2 , the tangential direction of the second-transfer member  5  at the entrance of the second-transfer region shifts toward the intermediate transfer body  2 . Accordingly, the recording medium S enters the second-transfer region along a path nearer to the intermediate transfer body  2 . The movement amount of the surface-positioning member  7  in a direction away from the intermediate transfer body  2  at this time may be selected as appropriate. As long as the angle between the intermediate transfer body  2  and the tangential direction of the second-transfer member  5  is maintained to be substantially constant, discharge due to a transfer electric field does not occur, because the distance between the intermediate transfer body  2  and the second-transfer member  5  at a position immediately in front of the entrance of the second-transfer region is not excessively small. 
     The image forming apparatus may further include a tension adjustment member  13  that adjusts the tension of the intermediate transfer body  2  so as to cancel out a decrease in the tension of the intermediate transfer body  2  due to movement of the surface-positioning member  7  when the determination device  11  determines that the recording medium S is of a type having a basis weight or a thickness that is less than or equal to a predetermined value. 
     The tension adjustment member  13  may be any member that is capable of canceling out a decrease in the tension of the intermediate transfer body  2  due to movement of the surface-positioning member  7 . The tension adjustment member  13  may be disposed at any position inside or outside of the intermediate transfer body  2 , as long as the tension adjustment member  13  does not interfere with a first-transfer operation, a second-transfer operation, and the function of the surface-positioning member  7  for positioning the surface of the intermediate transfer body  2 . When the surface-positioning member  7  moves, the tension of the intermediate transfer body  2  decreases. In this case, the tension adjustment member  13  cancels out the decrease in the tension and maintains the tension of the intermediate transfer body  2 . 
     The tension adjustment member  13  may be disposed at a position that is downstream of the second-transfer region in the transport direction of the intermediate transfer body  2  and that is upstream of one of the span members  3  (in this example,  3   a ) in the transport direction of the intermediate transfer body  2 , the one of the span members  3  being disposed upstream of one of the image carriers  1  (in this example,  1   a ) that is located most upstream in the transport direction of the intermediate transfer body  2 . 
     In this case, the tension adjustment member  13  is disposed downstream of the second-transfer region in the transport direction of the intermediate transfer body  2 . 
     If the tension adjustment member  13  were disposed downstream of one of the span members  3  (in this example,  3   a ) in the transport direction of the intermediate transfer body  2 , the one of the span members  3  being disposed upstream of one of the image carriers  1  (in this example,  1   a ) that is located most upstream in the transport direction of the intermediate transfer body  2 , the first-transfer region between the image carrier  1  and a corresponding one of the first-transfer members  4  might become displaced as the tension adjustment member  13  becomes displaced. As a result, an image might not be properly first-transferred in the first-transfer region. The tension adjustment member  13  may be moved in a direction that intersects the in-plane direction of the intermediate transfer body  2 . However, in order to effectively prevent the recording medium S from adhering to the intermediate transfer body  2 , the tension adjustment member  13  may be moved so that the intermediate transfer body  2  does not become too close to the recording medium S that has passed through the second-transfer region. 
     The tension adjustment member  13  may move in such a way that the angle between the intermediate transfer body  2  and the tangential direction of the second-transfer member  5  on an exit side of the second-transfer region is maintained substantially constant. 
     The tension adjustment member  13  may move in any direction. In order to effectively prevent the recording medium S from adhering to the intermediate transfer body  2 , it is necessary that the intermediate transfer body  2  does not move excessively in a direction such that the intermediate transfer body  2  approaches the recording medium S that is passing through the second-transfer region. Therefore, the tension adjustment member  13  may move in such a way that the angle between the intermediate transfer body  2  and the recording medium S that has passed through the second-transfer region be maintained substantially constant. Here, the term “substantially constant” not only has a meaning that the angle between the intermediate transfer body  2  and the recording sheet S does not change but also has a meaning that the angle between the intermediate transfer body  2  and the recording sheet S changes only slightly. 
     In this case, the tension adjustment member  13  may be moved in any of the following ways: (1) the tension adjustment member  13  is moved in the in-plane direction of a part of the intermediate transfer body  2  between the second-transfer member  5  and the tension adjustment member  13 ; (2) the tension adjustment member  13  is moved in a direction that intersects the in-plane direction of the intermediate transfer body  2  at a position sufficiently separated from the second-transfer region; and (3) a positioning member is provided at a position upstream of the tension adjustment member  13  in the transport direction of the intermediate transfer body  2  so as to maintain the inclination of the intermediate transfer body  2  with respect to the second-transfer region to be constant, and the tension adjustment member  13  is moved in a direction that intersects the in-plane direction of the intermediate transfer body  2 . 
     The tension adjustment member  13  may be disposed at a position that is upstream of the surface-positioning member  7  in the transport direction of the intermediate transfer body  2  and that is downstream of one of the span members  3  (in this example,  3   b ) in the transport direction of the intermediate transfer body  2 , the one of the span members  3  being disposed downstream of one of the image carriers  1  (in this example,  1   d ) that is located most downstream in the transport direction of the intermediate transfer body  2 . 
     In this case, the tension adjustment member  13  is disposed upstream of the second-transfer region in the transport direction of the intermediate transfer body  2 . 
     In this case, it is necessary to dispose the tension adjustment member  13  upstream of the surface-positioning member  7  in the transport direction of the intermediate transfer body  2  so that the tension adjustment member  13  does not deform the path of the intermediate transfer body  2  extending to the second transfer region. Moreover, it is necessary to dispose the tension adjustment member  13  downstream of one of the span members  3  (in this example,  3   b ) in the transport direction of the intermediate transfer body  2 , the one of the span members  3  being disposed downstream of one of the image carriers  1  (in this example,  1   d ) that is located most downstream in the transport direction of the intermediate transfer body  2  so that the tension adjust member  13  does not influence on an operation of transferring an image in the first-transfer region. 
     The surface-positioning member  7  may move in plural steps when the determination device  11  determines that the recording medium S is of a type having a basis weight or a thickness that is less than or equal to a predetermined value. 
     The image forming apparatus may further include a detector  14  that is capable of detecting environmental conditions including temperature and humidity. When the determination device  11  determines that the recording medium S is of a type having a basis weight or a thickness that is less than or equal to a predetermined value, the controller  12  sets a movement amount of the surface-positioning member  7  under a predetermined low-temperature and low-humidity environmental condition to be larger than that under other environmental conditions. 
     In this case, the detector  14  detects temperature and humidity, and the controller  12  sets a movement amount of the surface-positioning member  7  under a predetermined low-temperature and low-humidity environmental condition to be greater than that under other environmental conditions and sets the inclination angle at which the intermediate transfer body  2  enters the second-transfer region with respect to the recording medium S to be greater than that under other environmental conditions. That is, because the recording medium S tends to be electrically charged in a low-temperature and low-humidity environment, discharge between the intermediate transfer body  2  and the recording medium S may occur near the entrance of the second-transfer region. In order to avoid such discharge, the inclination angle at which the intermediate transfer body  2  enters the second-transfer region with respect to the recording medium S is increased. 
     One of the span members  3  (for example,  3   b ) may also serve as a tension applying member. 
     In an image forming apparatus of this type, one of the span members  3  may also serve as a tension applying member that applies a predetermined tension to the intermediate transfer body  2 , and a displacement amount of the tension adjustment member  13  may be larger than a displacement amount of the tension applying member. 
     In the case where the span member  3  also serves as the tension applying member, for example, when the first-transfer member  4  becomes separated from the intermediate transfer body  2 , the tension of the intermediate transfer body  2  decreases. However, the displacement of the intermediate transfer body  2  due to the decrease in the tension, which is typically about 1 mm, is canceled out by the tension applying member. 
     Here, if the tension applying member were to also serve as the tension adjustment member  13 , it would be necessary to move the tension applying member by 10 mm or more in order to cancel out the distance when the surface-positioning member  7  is moved backward. Then, the length of a portion of the intermediate transfer body  2  between the image carrier  1  and the tension applying member would increase, the intermediate transfer body  2  would become warped substantially, and disturbance of an image due to discharge would occur in the first-transfer regions between the image carriers  1  and the intermediate transfer body  2 . Therefore, it is difficult to dispose the tension adjustment member  13  on a portion of the intermediate transfer body  2  that forms a first-transfer surface. Accordingly, even in the case where the span member  3  for forming the first-transfer surface also serves as the tension applying member, the tension adjustment member  13  may be provided independently from the tension applying member. 
     A relationship Ra&gt;Rb may be satisfied, where Ra is the resistance of the second-transfer member  5  and Rb is the resistance of one of the span members  3  (in this example,  3   c ) facing the second-transfer member  5 . 
