Patent Publication Number: US-9897962-B2

Title: Sheet processing device and image forming apparatus provided with the same

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a sheet processing device that folds a sheet bundle formed by sheets that are sequentially carried out from an image forming device such as a copier or a printer and accumulated in a bundle and, more particularly, to a sheet processing device that performs processing for preventing the folded sheet bundle from being opened after discharge by pressing a fold of a folded sheet bundle while preventing damage of a sheet end portion and deviation of the folded sheet bundle from a proper position. 
     Description of the Related Art 
     There are widely known processing devices that align sheets carried out from an image forming device and staple them, or fold them into a booklet. Among them, some processing devices are configured to saddle-stitch the sheet bundle with a staple or an adhesive and fold the sheet bundle into a booklet. Such a device performs folding sheet bundle formed by 2 or 3 sheets to about 30 sheets in two; however, there may be a case where the two-folded sheet bundle is unintentionally opened after being discharged from the device, degrading aligning property of the sheet bundle, which results in reduction in accumulation amount. 
     Thus, as processing to cope with this problem, it is widely known that the fold of the two-folded sheet bundle that has once been subjected to folding is subjected to pressing from front and back sides thereof. 
     For example, as illustrated in FIG. 21, Japanese Patent No. 4,514,217 discloses a device that presses from above and below the fold of a folded sheet bundle BS by means of a pressing roller moving along the fold. The pressing roller is a pair of pressing rollers 361 a  and 361 b . The pressing roller pair 361 a  and 361 b  are supported by a holder 362 and moved thereby along a fold direction F. With this configuration, the folded sheet is prevented from being opened after discharge. 
     Further, as illustrated in FIGS. 22A and 22B, Jpn. Pat. Appln. Laid-Open Publication No. 2012-201462 disclose a device in which two pressing roller pairs 461 and 463 are provided in a holder 462 that is moved along the fold of the sheet bundle. The pressing roller pair 461 is long in a sheet conveying direction and the processing roller pair 463 is short in the sheet conveying direction. In this device, depending on the number of sheets forming the sheet bundle, a position of the fold of the sheet bundle is moved forward and backward. When the number of sheets is small, the fold is pressed only by the long roller pair, and when the number of sheets is large, both the long and short pressing roller pairs 461 and 463 are used to press twice the fold. Thus, when the number of sheets is large, the fold is pressed at two positions and thereby strengthened. 
     As another embodiment, there is disclosed a configuration in which one of pressing roller pairs disposed in the fold direction is designed such that opposing rollers can be moved in a direction separating from each other. That is, when the number of sheets is large, the fold is pressed by the pressing roller pairs of two rows; on the other hand, when the number of sheets is small, the fold is pressed only by the pressing roller pairs of one row. 
     Further, as illustrated in FIGS. 23A and 23B, Jpn. Pat. Appln. Laid-Open Publication No. 2014-76903 discloses a processing device provided with a pair of pressing rollers 561 a  and 562 a  that press the fold of a two-folded sheet bundle in a sheet thickness direction and a holder 560 that reciprocates the pressing roller pair in a sheet width direction. Further, the processing device is configured to move the rollers of the pressing roller pair between a position where they are separated away from each other and a position where they are brought into pressure contact with each other. Upon re-folding the sheet bundle, the holder 560 is moved inward of the sheet bundle from an end portion thereof in the sheet width direction with the rollers of the pressing roller pair separated away from each other, as illustrated in FIG. 23A. After the holder 560 is moved inward, the rollers of the pressing roller pair are brought into pressure contact with each other to press one side of the sheet bundle. After the holder 560 passes through one end portion of the sheet bundle, the pressure contact state between the pair of pressing rollers are released to separate the rollers of the roller pair from each other and, when the holder 560 is moved toward the other side, the pressing roller pair is made to pass through the end portion of the sheet bundle in the separated state. Thereafter, the rollers of the pressing roller pair are brought into pressure contact with each other once again to press the other side of the sheet bundle. In other words, the pressing against the fold of the sheet bundle is started from the inside of the sheet bundle in the sheet width direction to the end portion thereof. 
     The above-described devices that perform pressing against the fold of the folded sheet bundle carried out from an image forming device or the like have the following problems. 
     In the device disclosed in Japanese Patent No. 4,514,217, the pair of pressing rollers 361 a  and 361 b  are moved inward from the sheet bundle end portion in the sheet width direction in the pressure contact state, so that the rollers collide with an end portion 371 of the sheet bundle, with the result that the sheet may be torn or damaged due to pressing. In addition, the collision may cause the sheet bundle to be inclined, so that a large holding mechanism is required for preventing the inclination. In particular, in this invention, a support fulcrum of the pair of pressing rollers 361 a  and 361 b  is positioned at a downstream side of the holder 362 in the moving direction, so that when the rollers collide with the end portion 371 of the sheet bundle, they are applied with a mutually approaching (closing) direction force, which increases impact on the sheet end portion. 
     In the device disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2012-201462, the position of the sheet bundle is changed depending on the number of sheets forming the sheet bundle BS. That is, when the number of sheets is small, the fold is pressed only by the long roller pair 461, and when the number of sheets is large, both the long and short pressing roller pairs 461 and 463 are used to press twice the fold. However, also in this case, the rollers of the pressing roller pairs 461 and 463 of respective rows are brought into pressure contact with each other, so that when, in particular, the number of sheets is large, the pressing roller pairs 461 and 463 collide with the sheet end portion, with the result that the sheet may be torn or damaged due to pressing. In addition, the collision may cause the sheet bundle to be inclined. 
     In the device disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2014-76903, the rollers of the pressing roller pair are separated from each other at a standby position outside the sheet bundle in the sheet width direction. Then, after being moved to the width direction inside of the sheet bundle having the fold in the separated state, the rollers of the pressing roller pair are brought into pressure contact with each other for sheet bundle pressing. When passing through the end portion in the sheet width direction, the rollers are separated from each other, making it possible to suppress the end portion of the sheet bundle from being torn or damaged due to pressing. However, the separation of the rollers is always made irrespective of whether the number of sheets is large or small, a separation mechanism is required, resulting in structural complication. In addition, every time the rollers are returned in its reciprocation, they need to pass through the end portion of the sheet bundle, irrespective of the number of sheets, so that it takes time to complete the pressing processing. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above problems and has a configuration in which a pressing member pair for pressing a fold of a folded sheet bundle has a first region where members thereof are brought into pressure contact with each other and a second region where members thereof are separated from each other. Thus, the folded sheet bundle formed by a small number of sheets is pressed in the first region; while the folded sheet bundle formed by a large number of sheets is pressed in the second region, whereby impact due to collision between the pressing member pair and a sheet end portion can be reduced even when the sheet end portion has a large thickness. Further, since the folded sheet bundle formed by a small number of sheets is pressed in the first region, the fold can surely be pressed. 
     That is, an object of the present invention is to provide a sheet processing device capable of minimizing damage on the sheet end portion and reducing a moving load of the pressing member pair when the pressing member pair is moved along the fold for pressing, and an image forming apparatus provided with the sheet processing device. 
     To solve the above problems, there is provided a sheet processing device that presses a fold of a folded sheet bundle, the device including: a pair of pressing members that press, in a fold thickness direction, the fold of the folded sheet bundle conveyed in a folded state; and a moving member that moves the pressing roller pair in a sheet fold direction while supporting the same. The pressing member pair presses the fold in one of a first region where members of the pressing member pair are brought into pressure contact with each other and a second region adjacent to the first region on an upstream side thereof where members of the pressing member pair are opposed to each other at an interval. 
     With this configuration, the pressing member pair has the first region where members of the pressing member pair are brought into pressure contact with each other and the second region where members of the pressing member pair are opposed to each other at an interval, so that it is possible to reduce impact generated when the pressing member pair abuts against the end portion of the sheet bundle and a moving load of the pressing member pair. 