     By setting the resistance Ra of the second-transfer member  5  to be higher than the resistance Rb of the opposing member (span member), the discharge amount on the front surface of the recording medium S is increased so that the entirety of the recording medium S may have a weak positive charge. That is, when the second-transfer member  5  becomes displaced so as to be offset upstream in the transport direction of the intermediate transfer body  2 , the recording medium S is first peeled off the second-transfer member  5  and then peeled off the intermediate transfer body  2 . At this time, because discharge that causes the back surface of the recording medium S to be positively charged occurs first, the entirety of the recording medium S become positively charged. Subsequently, discharge that causes the front surface of the recording medium S to be negatively charged occurs when the recording medium S is peeled off the intermediate transfer body  2 . If the recording medium S were positively charged excessively, the recording medium S wound be electrostatically attracted to and adhere to the intermediate transfer body  2 . Therefore, in order to control the recording medium S to be weakly positively charged, the resistance Ra of the second-transfer member  5  is made greater than the resistance Rb of the span member  3 , which faces the second-transfer member  5 , so as to reduce discharge that occurs when the recording medium S is peeled off the second-transfer member  5 . In this example, the position of the thin recording medium S is changed in a direction such that the recording medium S becomes separated from the intermediate transfer body  2 . Therefore, when the second-transfer member  5  and the span member  3  ( 3   c ) have resistances that satisfy the above relationship, a leading end portion of the recording medium S is attracted toward the intermediate transfer body  2 , and thereby the recording medium S is prevented from becoming wound around the second-transfer member  5 . 
     The image forming apparatus may further include a preprocessing unit (not shown) that is disposed in front of the second-transfer region in a transport path of the recording medium S and that preprocesses the recording medium S so as to provide a curl at a leading end portion of the recording medium S, the curl being convex toward the second-transfer member  5 . 
     In this case, because the preprocessing unit forms a curl at the leading end portion of the recording medium S, the curl being convex toward the second-transfer member  5 , the leading end portion of the recording medium S rises above the second-transfer member  5  when the recording medium S passes through the second-transfer region. Therefore, for example, it is possible to remove static electricity from the leading end portion of the recording medium S by using a charge adjusting unit  15  (described below), and therefore the recording medium S is easily and reliably peeled off the second-transfer member  5 . 
     The preprocessing unit may also perform a charging operation of negatively charging a surface of the recording medium S facing the second-transfer member  5 . In this case, a thin recording medium S is not likely to adhere to the intermediate transfer body  2 . Even if the thin recording medium S adheres to the second-transfer member  5  and passes through the second-transfer region, the leading end portion of the recording medium S rises above the second-transfer member  5 . Therefore, the recording medium S does not adhere to the intermediate transfer body  2  and is reliably peeled off the second-transfer member  5 . 
     The image forming apparatus may further include the charge adjusting unit  15  that is disposed at a position beyond the second-transfer region in the transport path of the recording medium S and that is capable of adjusting a charged state of the recording medium S. 
     In this case, where the charge adjusting unit  15  (such as a needle or a plate for removing static electricity) is additionally provided, it is possible to adjust the charge of the recording medium S that has passed through the second-transfer region. For example, it is possible to eliminate the charge of the recording medium S. 
     In the case where the charge adjusting unit  15  is additionally provided, the controller  12  may set an adjustment amount of the charge adjusting unit  15  in accordance with a displacement amount of the second-transfer member  5  when the determination device  11  determines that the recording medium S is of a type having a basis weight or a thickness that is less than or equal to a predetermined value. 
     The output angle of the thin recording medium S changes in accordance with the displacement amount of the second-transfer member  5 . Therefore, in order to accurately adjust the output angle of the recording medium S, for example, the amount of charge adjusted by the charge adjusting unit  15  (for example, the amount of static electricity to be removed) may be determined in accordance with the displacement amount of the second-transfer member  5 . 
     Hereinafter, first to fifth exemplary embodiments of the present invention, which are illustrated in the drawings, will be described in more detail. 
     First Exemplary Embodiment 
     Overall Structure of Image Forming Apparatus 
       FIG. 3  illustrates the overall structure of an image forming apparatus  20  according to the first exemplary embodiment. 
     Referring to  FIG. 3 , the image forming apparatus  20  is a so-called tandem-type intermediate-transfer image forming apparatus. The image forming apparatus  20  includes image forming units  21 , an intermediate transfer body  22 , first-transfer devices  23 , and a second-transfer device  25 . The image forming units  21  (to be specific,  21   a  to  21   d ), for plural color components (in this example, yellow (Y), magenta (M), cyan (C), and black (K),), are arranged in a substantially horizontal direction. The intermediate transfer body  22 , which has a belt-like shape and is rotatable, is disposed so as to face the image forming units  21 . The first-transfer devices  23  (to be specific,  23   a  to  23   d ) are disposed so as to be in contact with the back surface of the intermediate transfer body  22  at positions corresponding to the image forming units  21 . The first-transfer devices  23  transfer color component images, which are formed from color component toners by the image forming units  21 , to the intermediate transfer body  22 . The second-transfer device  25  is disposed so as to be in contact with the intermediate transfer body  22  at a position downstream of one of the image forming units  21  (in this example,  21   d ) that is located most downstream in the movement direction of the intermediate transfer body  22 . The second-transfer device  25  second-transfers (simultaneously transfers) the color component images, which have been first-transferred to the intermediate transfer body  22 , to a sheet S, which is an example of a recording medium. 
     The image forming apparatus  20  further includes a fixing device  27  and a sheet transport system  28 . The fixing device  27  fixes the images, which have been simultaneously transferred by the second-transfer device  25 , onto the sheet S. The sheet transport system  28  transports the sheet S to a transfer region for the second-transfer device  25  and a fixing region of the fixing device  27 . 
     In the present exemplary embodiment, each of the image forming units  21  ( 21   a  to  21   d ) includes a photoconductor  31  having a drum-like shape and the following devices, which are disposed so as to surround the photoconductor  31 : a charger  32 , such as a corotron, that charges the photoconductor  31 ; an exposure device  33 , such as a laser exposure device, that forms an electrostatic latent image on the charged photoconductor  31 ; a developing device  34  that develops the electrostatic latent image, formed on the photoconductor  31 , by using a color component toner; and a cleaner  35  that removes toner remaining on the photoconductor  31 . 
     The intermediate transfer body  22  is, for example, a belt-like member made of a rubber or a resin material. The intermediate transfer body  22  is looped over plural (in the present exemplary embodiment, three) span rollers  41  to  43 . The span roller  41  is a driving roller rotated by a driving motor (not shown), and the span rollers  42  and  43  are driven rollers. The span rollers  41  and  42  form a first-transfer surface for the photoconductors  31 . The span roller  43  is an opposing roller for the second-transfer device  25 . A cleaner  48  is provided on the front surface of a portion of the intermediate transfer body  22  facing the span roller  41 . The cleaner  48  removes toner remaining on the front surface of the intermediate transfer body  22  after second-transfer has been finished. 
     In the present exemplary embodiment, each of the first-transfer devices  23  includes a first-transfer roller  51 . The first-transfer roller  51  is disposed so as to correspond to one of the photoconductors  31  and so as to be in contact with the back surface of the intermediate transfer body  22 . By pressing the first-transfer roller  51  against the photoconductor  31  with a predetermined load, a contact region (nip region), which functions as a first-transfer region, is formed between the photoconductor  31  and the intermediate transfer body  22 . Moreover, by supplying a predetermined first transfer current to the first-transfer roller  51 , a first transfer electric field is generated in the first-transfer region, and an image on the photoconductor  31 , which is formed from a color component toner, is transferred to the intermediate transfer body  22 . 
     As illustrated in  FIGS. 3, 7A, and 7B , the second-transfer device  25  includes a second-transfer roller  71 . The second-transfer roller  71  is disposed so as to be in contact with a portion of the front surface of the intermediate transfer body  22  corresponding to the span roller  43 . A contact region (nip region), which functions as a second-transfer region, is formed between the second-transfer roller  71  and the intermediate transfer body  22 . An electricity feed roller  73  is disposed so as to be in contact with the span roller  43 , which is an opposing roller  72  for the second-transfer roller  71 . By applying a predetermined second transfer voltage Vt to the electricity feed roller  73  and by grounding the second-transfer roller  71 , an electric field is generated in the second-transfer region, and the color component toner images on the intermediate transfer body  22  are transferred to the sheet S. 
     A surface-positioning roller  130 , which is grounded, is disposed on the back side of a portion of the intermediate transfer body  22  that is located upstream of the second-transfer region in the transport direction of the intermediate transfer body  22  and that is between the span rollers  42  and  43 . The surface-positioning roller  130  moves forward and backward in a direction that intersects the in-plane direction of the intermediate transfer body  22  (in this example, in the thickness direction of the intermediate transfer body  22 ). Thus, the surface-positioning roller  130  forms, in a changeable manner, a transport path surface of the intermediate transfer body  22  extending to the second-transfer region. 
     A tension adjustment roller  150 , which is grounded, is disposed on the back surface of a portion the intermediate transfer body  22  that is located downstream of the second-transfer region in the transport direction of the intermediate transfer body  22  and that is between the span rollers  41  and  43 . As the surface-positioning roller  130  moves forward and backward, the tension of the intermediate transfer body  22  may decrease and the intermediate transfer body  22  may become deformed. If this occurs, the tension adjustment roller  150  adjusts the tension of the intermediate transfer body  22  so as to cancel out the decrease in the tension. 
     The fixing device  27  includes a heat fixing roller  81  and a press fixing roller  82 . The heat fixing roller  81  has a heater, for example, inside thereof. The press fixing roller  82  is disposed so as to be in pressed-contact with the heat fixing roller  81  and is rotated by the heat fixing roller  81 . The fixing device  27  applies heat and pressure to an unfixed image on the sheet S in a region between the fixing rollers  81  and  82  to fix the unfixed image onto the sheet S. 
     The sheet transport system  28  includes a feed roller  92 , an appropriate number of pairs of transport rollers  93 , a pair of positioning rollers  94 , and transfer belts  95 . The feed roller  92  feeds a sheet S, which is stored in a sheet container  91 , to a sheet transport path. The transport rollers  93  are disposed along the sheet transport path. The positioning rollers  94  are disposed in the sheet transport path at a position immediately in front of the second-transfer region. The positioning rollers  94  adjust the position the sheet S, and then feed the sheet S to the second-transfer region at a predetermined timing. The transfer belts  95  are disposed downstream of the second-transfer region in the sheet transport path, and transport the sheet S toward the fixing device  27 . 
     In this example, the positioning rollers  94  also serve as a curl adjuster that provides a predetermined curl (in this example, a downwardly convex curl) to a leading end portion of the sheet S and as a pre-transfer charger that charges the sheet S beforehand. A lower one of the positioning rollers  94  is grounded so that the back surface of the sheet S is negatively charged, and an upper one of the positioning rollers  94  is provided with a positive charging voltage. The positioning rollers  94  nip the sheet S therebetween with a predetermined pressing force and transport the sheet S. 
     In this example, the thin sheet S (thin paper) is preprocessed to electrostatically adhere to the second-transfer roller  71  in order to prevent the thin sheet S from adhering to the intermediate transfer body  22 . That is, because the surface of the second-transfer roller  71  is positively charged, the back surface of the sheet S is negatively charged beforehand. 
     However, by just making the sheet S adhere to the second-transfer roller  71 , the sheet S might not be separated from the second-transfer roller  71  and may become wound around the second-transfer roller  71 . Therefore, a predetermined curl is provided to the leading end portion of the sheet S in the preprocessing operation so as to prevent the leading end portion of the sheet S from adhering to the second-transfer roller  71 . Therefore, the positioning rollers  94  used in the present exemplary embodiment have a function of adjusting a curl and adjusting the amount of charge. Alternatively, a curl adjustment unit and a pre-transfer charging unit may be provided independently from the positioning rollers  94 . 
     In this example, a charge adjustment device  96  that adjusts the charge of the sheet S is disposed in the sheet transport path at a position immediately behind the second-transfer region. The charge adjustment device  96  is typically a static electricity remover  97  that reduces the charge of the sheet S. The static electricity remover  97  is, for example, a saw-tooth shaped needle for removing static electricity, to which a voltage for removing static electricity is applied. 
     When the sheet S is output from the second-transfer region toward the second-transfer roller  71 , it is possible for the static electricity remover  97  to remove static electricity from the sheet S to prevent the sheet from adhering to the intermediate transfer body  22 . However, if the sheet S becomes wound around the second-transfer roller  71  when the sheet S is output from the second-transfer region, it is not possible for the static electricity remover  97  to remove static electricity from the sheet S. In this case, it is difficult to peel off (separate) the sheet S from the second-transfer roller  71 . Therefore, it is necessary to appropriately adjust the output direction of the sheet S. 
     After the sheet S has passed through the fixing device  27 , the sheet S is output to a sheet output container (not shown) by, for example, an output roller (not shown). 
     Drive Control System of Image Forming Apparatus 
       FIG. 4  illustrates a drive control system of the image forming apparatus according to the first exemplary embodiment. 
     Referring to  FIG. 4 , a controller  100 , which controls an image-forming operation of the image forming apparatus, is a microcomputer including a CPU, a ROM, a RAM, an input/output interface, and the like. The controller  100  receives switch signals and various input signals from an input/output interface (not shown). The switch signals are sent from, for example, a start switch and an image forming mode switch for selecting an image forming mode. The input signals are, for example, sensor signals and a sheet-type-determination signal for determining whether or not the sheet S is of a type having a basis weight or a thickness that is less than or equal to a predetermined value (a so-called thin sheet or a thick sheet). The CPU executes an image forming process control program (see  FIG. 11 ) stored beforehand in the ROM. The controller  100  generates control signals for controlling control targets and sends the control signals to the control targets. 
     Here, the “sheet-type-determination signal” input to the controller of  FIG. 4  may be any signal sent from a determination device  101  that is capable of determining the type of the sheet S. The determination device  101  may be a selection switch that allows a user to select the type of the sheet S or may be a detector that is capable of detecting the basis weight or the thickness of the sheet S. 
     Referring to  FIG. 4 , control targets controlled by the controller  100  are as follows: a photoconductor drive system  102 , an intermediate transfer body drive system  103 , a retraction mechanism  104 , a current supply device  106 , a voltage application device  107 , a support mechanism  108 , and an advancing-withdrawing mechanism  109 . The photoconductor drive system  102  drives the photoconductors  31  of the image forming units  21  ( 21   a  to  21   d ). The intermediate transfer body drive system  103  rotates the intermediate transfer body  22  by, for example, rotating the span roller  41 , which is a driving roller. The retraction mechanism  104  causes the intermediate transfer body  22  to be in contact with or separated from the photoconductors  31  of the image forming units  21  ( 21   a  to  21   d ). The current supply device  106  supplies a first transfer current to the first-transfer rollers  51  of the first-transfer devices  23  corresponding to the image forming units  21 . The voltage application device  107  applies a second transfer voltage to the electricity feed roller  73  of the second-transfer device  25 . The support mechanism  108  supports the second-transfer roller  71  so that the second-transfer roller  71  is movable along the transport path of the intermediate transfer body  22 . The advancing-withdrawing mechanism  109  moves the surface-positioning roller  130  forward and backward. 
     Retraction Mechanism 
       FIGS. 5A and 5B  illustrate the details of the retraction mechanism  104  used in the present exemplary embodiment. 
     Referring to  FIGS. 5A and 5B , the retraction mechanism  104  causes the intermediate transfer body  22  to be into contact with or to be separated from the photoconductors  31  of the image forming units  21   a  to  21   c . However, the retraction mechanism  104  does not cause the intermediate transfer body  22  to be separated from the image forming unit  21   d , which is one of the image forming units  21  that is located most downstream in the movement direction of the intermediate transfer body  22 . In this example, when the retraction mechanism  104  retracts the intermediate transfer body  22  from the photoconductors  31  of the image forming units  21   a  to  21   c , the retraction mechanism  104  also retracts the first-transfer rollers  51  of the first-transfer devices  23  corresponding to the image forming units  21   a  to  21   c  to positions such that the photoconductors  31  of the image forming units  21   a  to  21   c  are not in contact with the intermediate transfer body  22 . 
     The retraction mechanism  104  includes an intermediate transfer body contact/separation mechanism  110  and a link mechanism  120 . The intermediate transfer body contact/separation mechanism  110  causes the intermediate transfer body  22  to be into contact with or separated from the photoconductors  31  of the image forming units  21  (in this example,  21   a  to  21   c ). The link mechanism  120 , which is linked with the intermediate transfer body contact/separation mechanism  110 , causes the first-transfer devices  23  (in this example,  23   a  to  23   c ) of the image forming units  21  ( 21   a  to  21   c ) to be in contact with or separated from the intermediate transfer body  22 . 
     Here, the intermediate transfer body contact/separation mechanism  110  includes an immovable positioning roller  111  and a movable positioning roller  112 . The immovable positioning roller  111  is disposed at a fixed position that is located in the movement path of the intermediate transfer body  22  and that is between the image forming units  21   c  and  21   d  so as to be in contact with the back surface of the intermediate transfer body  22 . The movable positioning roller  112  is disposed so as to be movable in a region that is located upstream of the image forming unit  21   a  in the movement direction of the intermediate transfer body  22  so as to be in contact with the back surface of the intermediate transfer body  22 . Here, the image forming unit  21   a  is one of the image forming units  21  that is located most upstream in the movement direction of the intermediate transfer body  22 . The movable positioning roller  112  is supported by a swing base  113  that is swingable about a swing pivot  114 . 