     Thus, there can be provided a sheet processing device capable of minimizing damage on the sheet end portion due to collision with the pressing member pair that is moved along the fold for pressing, and an image forming apparatus provided with the sheet processing device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory view illustrating an entire configuration of an image forming system including an image forming apparatus according to the present invention and a sheet processing device incorporating a stepwise folding unit; 
         FIG. 2  is an explanatory view of an entire configuration of the sheet processing device incorporating the stepwise folding unit according to the present invention; 
         FIGS. 3A to 3D  are views explaining folding processing performed by a folding roller in the sheet processing device; 
         FIG. 4  is a perspective view of the stepwise folding unit of  FIG. 2  as viewed from a bundle discharge roller side; 
         FIG. 5  is a perspective view of the stepwise folding unit of  FIG. 2  as viewed from a folding roller side; 
         FIG. 6  is an explanatory view illustrating an inside of the stepwise folding unit of  FIG. 4 ; 
         FIG. 7  is a front view of the stepwise folding unit of  FIG. 4  as viewed from the folding roller side; 
         FIG. 8  is a front view illustrating a state where the pressing roller unit illustrated in  FIGS. 4 to 7  is situated at a start position; 
         FIG. 9  is a front view illustrating a state where the pressing roller unit illustrated in  FIGS. 4 to 7  is moved in the middle of the folded sheet bundle in a width direction thereof; 
         FIG. 10  is a front view illustrating a state where the pressing roller unit illustrated in  FIGS. 4 to 7  is situated at an end position in the width direction; 
         FIGS. 11A to 11C  are explanatory views illustrating a folded sheet bundle having a plurality of fold lines as a result of stepwise folding performed by the stepwise folding unit of  FIGS. 7 to 9 , in which  FIG. 11A  is a view illustrating a state where the folded sheet bundle is pressed between first upper and lower pressing rollers,  FIG. 11B  is a view illustrating a state where the folded sheet bundle is pressed between second upper and lower pressing rollers, and  FIG. 11C  is a view illustrating a state where the folded sheet bundle is pressed between third upper and lower pressing rollers; 
         FIG. 12  is a view illustrating a booklet obtained as a result of the stepwise folding of  FIGS. 11A to 11C ; 
         FIG. 13  is a conceptual view for explaining a relationship between final-stage pressing roller pair of the stepwise folding unit and folded sheet bundle; 
         FIGS. 14A to 14D  are views illustrating the pressing roller pairs of a plurality of rows provided in the stepwise folding unit, in which  FIG. 14A  is a view for explaining stepwise folding for a large number of sheets (equal to or more than a predetermined number of sheets),  14 B is a plan view illustrating a state before the stepwise folding of  FIG. 14A ,  FIG. 14C  is a view for explaining stepwise folding for a small number of sheets (less than a predetermined number of sheets),  14 D is a plan view illustrating a state before the stepwise folding of  FIG. 14C ; 
         FIG. 15  is a flowchart illustrating the stepwise folding to be executed for a large number of sheets (equal to or more than a predetermined number of sheets) and a small number of sheets (less than a predetermined number of sheets); 
         FIG. 16  is a flowchart continuing from  FIG. 15 ; 
         FIG. 17  is an explanatory view of a control configuration of the sheet processing device of  FIG. 2 ; 
         FIG. 18  is an explanatory view of a stepwise folding unit according to a first modification; 
         FIG. 19  is a view illustrating the entire configuration a stepwise folding unit according to a second modification; 
         FIG. 20  is a cross-sectional view of the stepwise folding unit according to the second modification; 
         FIG. 21  is an explanatory view of a reference 1; 
         FIGS. 22A and 22B  are explanatory views of a reference 2; and 
         FIGS. 23A and 23B  are explanatory views of a reference 3. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, the present invention will be described in detail based on illustrated embodiments.  FIG. 1  illustrates an entire configuration of an image forming system including an image forming apparatus according to the present invention,  FIG. 2  is an explanatory view of an entire configuration of a sheet processing device, and  FIGS. 3A to 3D  are views explaining a folded state of a sheet bundle in the sheet processing device.  FIG. 4  is a perspective view of a stepwise folding unit incorporated in the sheet processing device and configured to stepwise fold a sheet bundle, as viewed from a discharge side thereof, and  FIG. 5  is a perspective view of the stepwise folding unit as viewed from a folding roller side. 
     The image forming system illustrated in  FIG. 1  is constituted by an image forming device A and a sheet processing device B, and a stepwise folding unit  50  is incorporated in the sheet processing device B. 
     [Configuration of Image Forming Device] 
     The image forming device A illustrated in  FIG. 1  feeds a sheet from a sheet supply section  1  to an image forming section  2 , performing printing for the sheet in the image forming section  2 , and carries out the resultant sheet from a main body sheet discharge port  3 . In the sheet supply section  1 , sheets of a plurality of sizes are housed in sheet cassettes  1   a  and  1   b , and sheets of a designated size are fed to the image forming section  2  while being separated one from another. The image forming section  2  includes, for example, an electrostatic drum  4 , a print head (laser emitter)  5 , a developing unit  6 , a transfer charger  7 , and a fixing unit  8 . The print head  5 , developing unit  6 , transfer charger  7 , and fixing unit  8  are disposed around the electrostatic drum  4 . In this image forming section  2 , the laser emitter  5  forms an electrostatic latent image on the electrostatic drum  4 , the developing unit  6  makes toner adhere to the obtained electrostatic latent image, the transfer charger  7  transfers an image onto the sheet, and the fixing unit  8  thermally fixes the image to the sheet. The sheets with the thus formed image are sequentially carried out from the main body sheet discharge port  3 . A reference numeral  9  is a circulation path for two-sided printing. More specifically, the sheet on a front side of which the image has been printed is fed from the fixing unit  8 , reversed in a main body switchback path  10 , and made to enter the circulation path  9 , along which the sheet is fed once again to the image forming section  2  for printing on a back side of the sheet. The thus two-sided printed sheet is reversed in the main body switchback path  10  and carried out from the main body sheet discharge port  3 . 
     A reference numeral  11  is an image reading device. The image reading device  11  scans a document sheet set on a platen  12  using a scan unit  13 , and light reflected from the document and passing through a reflective mirror and a condenser lens is electrically read by a photoelectric conversion element  14 . This image data is subjected to, e.g., digital processing in an image processing section, transferred to a data storage section  17 , from which an image signal corresponding to the resultant image data is transmitted to the laser emitter  5 . A reference numeral  15  is a document feeder that feeds a document sheet housed in a document stacker  16  to the platen  12 . 
     A controller is provided in the image forming device A having the above configuration, and image formation conditions, for example, print-out conditions such as sheet size designation, color/monochrome printing designation, number-of-copies designation, one-sided/two-sided printing designation, and scaling printing designation are set through a controller panel  18 . Further, in the image forming device A, image data read by the scan unit  13  or image data transferred from an external network is stored in the data storage section  17 . The image data is transferred from the data storage section  17  to a buffer memory  19 , from which data signals corresponding to the image data are sequentially transmitted to the laser emitter  5 . 
     Simultaneously with the image forming conditions, sheet processing conditions (post-processing conditions) are input and designated via the control panel  18 . For example, (1) “printout mode”, (2) “stapling mode”, (3) “saddle stitching and folding mode”, and (4) “stepwise folding mode” are designated as the post-processing conditions. The image forming device A forms an image on the sheet in accordance with the image forming conditions and the post-processing conditions. 
     [Configuration of Sheet Processing Device] 
     The sheet processing device B connected to the above-described image forming device A is configured to receive the image-formed sheet from the main body sheet discharge port  3  of the image forming device A and perform sheet processing according to one of the following modes. 
     The modes include: (1) A mode in which the image-formed sheet is stored in a first sheet discharge tray  21  (“print-out mode” described above); (2) A mode in which the sheets from the main body sheet discharge port  3  are aligned in a bundle, bound by an end face stapler  33 , and stored in the first sheet discharge tray  21  (“stapling mode” described above); (3) A mode in which the sheets from the main body sheet discharge port  3  are aligned in a bundle in a stacker section  35  which is a second processing tray, bound at a portion near a center thereof using a saddle stitching stapler  40 , folded into a booklet, and stored in a second sheet discharge tray  22  (“saddle stitching and folding mode” described above); and (4) A mode in which a sheet loop is folded stepwise at a fold of a folded sheet bundle that has been saddle-stitched and folded into a booklet and then stored in the second sheet discharge tray  22  (“stepwise folding mode” described above). 
     As illustrated in  FIG. 2 , the sheet processing device B includes, in a casing  20  thereof, the first and second sheet discharge trays  21  and  22  and a sheet carry-in path P 1  having a carry-in port  23  connected to the main body sheet discharge port  3 . The sheet carry-in port P 1  is constituted by a straight line path extending in a substantially horizontal direction. Further, first and second switchback conveying paths SP 1  and SP 2  are branched from the sheet carry-in path P 1 , along which the sheet is conveyed in a reverse direction. The first switchback conveying path SP 1  is branched from the sheet carry-in path P 1  at a path downstream side, and the second switchback path SP 2  is branched from the sheet carry-in path P 1  at a path upstream side. The first and second switchback conveying paths SP 1  and SP 2  are distanced from each other. 