     As illustrated in  FIG. 5B , a drive system of the intermediate transfer body contact/separation mechanism  110  includes a driving motor  115  that is activated by a control signal sent from the controller  100 . A driving force from the driving motor  115  is transmitted through a drive transmission mechanism  116 , such as a gear and a belt, to the swing pivot  114  of the swing base  113 . 
     The link mechanism  120  includes a swing plate  121 , a swing pivot  122 , an urging spring  123 , a rotation member  124 , and a contact tab  125 . The swing plate  121  is swingable around the swing pivot  122  in a space surrounded by the intermediate transfer body  22 . The first-transfer devices  23   a  to  23   c  are fixed to the swing plate  121 . The swing pivot  122  is located between the image forming units  21   c  and  21   d . The urging spring  123  urges the swing plate  121  toward the intermediate transfer body  22 . The rotation member  124 , which rotates as the swing base  113  swings, is fixed to the swing pivot  114  of the swing base  113  of the intermediate transfer body contact/separation mechanism  110 . The contact tab  125  is disposed at a position separated from the swing pivot  114  of the rotation member  124 . The contact tab  125  is in contact with a free end of the swing plate  121 . 
     Referring to  FIG. 5B , when bringing the intermediate transfer body  22  into contact with the photoconductors  31  of all the image forming units  21  ( 21   a  to  21   d ), the retraction mechanism  104  moves the movable positioning roller  112  of the intermediate transfer body contact/separation mechanism  110  to an advanced position shown by a solid line. 
     At this time, a portion of the intermediate transfer body  22  corresponding to the image forming units  21   a  to  21   c  is positioned by the immovable positioning roller  111  and the movable positioning roller  112 , the photoconductors  31  of the image forming units  21  ( 21   a  to  21   c ) are in contact with the intermediate transfer body  22 , and the first-transfer rollers  51  of the first-transfer devices  23  ( 23   a  to  23   c ) corresponding to the image forming units  21  ( 21   a  to  21   c ) are in contact with the intermediate transfer body  22 . 
     Referring to  FIG. 5B , when separating the intermediate transfer body  22  from the photoconductors  31  of the image forming units  21  ( 21   a  to  21   c ), excluding the most downstream image forming unit  21   d , the retraction mechanism  104  retracts the movable positioning roller  112  of the intermediate transfer body contact/separation mechanism  110  to a retraction position shown by a two-dot chain line. 
     At this time, a portion of the intermediate transfer body  22  corresponding to the image forming units  21   a  to  21   c  is positioned by the immovable positioning roller  111  and the span roller  41 , the photoconductors  31  of the image forming units  21  ( 21   a  to  21   c ) are not in contact with the intermediate transfer body  22 , and the intermediate transfer body  22  is not in contact with the movable positioning roller  112 , which is located at the retraction position. As illustrated in  FIG. 5B , when the movable positioning roller  112  moves to the retraction position, the rotation member  124  of the link mechanism  120  is moved to a position shown by a two-dot chain line. The rotation member  124  presses the swing plate  121  through the contact tab  125  so that the swing plate  121  rotates downward around the swing pivot  122 . As a result, the first-transfer devices  23  (in this example,  23   a  to  23   c ), which are disposed on the swing plate  121 , become separated from the intermediate transfer body  22 . 
     Support Structure for Supporting Span Roller 
     In this example, a support structure for supporting the span roller  42  for the intermediate transfer body  22  may be appropriately selected.  FIGS. 6A to 6C  illustrate examples of the support structure. 
       FIG. 6A  illustrates a support structure in which the span roller  42  also serves a tension applying roller. Both ends of the span roller  42  are urged by the urging springs  45 , so that a predetermined tension is applied to the intermediate transfer body  22 . Moreover, one of the ends of the span roller  42  is swingably supported by a steering mechanism  46  so that meandering of the intermediate transfer body  22  may be corrected. 
     A bearing  47  rotatably supports the span roller  42 . 
       FIG. 6B  illustrates a support structure that does not have the steering mechanism  46 . Both ends of the span roller  42  are urged by the urging springs  45 , so that the span roller  42  also serves as a tension applying roller. 
     In this case, for example, guide members for guiding the transport path of the intermediate transfer body  22  may be provided at both ends of the span roller  42 , and meandering of the intermediate transfer body  22  may be prevented using the guide members. 
       FIG. 6C  illustrates a support structure in which the span roller  42  does not serve as a tension applying roller. The steering mechanism  46  supports the span roller  42  so that the span roller  42  may swing around one end of the span roller  42  and meandering of the intermediate transfer body  22  may be corrected by the steering mechanism  46 . 
     Exemplary Structure of Second-Transfer Device 
     As illustrated in  FIGS. 7A and 7B , in the present exemplary embodiment, the second-transfer device  25  has a contact region (nip region), which is a second-transfer region n, in a space between the second-transfer roller  71  and the opposing roller  72  (which is the same as the span roller  43 ). 
     The shape of the contact region, which is the second-transfer region n, may be selected as appropriate. In this example, the second-transfer roller  71  and the opposing roller  72  are selected so that the following relationships are satisfied:
 
Rt&gt;Rb
 
Ht&gt;Hb
 
dt&gt;db
 
     where Rt, Ht, and dt are respectively the resistance (volume resistivity), the hardness, and the diameter of the second-transfer roller  71 ; and Rb, Hb, and db are respectively the resistance (volume resistivity), the hardness, and the diameter of the opposing roller  72 . 
     Because the second-transfer roller  71  and the opposing roller  72  have diameters and harnesses that satisfy the above relationships, the shape of the contact region (nip), which is the second-transfer region n, is convex toward the opposing roller  72 . Therefore, as shown by a solid line in  FIG. 7B , the sheet S, which has passed through the second-transfer region n, is output in a direction away from the intermediate transfer body  22 , that is, in a direction toward the second-transfer roller  71 . 
     Moreover, in this example, because the resistance Rb of the opposing roller  72  is lower than the resistance Rt of the second-transfer roller  71 , discharge between the opposing roller  72  and the sheet S is more likely to occur in a region U, which is located immediately behind the exit of the second-transfer region n, and the sheet S becomes slightly negatively charged. As shown by an alternate long and short dash line in  FIG. 7B , a sheet S′, which has passed through the second-transfer region n, is electrostatically attracted toward the intermediate transfer body  22 , which is in contact with the opposing roller  72 . Thus, the sheet S′ becomes deformed so as to form a curled portion Sa that is curled in such a way that the leading end of the sheet S′ is located on a reference line L, which extends substantially perpendicular to a central reference line O, which connects the center of the second-transfer roller  71  to the center of the opposing roller  72 . 
     Support Mechanism for Supporting Second-Transfer Roller 
     In the present exemplary embodiment, the support mechanism  108  for supporting the second-transfer roller  71  has a structure illustrated in  FIG. 8A . 
     It is necessary that the support mechanism  108  moves the second-transfer roller  71  upstream in the transport direction of the intermediate transfer body  22  while the second-transfer roller  71  is in contact with the intermediate transfer body  22  in an upstream portion of the second-transfer region. 
     The support mechanism  108  includes a pair of pressing levers  170 , a fixed support shaft  172 , tension springs  173 , compression springs  174 , an actuator  175 , and a drive rod  176 . The pressing levers  170  are disposed at both ends of the second-transfer roller  71 . An elongated hole  171 , which allows the fixed support shaft  172  to move therein by a predetermined distance, is formed in a base end portion of each of the pressing levers  170 . The fixed support shaft  172  is inserted into the elongated holes  171  so as to be relatively movable. Shafts at both ends of the second-transfer roller  71  are rotatably supported at free end portions of the pressing levers  170 . The tension springs  173  urge the base end portions of the pressing levers  170  downward. The compression springs  174  press the free end portions of the pressing levers  170 , so that the second-transfer roller  71  is pressed against the opposing roller  72 . The actuator  175  is connected to the base end portions of the pressing levers  170  and moves the drive rod  176  forward and backward in the direction in which the elongated holes  171  extend. When the drive rod  176  of the actuator  175  is advanced, the pressing levers  170  are located at a position such that the fixed support shaft  172  abuts against the upper edges of the elongated holes  171  of the pressing levers  170  due to the urging force of the tension springs  173 . When the drive rod  176  of the actuator  175  is retracted against the urging force of the tension springs  173 , the pressing levers  170  are located at positions such that the fixed support shaft  172  abuts against the lower edges of the elongated holes  171  of the pressing levers  170 . 
     In this example, when the drive rod  176  of the actuator  175  is advanced, the second-transfer roller  71  is located at a predetermined initial position (the position A shown by a solid line in  FIG. 8B ). When the actuator  175  is withdrawn (retracted), the second-transfer roller  71  is located at a position that is upstream of the position A in the transport direction of the intermediate transfer body  22  (the position B shown by a two-dot chain line in  FIG. 8B ). 