     In such a path configuration, in the sheet carry-in path P 1 , there are disposed a carry-in roller  24  and a sheet discharge roller  25 . The sheet discharge roller  25  is configured to be rotatable in normal and reverse directions. Further, in the sheet carry-in path P 1 , there is disposed a path switching piece (not illustrated) for guiding the sheet to the second switchback conveying path SP 2 , and the piece is coupled to an operation section such as a solenoid. Further, the sheet carry-in path P 1  has, on the downstream side of the carry-in roller  24 , a stamp section that performs stamping on the sheet fed from the carry-in port  23  or a single-sheet punching unit  28  that punches the sheets fed from the carry-in port  23  one by one. 
     [Configuration of First Switchback Conveying Path SP 1 ] 
     The first switchback conveying path SP 1  disposed on the downstream side (rear end portion of the device) of the sheet carry-in path P 1  as illustrated in  FIG. 2  is configured as described below. The sheet carry-in path P 1  is provided, at its exit end, with the sheet discharge roller  25  and a processing tray  29  on which the sheets fed by the sheet discharge roller  25  are stacked and supported. There is disposed, above the processing tray  29 , a normal/reverse rotation roller  30  capable of moving up and down between a position to come into contact with the sheet on the tray and a standby position spaced apart therefrom. The normal/reverse rotation roller  30  is controlled to rotate in a clockwise direction in  FIG. 2  when the sheet approaches the processing tray  29  to which the normal/reverse rotation roller  30  is coupled, and to rotate in a counterclockwise direction after a sheet rear end passing through the sheet discharge roller  25  enters the processing tray  29 . Thus, the first switchback conveying path SP 1  is positioned above the processing tray  29 . 
     Further, the first sheet discharge tray  21  is located downstream of the first switchback conveying path SP 1  and is configured to support a leading end of the sheet to be guided to the first switchback conveying path SP 1  and the second switchback conveying path SP 2 . 
     An end face stapler  33  is disposed at a rear end portion of the processing tray  29  in the sheet discharge direction. The illustrated end face stapler  33  staples a sheet bundle on the processing tray  29  at one or more positions of a rear end edge of the sheet bundle. The staple-bound sheet bundle is discharged onto the first sheet discharge tray  21 . 
     The first switchback conveying path SP 1  configured as described above aligns the sheets fed by the sheet discharge roller  25  on the processing tray  29  in the “(2) staple-binding mode” as described above, and the end face stapler  33  staples the sheet bundle at one or more positions of the rear end edge of this sheet bundle. In the “(1) print-out mode”, the sheet fed by the sheet discharge roller  25  is not subjected to the switchback, but conveyed along the processing tray  29  and discharged to the first sheet discharge tray  21  by a rotation of the normal/reverse rotation roller  30  in a clockwise direction in  FIG. 2 . 
     [Configuration of Second Switchback Conveying Path SP 2 ] 
     The following describes a configuration of the second switchback conveying path SP 2  branched from the sheet carry-in path P 1 . The second switchback conveying path SP 2  is a conveying path for guiding a switchback-conveyed sheet. That is, in a state where the sheet is nipped by the sheet discharge roller  25 , rotation of the sheet discharge roller  25  is changed from the normal rotation to the reverse rotation, with the result that the sheet is switchback-conveyed along the switchback conveying path SP 2 . As illustrated in  FIG. 2 , the second switchback conveying path SP 2  is located in a substantially vertical direction inside the casing  20 . A conveying roller  36  is located at an inlet of the second switchback conveying path SP 2 , and an exit conveying roller  37  is located at an outlet of the second switchback conveying path SP 2 . A stacker section  35  constituting a second processing tray that aligns and temporarily stacks the sheets fed along the second switchback conveying path SP 2  is provided downstream of the second switchback conveying path SP 2 . The illustrated stacker section  35  includes a conveying guide that transfers the sheet. A saddle stitching stapler  40  and a folding roller  45  are arranged along the stacker section  35 . The configurations of these components will be sequentially described below. 
     [Configuration of Stacker Section] 
     The stacker section  35  is formed of a guide member that guides the sheet being conveyed. The stacker section  35  is configured such that the sheets are stacked and housed thereon. The illustrated stacker section  35  is connected to the second switchback conveying path SP 2  and located in a center portion of the casing  20  so as to extend in the substantially vertical direction. This allows the device to be compactly configured. The stacker section  35  is shaped to have an appropriate size to house maximum sized sheets. In particular, the illustrated stacker section  35  is curved or bent so as to project toward an area in which the saddle stitching stapler  40  and the folding roller  45  ( 45   a ,  45   b ) to be described later are arranged. 
     A switchback approaching path  35   a  is connected to a conveying direction rear end of the stacker section  35 . The switchback approaching path  35   a  overlaps the outlet end of the second switchback conveying path SP 2 . This is to allow the leading end of a carried-in (succeeding) sheet fed from the exit conveying roller  37  on the second switchback conveying path SP 2  to overlap the rear end of the stacked (preceding) sheets supported on the stacker section  35  to ensure the page order of the stacked sheets. A leading end regulating member (hereinafter, referred to as stopper  38 ) serving as a stopper that regulates a leading end of the sheet in the conveying direction is located downstream of the stacker section  35 . The stopper  38  is supported by a guide rail and the like so as to be movable along the stacker section  35 . The stopper  38  is configured to be movable to a position where the sheet is carried in the stacker section  35  by means of a not illustrated shift means, a position where the sheet bundle is bound at a center thereof in the stacking direction, and a position where the sheet bundle is folded by the folding roller  45 . Further, an aligning member  39  for aligning the sheets is provided in the middle of the stacker section  35  in the sheet conveying direction. The aligning member  39  presses a side edge of the sheet for alignment every time the sheet is carried-in. 
     [Saddle Stitching Stapler] 
     The saddle stitching stapler  40  positioned above the stacker section  35  includes a driver unit  41  and a clincher unit  42  which are arranged opposite to each other with respect to the stacker section  35 . The driver unit  41  drives a staple into a sheet bundle. The clincher unit  42  bends leg portions of the driven staple in a direction facing each other. With this configuration, the sheet bundle is bound at a binding position X illustrated in  FIG. 2  corresponding to the half of a sheet length. The saddle stitching stapler  40  may perform the binding not only by using a metallic staple, but also by using a paper-made staple, by performing press-bonding (without use of the metallic or paper-made staple), or by forming a cut in the paper sheets (without use of the metallic or paper-made staple). 
     [Folding Roller] 
     The following describes a configuration of the folding roller  45 . As illustrated in  FIG. 2 , the folding roller  45  for folding the sheet bundle and a folding blade  46  for inserting the sheet bundle into a nip position of the folding roller  45  are disposed at a folding position Y set on the downstream side of the above-described saddle stitching stapler  40 . As illustrated in  FIGS. 3A to 3D , the folding roller  45  is constituted by an upper pressure contact roller  45   a  and a lower pressure contact roller  45   b  which are brought into pressure contact with each other. The upper and lower pressure contact rollers  45   a  and  45   b  each have a length slightly longer than the maximum width of the sheet. The rollers of the folding roller pair  45  are biased in the pressure contact direction by a not illustrated compression spring. The folding roller pair  45  is formed of a material, such as rubber, having a comparatively large friction coefficient. 
     Further, the folding blade  46  configured to be moved toward the pressure contact position of the folding roller pair  45  is disposed so as to be capable of advancing and retreating. After the sheet bundle is saddle-stitched by the saddle stitching stapler  40 , the folding blade  46  is moved to push the binding position between the rollers of the folding roller pair  45  to cause the rollers of the folding roller pair  45  to be rotated while being pressed against each other, whereby the saddle-stitched sheet bundle is two-folded. In the middle of the above folding processing, the folding blade  46  is returned to its original position and waits for the next sheet bundle. A movement path along which the folding blade  46  is moved is illustrated as the folding position Y which coincides with the binding position X of the sheet bundle. 