     In the present exemplary embodiment, an attachment tab  177  protrudes from the free end portion of each of the pressing levers  170 . The static electricity remover  97 , which is the charge adjustment device  96 , is fixed to the attachment tab  177 . Therefore, when the position of the second-transfer roller  71  changes due to a change in the positions of the pressing levers  170 , the position of the static electricity remover  97  (charge adjustment device  96 ) changes as the position of the second transfer roller  71  changes. Therefore, even when the position of the second-transfer roller  71  changes, the relative positions of the second-transfer roller  71  and the static electricity remover  97  is maintained to be constant. 
     In  FIG. 8A , a transfer container  178  contains both of the second-transfer roller  71  and the static electricity remover  97 . 
     Advancing-withdrawing Mechanism for Surface-Positioning Roller 
       FIG. 9A  illustrates an example of the structure of the advancing-withdrawing mechanism  109  for the surface-positioning roller  130 . 
     Referring to  FIG. 9A , bearings  131 , which are disposed at both ends of the surface-positioning roller  130 , rotatably support shafts at both ends of the surface-positioning roller  130 . The advancing-withdrawing mechanism  109  includes the bearings  131 , urging springs  132 , an eccentric cam  133 , and a driving motor  134 . The urging springs  132  urge the bearings  131  so that the surface-positioning roller  130  is pressed against the back surface of the intermediate transfer body  22 . The eccentric cam  133 , having a rotation center is displaced from its center, is disposed so as to be in contact with one of the bearing  131  for the surface-positioning roller  130 . The driving motor  134  appropriately rotates the eccentric cam  133  so as to change the position of the surface-positioning roller  130  forward and backward. 
     As illustrated in  FIG. 9A , the eccentric cam  133  changes the position of the surface-positioning roller  130  as the distance h between the centers of the surface-positioning roller  130  and the eccentric cam  133  is changed. Accordingly, the surface-positioning roller  130  is moved forward and backward between a position shown in  FIG. 9B  and a position shown in  FIG. 9C . At the position shown in  FIG. 9B , the distance h between the centers of the surface-positioning roller  130  and the eccentric cam  133  is the maximum distance h 1 . At the position shown in  FIG. 9C , the distance h between the centers of the surface-positioning roller  130  and the eccentric cam  133  is the minimum distance h 2 . 
     In order to stabilize the movement path of the surface-positioning roller  130 , for example, the path of the surface-positioning roller  130  may be restricted by using guide rails (not shown). 
     Therefore, in this example, it is possible to move the surface-positioning roller  130  to any position within the range of the aforementioned forward and backward movement by adjusting the angular position of the eccentric cam  133 . For example, by appropriately determining the distance h between the centers of the surface-positioning roller  130  and the eccentric cam  133 , the initial position (position C) of the surface-positioning roller  130  corresponding to the initial position (position A) of the second-transfer roller  71  and a displaced position (position D) of the surface-positioning roller  130  corresponding to the displaced position of (position B) of the second-transfer roller  71  may be determined beforehand. 
     Support Structure for Supporting Tension Adjustment Roller 
       FIG. 10A  illustrates a support structure for supporting the tension adjustment roller  150 . 
     Referring to  FIG. 10A , at least a part of the tension adjustment roller  150  protrudes outward from a tangential reference line J connecting the span rollers  41  and  43  for the intermediate transfer body  22 . Shafts at both end of the tension adjustment roller  150  are supported so as to be slidable along guide rails  151 . An urging spring  152  urges the tension adjustment roller  150  against the back surface of the intermediate transfer body  22 . 
     In particular, a part of the intermediate transfer body  22  extending between the span roller  43  and the tension adjustment roller  150  forms an angle θ with respect to a horizontal reference line Lh. The angle θ may be appropriately determined so that the sheet S does not adhere to the intermediate transfer body  22  after passing through the second-transfer region n. The angle θ may be, for example, 10° or more, or preferably 20° or more. 
     In this example, the guide rails  151  are disposed so as to extend substantially parallel to the movement path of the intermediate transfer body  22  between the span roller  43  and the tension adjustment roller  150 . The spring constant of the urging spring  152 , which urges the tension adjustment roller  150 , is greater than the spring constant of the urging springs  45  attached to the span roller  42 , which also serves as a tension applying roller. 
     When, for example, the surface-positioning roller  130  moves from the initial position (position C) to the displaced position (position D), the tension of the intermediate transfer body  22  decreases. In this example, as illustrated in  FIGS. 10A and 10B , the urging spring  152  urges the tension adjustment roller  150  so that the tension adjustment roller  150  moves from a position shown by a two-dot chain line to a position shown by a solid ling along the guide rails  151 . As a result, the intermediate transfer body  22  becomes stretched and the tension of the intermediate transfer body  22  is adjusted to a predetermined level. 
     At this time, even when the tension adjustment roller  150  moves, the angle between a part of the intermediate transfer body  22  immediately behind the second-transfer region n and the horizontal reference line Lh does not change from that before the tension adjustment roller  150  moves. That is, the angle is maintained to be θ with respect to the horizontal reference line Lh. Therefore, the sheet S that has passed through the second-transfer region n is not likely to adhere to the intermediate transfer body  22  as the tension adjustment roller  150  moves. 
     Operation of Image Forming Apparatus 
     Next, an operation of the image forming apparatus according to the present exemplary embodiment will be described. 
       FIG. 11  is a flowchart showing an example of an image forming control process of the image forming apparatus according to the present exemplary embodiment. 
     A user selects a full color mode (FC mode) or a monochrome mode (K mode) by operating an image forming mode switch (not shown). 
     Setting Image Forming Mode 
     When an FC mode is selected, the controller  100  determines that the image forming mode is the FC mode and selects an FC mode process. In this state, the controller  100  causes the retraction mechanism  104  to bring the intermediate transfer body  22  into contact with the photoconductors  31  of all of the image forming units  21  ( 21   a  to  21   d ), as illustrated in  FIGS. 4 and 12A . 
     When a monochrome mode is selected, the controller  100  determines that the image forming mode is the monochrome mode and selects a monochrome process. In this state, the controller  100  causes the retraction mechanism  104  to bring the intermediate transfer body  22  into contact with the photoconductors  31  of some of the image forming units  21  ( 21   a  to  21   c ), excluding the most downstream image forming unit  21   d , as illustrated in  FIGS. 4 and 12B . 
     In the case where the monochrome mode process is selected, the relationship between the most downstream image forming unit  21   d  and the span roller  42 , which is located downstream of the image forming unit  21   d , is as follows. 
     In the monochrome mode, the retraction mechanism  104  causes the photoconductors  31  of the image forming units  21  ( 21   a  to  21   c ), excluding the most downstream image forming unit  21   d , to be separated from the intermediate transfer body  22  and causes the first-transfer rollers  51  to be separated from the back surface of the intermediate transfer body  22 . Therefore, the tension of the intermediate transfer body  22  decreases. In the case where the span roller  42  also serves as a tension applying roller, the span roller  42  cancels out the decrease in the tension of the intermediate transfer body  22 . At this time, the displacement amount of the span roller  42  is as small as about 1 mm. Therefore, a span m of the intermediate transfer body  22  between the span roller  42  and most downstream image forming unit  21   d  (to be specific, the first-transfer region between the photoconductor  31  and the first-transfer roller  51 ) does not increase. 
       FIG. 12C  schematically illustrates how the span roller  42  cancels out a decrease in the tension of the intermediate transfer body  22  when the monochrome mode is selected and how a tension T is applied to the intermediate transfer body  22 . This corresponds to a case where the span m of a part of the intermediate transfer body  22  between the most downstream image forming unit  21   d  and the span roller  42  is small (m=m1). Even if a predetermined pressing force P is applied to the intermediate transfer body  22  due to vibrations or the like, the degree of warping of the intermediate transfer body  22  is not considerably large. 
     In contrast, in the case where, for example, a monochrome mode is selected, it is necessary that the movement amount of the span roller  42  be about 10 mm in order that the span roller  42  may cancel out a decrease in the tension of the intermediate transfer body  22  due to the movement of the surface-positioning roller  130  and to apply a tension T to the intermediate transfer body  22 .  FIG. 12D  illustrates how the span roller  42  cancels out the decrease in the tension of the intermediate transfer body  22 . This corresponds to a case where the span m of a part of the intermediate transfer body  22  between the most downstream image forming unit  21   d  and the span roller  42  is large (m=m2&gt;m1). If a predetermined pressing force P is applied to the intermediate transfer body  22  due to vibrations of the like, the degree of warping of the part of the intermediate transfer body  22  between the image forming unit  21   d  and the span roller  42  is large. Therefore, the degree of warping of the intermediate transfer body  22  due to vibrations is large in a region near the exit of the first-transfer region of the image forming unit  21   d . Thus, discharge due to a transfer electric field may occur and such discharge my cause disturbance of an image transferred onto the intermediate transfer body  22 . 
     Thus, even when the span roller  42  also serves as a tension applying roller, it is substantially difficult for the span roller  42  to cancel out a decrease in the tension of the intermediate transfer body  22 , which occurs when the surface-positioning roller  130  moves forward and backward. 
     As described above, when an image forming mode is selected, the controller  100  determines a sheet type on the basis of information from the determination device  101  shown in  FIG. 4 . 
     At this time, the controller  100  determines that the sheet S is a “thin sheet” when the sheet S is of a type having a basis weight or a thickness that is less than or equal to a predetermined value and otherwise determines that the sheet S is a “thick sheet”. 
     When it is determined that the sheet S is a “thick sheet”, as shown by two-dot chain lines in  FIG. 13 , the controller  100  sets the second-transfer roller  71  at the predetermined position A and sets the surface-positioning roller  130  at the predetermined position C. Moreover, the controller  100  sets the voltage of the static electricity remover  97  for removing static electricity at a predetermined voltage Vd 1 . 
     In this state, the tension adjustment roller  150  is urged by the urging spring  152 . Therefore, in accordance with the position of the surface-positioning roller  130 , the tension adjustment roller  150  is disposed at a position E shown by a two-dot chain line in  FIG. 13  so as to be in pressed contact with the back surface of the intermediate transfer body  22 . 
     When it is determined that the sheet S is a “thin sheet”, as shown by solid lines in  FIG. 13 , the controller  100  sets the second-transfer roller  71  at the predetermined position B (located upstream of the position A in the transport direction of the intermediate transfer body  22 ) and sets the surface-positioning roller  130  at the predetermined position D (separated from the position C by a predetermined distance). Moreover, the controller  100  sets the voltage of the static electricity remover  97  for removing static electricity at a predetermined voltage Vd 2  (in this example, |Vd 1 |&gt;|Vd 2 |). 
     In this state, because the surface-positioning roller  130  moves from the position C to the position D, the tension of the intermediate transfer body  22  decreases. In this example, the tension adjustment roller  150 , which is urged by the urging spring  152 , moves to a position F shown by a solid line in  FIG. 13  as the position of the surface-positioning roller  130  changes, and adjusts the decrease in the tension of the intermediate transfer body  22 . 
     Then, an image forming process is started. In the second-transfer region, a second transfer voltage is applied to the second-transfer roller  71 , a voltage for removing static electricity is applied to the static electricity remover  97 , images formed by the image forming units  21  in each image forming mode are first-transferred to the intermediate transfer body  22  in the first-transfer region and then transferred from the intermediate transfer body  22  to the sheet S in the second-transfer region. 
     How the sheet S passes through the second-transfer region will be described. Thick Sheet 
     When the sheet S is a thick sheet, a reference line L 1  is set as shown by a two-dot chain line in  FIG. 13 . The reference line L 1  is substantially perpendicular to the central reference line O 1 , which connects the centers of the second-transfer roller  71  and the opposing roller  72  (span roller  43 ). The sheet S, which is a thick sheet and is relatively rigid, passes through the second-transfer region while being subjected to a second transfer electric field. Then, the static electricity remover  97  removes static electricity from the sheet S, and the sheet S is output along the reference line L 1 . 
     At this time, the inclination of a part of the intermediate transfer body  22  on the entrance side of the second-transfer region is adjusted beforehand so as to have a sufficient angle with respect to the second-transfer roller  71 . Moreover, a part of the intermediate transfer body  22  on the exit side of the second-transfer region has a sufficient angle θ with respect to the horizontal reference line Lh. Therefore, it is not likely that disturbance of an image due to discharge caused by a transfer electric field occurs near the second-transfer region. 
     Thin Sheet 
     When the sheet S is a thin sheet, because the position of the second-transfer roller  71  moves from the position A to the position B, the central reference line O 2 , which connects the centers of the second-transfer roller  71  and the opposing roller  72 , becomes inclined rightward by angle β with respect to the central reference ling O 1  in  FIG. 13 . Accordingly, the reference line L 2 , which is substantially perpendicular to the central reference line O 2 , becomes inclined so as to be separated from the intermediate transfer body  22  as compared with the reference line L 1 . 
     The sheet S, which is a thin sheet and is relatively flexible, passes through the second-transfer region while being subjected to a second transfer electric field. Then, the static electricity remover  97  removes static electricity from the sheet S, and the sheet S is output along the reference line L 2 . 
     At this time, a leading end portion of the sheet S, which is a thin sheet, becomes curled so as to be convex downward due to preprocessing. Therefore, the sheet S, which is a thin sheet, is output while being separated from the intermediate transfer body  22  by a sufficient distance so that the sheet S may not adhere to the intermediate transfer body  22 . Moreover, a curl is formed at the leading end portion of the sheet S so that the sheet S may not become wound around the second-transfer roller  71 . 
     Furthermore, in this example, because the discharging voltage Vd 2  applied to the static electricity remover  97  is lower than Vd 1  in the case of a thick sheet, the effect of removing static electricity from the sheet S, which is a thin sheet, is suppressed as compared with that for a thick sheet. 
     Because the surface-positioning roller  130  moves from the position C to the position D, the angle between the horizontal reference line Lh and a part of the intermediate transfer body  22  on the entrance side of the second-transfer region is increased. Therefore, the angle formed between the second-transfer roller  71  and a part of the intermediate transfer body  22  on the entrance side of the second-transfer region does not become excessively small. As a result, it is not likely that discharge due to a transfer electric field occurs at the entrance of the second-transfer region and it is not likely that disturbance of an image on the intermediate transfer body  22  occurs. 
     When the tension adjustment roller  150  moves from the position E to the position F as the surface-positioning roller  130  moves, the inclination of a part of the intermediate transfer body  22  on the exit side of the second-transfer region does not change and remains constant. Therefore, it is not likely that the sheet S, which is a thin sheet, adheres to the intermediate transfer body  22  after passing through the second-transfer region. 
     Thus, depending on whether the type of the sheet S is a “thick sheet” or a “thin sheet”, the positions of the second-transfer roller  71  and the surface-positioning roller  130  are adjusted, and the effect of removing static electricity from the sheet S by the static electricity remover  97  is adjusted. As a result, after passing through the second-transfer region, the sheet S is peeled off and output from the second-transfer region without adhering to the intermediate transfer body  22  and without becoming wound around the second-transfer roller  71 . 
     Such an operation is continued until all sheets to be processed in an image forming job are output. 
     In the present exemplary embodiment, the tension adjustment roller  150  moves along the guide rails  151  to control the movement path of the intermediate transfer body  22 . However, this is not necessarily the case.  FIGS. 14A and 14B  illustrate first and second modifications regarding the tension adjustment roller  150 . 
     First Modification 
       FIG. 14A  illustrates a first modification in which, as in the first exemplary embodiment, at least a part the tension adjustment roller  150  protrudes outward from the tangential reference line J connecting the span rollers  41  and  43  for the intermediate transfer body  22 . The first modification differs from the first exemplary embodiment in the following two respects. First, the tension adjustment roller  150  is disposed at a position sufficiently separated from the second-transfer region, such as a position near the span roller  41 . (The position is, for example, a position at which s1&gt;s2 is satisfied, where s1 is the distance between the centers of the span roller  43  and the tension adjustment roller  150  along the tangential reference line J, and s2 is the distance between the centers of the tension adjustment roller  150  and the span roller  41  along the tangential reference line J.) Second, the tension adjustment roller  150  is movable forward and backward along guide rails (not shown) in a direction that intersects the in-plane direction of the intermediate transfer body  22 , and an urging spring (not shown) urges the tension adjustment roller  150  against the back surface of the intermediate transfer body  22 . 
     With the present modification, for example, when a surface-positioning roller (not shown) moves backward, the tension adjustment roller  150  moves from a position shown by a two-dot chain line to a position shown by a solid line. Accordingly, the angle between the horizontal reference line Lh and a part of the intermediate transfer body  22  on the exit-side of the second-transfer region is changed from θ to θ′ (θ&gt;θ′). However, because the tension adjustment roller  150  is disposed at a position sufficiently separated from the second-transfer region, the change in the angle Δθ (θ−θ′) is sufficiently small, so that it is not likely that the sheet S will adhere as the inclination of the intermediate transfer body  22  is changed. 
     Second Modification 
       FIG. 14B  illustrates a second modification in which, as in the first modification shown in  FIG. 14A , the tension adjustment roller  150 , which is movable forward and backward in a direction that intersects the in-plane direction of the intermediate transfer body  22 , is disposed between the span rollers  41  and  43  for the intermediate transfer body  22 . The second modification differs from the first modification shown in  FIG. 14A  in the following respect. A positioning roller  155 , which is rotatable, is provided at a fixed position between the span roller  43  and the tension adjustment roller  150  so as to be in contact with the back surface of the intermediate transfer body  22 . The positioning roller  155  maintains the inclination of a part of the intermediate transfer body  22  on the exit-side of the second-transfer region to be constant. 
     With the second modification, when the surface-positioning roller (not shown) moves backward, the tension adjustment roller  150  moves from a position shown by a two-dot chain line to a position shown by a solid line so as to adjust a decrease in the tension of the intermediate transfer body  22 . At this time, due to the presence of the positioning roller  155 , the inclination of a part of the intermediate transfer body  22  on the exit-side of the second-transfer region is maintained to be constant. 
     In the present modification, it is not necessary that the position of the tension adjustment roller  150  be near the span roller  41 . 
     Second Exemplary Embodiment 
       FIG. 15  illustrates a part of an image forming apparatus according to a second exemplary embodiment. 
     Referring to  FIG. 15 , the basic structure of the image forming apparatus is substantially the same as that of the first exemplary embodiment. The image forming apparatus includes a support mechanism (not shown) for the second-transfer roller  71 , the surface-positioning roller  130 , and the tension adjustment roller  150 . The second exemplary embodiment differs from the first exemplary embodiment in the method of moving the tension adjustment roller  150 . 
     The elements the same as those of the first exemplary embodiment will be denoted by the same numerals, and detailed descriptions of such elements will be omitted. 
     In this example, the tension adjustment roller  150  is moved, for example, by using the following method: a bearing  158  for a tension adjustment roller  150  is connected to an end of a drive rod  157  of an actuator  156 , and the tension adjustment roller  150  is moved forward and backward by appropriately moving the drive rod  157  forward and backward. 
     As in the first exemplary embodiment, at least a part of the tension adjustment roller  150  protrudes outward from the tangential reference line J connecting the span rollers  41  and  43  for the intermediate transfer body  22 . The tension adjustment roller  150  is movable in the transport direction of the intermediate transfer body  22 . 
     In this example, the actuator  156  is controlled by a controller (not shown). The actuator  156  moves the tension adjustment roller  150  between two predetermined positions (for example, the position E and the position F) in accordance with the position of the surface-positioning roller  130  (for example, the position C and the position D) so as to adjust the tension of the intermediate transfer body  22 . 
       FIG. 16  illustrates a process for controlling an image forming operation according to the present exemplary embodiment. 
     As illustrated in  FIG. 16 , a controller (not shown) sets an image forming mode (a FC mode or a monochrome mode) and then determines the sheet-type. When the sheet is a “thick sheet”, the controller sets the second-transfer roller  71  at the position A, the surface-positioning roller  130  at the position C, and the tension adjustment roller  150  at the position E, as shown by two-dot chain lines in  FIG. 15 . When the sheet is a “thin sheet”, the controller sets the second-transfer roller  71  at the position B, the surface-positioning roller  130  at the position D, and the tension adjustment roller  150  at the position F, as shown by solid lines in  FIG. 15 . 
     In the second-transfer region, a second transfer voltage is applied to the second-transfer roller  71 , and a predetermined discharging voltage is applied to the static electricity remover  97 . 
     In this state, an image forming process is performed as in the first exemplary embodiment. 
     Third Exemplary Embodiment 
       FIG. 17  illustrates a part of an image forming apparatus according to a third exemplary embodiment. 
     In  FIG. 17 , the basic structure of the image forming apparatus is substantially the same as those of the first and second exemplary embodiments. The image forming apparatus includes a support mechanism (not shown) for supporting the second-transfer roller  71 , the surface-positioning roller  130 , and the tension adjustment roller  150 . However, the position of the tension adjustment roller  150  differs from those of the first and second exemplary embodiments. The elements the same as those of the first and second exemplary embodiments will be denoted by the same numerals, and detailed descriptions of such elements will be omitted. 
     In this example, the tension adjustment roller  150  is disposed between the span rollers  41  and  43  for the intermediate transfer body  22 . In contrast to the first and second exemplary embodiments, the tension adjustment roller  150  is in contact with the front surface of the intermediate transfer body  22 . 
     The support structure for supporting the tension adjustment roller  150  may be the same as that of any one of the first or second exemplary embodiments. In this example, the tension adjustment roller  150  is disposed so as to be press the intermediate transfer body  22  inward from a tangential reference line (not shown) between the span rollers  41  and  43 . A part the intermediate transfer body  22  between the span roller  43  and the tension adjustment roller  150  has an angle cc with respect to a vertical reference line Lv. The tension adjustment roller  150  moves forward and backward while maintaining this positional relationship. 
     Therefore, also in the present exemplary embodiment, the position (A, B) of the second-transfer roller  71 , the position (C, D) of the surface-positioning roller  130 , and the position (E, F) of the tension adjustment roller  150  change depending on whether the type of the sheet S is a “thin sheet” or a “thick sheet” Moreover, the voltage of the static electricity remover  97  for removing static electricity is appropriately set, and motion of the sheet S passing through the second-transfer region is adjusted. 
     In this example, the movement path of the tension adjustment roller  150  is set so as to maintain the angle between the vertical reference line Lv and a part of the intermediate transfer body  22  between the span roller  43  and the tension adjustment roller  150  to be constant. However, this is not necessarily the case. Alternatively, the tension adjustment roller  150  may be moved in a direction such that the angle between the intermediate transfer body  22  and the vertical reference line Lv decreases. 
     Fourth Exemplary Embodiment 
       FIG. 18  illustrates a part of an image forming apparatus according to a fourth exemplary embodiment. 
     In  FIG. 18 , the basic structure of the image forming apparatus is substantially the same as those of the first and second exemplary embodiments. The image forming apparatus includes a support mechanism (not shown) for supporting the second-transfer roller  71 , the surface-positioning roller  130 , and the tension adjustment roller  150 . However, the position of the tension adjustment roller  150  differs from those of the first to third exemplary embodiments. The elements the same as those of the first to third exemplary embodiments will be denoted by the same numerals, and detailed descriptions of such elements will be omitted. 
     In this example, the tension adjustment roller  150  is disposed upstream of the second-transfer region in the transport direction of the intermediate transfer body  22 . To be specific, the tension adjustment roller  150  is disposed at a position that is downstream of the span roller  42  in the transport direction of the intermediate transfer body  22  and that is upstream of the surface-positioning roller  130  in the transport direction of the intermediate transfer body  22 . 
     The tension adjustment roller  150  is disposed so as to be in contact with the back surface of the intermediate transfer body  22 , so as to be movable forward and backward in a direction that intersects the in-plane direction of the intermediate transfer body  22 , and is pressed against the back surface of the intermediate transfer body  22  with a predetermined urging force by an urging spring (not shown). 
     An auxiliary span roller  49  supports a part of the intermediate transfer body  22  between the span rollers  41  and  43  on the exit-side of the second-transfer region. Depending on the positional relationship between the span roller  43  and the auxiliary span roller  49 , the inclination of a part of the intermediate transfer body  22  on the exit-side of the second-transfer region is appropriately determined. 
     Therefore, also in the present exemplary embodiment, the position (A, B) of the second-transfer roller  71 , the position (C, D) of the surface-positioning roller  130 , and the position (E, F) of the tension adjustment roller  150  are changed depending on whether the type of the sheet S is a “thin sheet” or a “thick sheet”. Moreover, the voltage of the static electricity remover  97  for removing static electricity is appropriately set, and motion of the sheet S passing through the second-transfer region is adjusted. 
     When the surface-positioning roller  130  moves backward from the position C to the position D, the tension of the intermediate transfer body  22  decreases, and the tension adjustment roller  150  moves from the position E to the position F to adjust the tension of the intermediate transfer body  22 . In this state, although the tension adjustment roller  150  is on the same surface of the intermediate transfer body  22  as the surface-positioning roller  130 , the inclination of a part of the intermediate transfer body  22  on the entrance side the second-transfer region does not change even when the tension adjustment roller  150  moves forward and backward. Therefore, motion of the sheet S in the second-transfer region is not negatively affected. 
     In this example, the auxiliary span roller  49  is disposed on the back surface of a part of the intermediate transfer body  22  on the exit-side of the second-transfer region. However, this is not necessarily the case. For example, as illustrated in  FIG. 19 , the auxiliary span roller  49  may be disposed on the front surface of the intermediate transfer body  22 , and the intermediate transfer body  22  may be bent inward from the tangential reference line (not shown) between the span rollers  41  and  43 . In this case, a space formed under a bent portion  22   a  of the intermediate transfer body  22  may be used as a space for installing another device. 
     Fifth Exemplary Embodiment 
       FIG. 20  illustrates a part of an image forming apparatus according to a fifth exemplary embodiment. 
     Referring to  FIG. 20 , the image forming apparatus includes, as in the first exemplary embodiment, the support mechanism  108  for the second-transfer roller  71 , the surface-positioning roller  130 , and the tension adjustment roller  150 . The fifth exemplary embodiment differs from the first exemplary embodiment in that the positions of the second-transfer roller  71 , the surface-positioning roller  130 , and the tension adjustment roller  150  are changed from their initial positions in plural steps. 
     In this example, the controller  100  determines whether or not the sheet S is a “thin sheet” or a “thick sheet” on the basis of information from the determination device  101 . Moreover, the controller  100  determines whether or not the environmental conditions are those of a predetermined “low-temperature and low-humidity environment” (where, in this example, the temperature is 10° C. or less and the relative humidity is 15% or less) on the basis of information from an environment sensor  180  that is capable of detecting temperature and humidity. The controller  100  controls the positions of the second-transfer roller  71 , the surface-positioning roller  130 , and the tension adjustment roller  150  in accordance with a table shown in  FIG. 21 . 
     In the present exemplary embodiment, the controller  100  determines whether or not the type of the sheet S is a “thin sheet” or a “thick sheet”. If the sheet S is a “thick sheet”, as shown by a two-dot chain line in  FIG. 20 , the controller  100  sets the second-transfer roller  71  at the position A, sets the surface-positioning roller  130  at the position C, sets the voltage of the static electricity remover  97  at Vd 1 , and performs an image forming process. 
     The position of the tension adjustment roller  150  is automatically adjusted to the position E, which corresponds to the position (position C) of the surface-positioning roller  130 . 
     If the sheet S is a “thin sheet”, the controller  100  checks the environmental conditions. If the environmental conditions are those of a non-low-temperature and non-low-humidity environment, as shown by an alternate long and short dash lines in  FIG. 20 , the controller  100  sets the second-transfer roller  71  at a position B 1 , sets the surface-positioning roller  130  at a position D 1 , sets the voltage of the static electricity remover  97  at Vd 2 , and performs an image forming process. The position of the tension adjustment roller  150  is automatically adjusted to a position F 1 , which corresponds to the position (position D 1 ) of the surface-positioning roller  130 . 
     If the sheet S is a “thin sheet” and the environmental conditions are those of a low-temperature and low-humidity environment, as shown by solid lines in  FIG. 20 , the controller  100  sets the second-transfer roller  71  at a position B 2 , sets the surface-positioning roller  130  at a position D 2 , sets the voltage of the static electricity remover  97  at Vd 2 , and performs an image forming process. The position of the tension adjustment roller  150  is automatically adjusted to a position F 2 , which corresponds to the position (position D 2 ) of the surface-positioning roller  130 . 
     When the environmental conditions are those of low-temperature and low-humidity environment, the resistance of the sheet S is high and it is not easy to remove static charges. Therefore, by setting the second-transfer roller  71  at the position B 2  (which is upstream of the position B 1  in the transport direction of the intermediate transfer body  22 ), the reference line L 2  extending from the second-transfer region is shifted further downward. By setting the surface-positioning roller  130  at the position D 2  (which is further withdrawn from the position D 1 ), the inclined position of a part of the intermediate transfer body  22  on the entrance side of the second-transfer region is further separated from the horizontal reference line Lh. 
     Therefore, with the present exemplary embodiment, if the sheet S passing through the second-transfer region is a “thick sheet”, the sheet S is output along the reference line L 1 , which is substantially perpendicular to the central reference line O 1  connecting the centers of the second-transfer roller  71  and the opposing roller  72 . If the sheet S is a “thin sheet” and the environment is a non-low-temperature and non-low-humidity environment, the sheet S is output along a reference line L 21 , which is substantially perpendicular to a central reference line O 21  connecting the centers of the second-transfer roller  71  and the opposing roller  72 . Moreover, if the sheet S is a “thin sheet” and the environment is a low-temperature and low-humidity environment, the sheet S is output along a reference line L 22 , which is substantially perpendicular to a central reference line O 22  connecting the centers of the second-transfer roller  71  and the opposing roller  72 . 
     In this example, the environmental conditions are divided into two types, and the position of the second-transfer roller  71  and the position of the surface-positioning roller  130  are changed in two steps from their initial positions. However, this is not necessarily the case. The environmental conditions may be divided into three types or more, the sheet type may be divided into a larger number of types, and, in accordance with such changes, the positions of the second-transfer roller  71  and the surface-positioning roller  130  may be changed from their initial positions in three steps or more. 
     If the sheet S is a “thin sheet”, even when the environmental conditions are different, the voltage of the static electricity remover  97  is set to Vd 2 . As necessary, the voltage of the static electricity remover  97  may be changed in accordance with the environmental conditions. 
     When the actuator  156  is used to move the tension adjustment roller  150  as in the second exemplary embodiment, the controller  100  may control not only the positions of the second-transfer roller  71  and the surface-positioning roller  130  but also the position of the tension adjustment roller  150 . 
     EXAMPLES 
     Example 1 
     In Example 1, an actual example of the image forming apparatus according to the first exemplary embodiment was operated, and the sheet-passing performance was evaluated. 
     The image forming apparatus used in Example 1 was as follows.
         process speed: 640 mm/sec   intermediate transfer body: made of a polyimide resin including carbon black; volume resistivity 10 log Ω·cm, thickness 80 μm, circumference 1350 mm, tension 65 N   second-transfer roller: φ24 mm, volume resistivity 7 log Ω, hardness 75° (Asker C)   opposing roller: φ20 mm, volume resistivity 6.5 log Ω, hardness 65° (Asker C)   surface-positioning roller: φ15 mm, grounded   tension adjustment roller: φ15 mm, grounded   angle between second-transfer roller and intermediate transfer body on the entrance side of the second-transfer region: 13.8°   voltage application device: a device that generates a transfer electric field by applying a negative second-transfer voltage to the opposing roller, while the second-transfer roller is grounded   span roller  42 : also serving as a tension applying roller   discharging device: voltage −4 kV in thick-sheet mode; voltage −3 kV in thin-sheet mode   pre-transfer charger (also serving as curl adjuster): a pair of positioning rollers each having φ14 mm, one of the rollers for negatively charging the back surface of the sheet is grounded, and +3 kV is applied to an upper roller, and both rollers are pressed against each other with a force of 60 N.   evaluation environment: temperature 22° C., relative humidity 55%       

     In Example 1, the sheet-passing performance for each type of sheet was evaluated for each of the cases where a pre-transfer charging operation using a pre-transfer charger was/was not performed and the position of the second-transfer roller was the position A or the position B. 
       FIG. 22  shows the results. In  FIG. 22 , “gsm” stands for the basis weight, which corresponds to “g/m 2 ”. 
     As shown in  FIG. 22 , when the sheet was a “thick sheet” (in this example, a normal sheet having a basis weight of 64 gsm), irrespective of the position of the second-transfer roller, the sheet did not adhere to the intermediate transfer body nor became wound around the second-transfer roller, and the sheet-passing performance was good. 
     In contrast, when the sheet was a “thin sheet”, an operation of moving the second-transfer roller to the position B (offset 5°) was effective in improving the sheet-passing performance. 
     Regarding a pre-transfer charging operation using a pre-transfer charger, the sheet-passing performance in a case where a pre-transfer charging operation was performed was better a case where such an operation was not performed. 
     Example 2 
     In Example 2, an image forming apparatus the same as that of Example 1 was used, and the sheet-passing performance was evaluated for each of the cases where a pre-transfer charging operation using a pre-transfer charger was/was not performed, the position of the second-transfer roller was changed, and the position of the surface-positioning roller was changed. 
       FIG. 23  shows the results. In  FIG. 23 , “gsm” stands for the basis weight, which corresponds to “g/m 2 ”. 
     As shown in  FIG. 23 , when the sheet was a “thick sheet” (in this example, a normal sheet having a basis weight of 64 gsm) and the second-transfer roller was at the position A, irrespective of the positions of the surface-positioning roller and the tension-adjustment roller, the sheet-passing performance and the image quality were good. When the second-transfer roller was moved to the position B and the surface-positioning roller and the tension adjustment roller were respectively moved to the position C and the position E, the image quality was bad. 
     In contrast, when the sheet was a “thin sheet” and the second-transfer roller was at the position A, the sheet-passing performance was bad and the image quality was not evaluated. 
     When the second-transfer roller was set at the position B and the surface-positioning roller and the tension adjustment roller were respectively set at the position D and the position F, both the sheet-passing performance and the image quality were mostly good. 
     Also in Example 2, the sheet-passing performance in a case where a pre-transfer charging operation was performed was better than in a case where such an operation was not performed, even for a thinner sheet. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.