     Hereinafter, a procedure of folding processing of the stacked sheet bundle or stacked and saddle-stitched sheet bundle will be described using  FIGS. 3A to 3D . The sheets are stopped by the stopper  38  and stacked as a sheet bundle. Then, the stopper  38  is moved upward to allow the saddle stitching stapler  40  to saddle-stitch the sheet bundle at about a center position of the sheet bundle in the sheet conveying direction. After the saddle stitching, the stopper  38  supporting the resultant sheet bundle is moved down and stopped at a position where the sheet binding position coincides with a folding position. This state is illustrated in  FIG. 3A . This position coincides with the pressure contact position between the upper and lower pressure contact rollers  45   a  and  45   b  constituting the folding roller  45 . Thereafter, the upper and lower pressure contact rollers  45   a  and  45   b  are rotated in the same direction by a not illustrated drive motor, and the folding blade  46  is moved so as to be pushed to the pressure contact position. This state is illustrated in  FIG. 3B . 
     Then, as illustrated in  FIG. 3C , the upper and lower pressure contact rollers  45   a  and  45   b  continue being rotated in the same direction, while the folding blade  46  is once stopped before the pressure contact position. Then, the folding blade  46  is retracted in a returning direction. Afterward, when the upper and lower pressure contact rollers  45   a  and  45   b  are further rotated in the same direction, a folded sheet bundle BS to be folded is folded while forming a certain loop BL as illustrated in  FIG. 3D . In this state, there are formed a folding loop leading end BL 1  which is a fold formed as a result of pushing of the folding blade  46 , an upper loop BL 2  swelling upward with the folding loop leading end BL 1  as a center, a lower loop BL 3  swelling downward, and a loop base end portion BL 4  that presses the sheets so as to maintain the loop, and the folding operation is once stopped. 
     The loop at the fold is generated due to action of the sheet bundle to be opened outward. Thus, the larger the number of the sheet bundle BS to be folded, the greater the force of the sheet bundle itself to be opened and spread. Accordingly, if no countermeasure is taken, a discharged sheet bundle is opened. So, in the present invention, the following stepwise folding in which the sheet bundle is subjected to stepwise folding is performed. 
     The folding roller  45  may be used for detecting a thickness of the sheet bundle BS to be folded. That is, by measuring a distance between axes of the upper and lower pressure contact rollers  45   a  and  45   b  in a state where the sheet bundle BS is not inserted and a distance therebetween in a state where the sheet bundle BS is inserted, the thickness of the folded sheet bundle BS can be detected. This point will be described later using  FIG. 13 . 
     [Stepwise Folding Unit] 
     Hereinafter, a stepwise folding unit  50  constituting a part of the sheet processing device according to the present invention will be described. The stepwise folding unit  50  is a unit for preventing the folded sheet bundle BS from being opened.  FIG. 4  is a perspective view of the stepwise folding unit  50  as viewed from the sheet discharge side,  FIG. 5  is a perspective view thereof as viewed from the folding roller  45  side. Further, a configuration of a pressing roller unit  56  will be described using  FIGS. 6 and 7 .  FIG. 6  is a perspective view of the pressing roller unit  56  as viewed from the folding roller  45  side, and  FIG. 7  is a front view thereof. Further, operation of the stepwise folding unit  50  will be described using  FIGS. 8 to 10 . 
     As illustrated in  FIG. 2 , the stepwise folding unit  50  is disposed so as to cross a folded sheet conveying path BP installed downstream of the folding roller  45 . More in detail, in the stepwise folding unit  50 , the sheet bundle BS folded by the folding roller  45  is pressed by a plurality of pressing roller pairs  70  having mutually different intervals as additional folding processing. The stepwise folding unit  50  faces the fold of the folded sheet bundle BS having the fold in the sheet width direction and a certain loop. 
     There are disposed, before and after the stepwise folding unit  50  of  FIG. 2 , a sheet bundle detection sensor (SEN 3 )  129  and a bundle discharge sensor (SEN 4 )  131 , respectively. The sheet bundle detection sensor  129  detects a back and a fore edge of the sheet bundle folded by the folding roller  45  and then conveyed. The bundle discharge sensor  131  detects discharge of the folded sheet bundle BS from the bundle discharge roller  49 . 
     The stepwise folding unit  50  illustrated in  FIG. 2  is installed between the folding roller  45  and the bundle discharge roller  49  as a discharge member that discharges the folded sheet bundle BS outside the device; alternatively however, the stepwise folding unit  50  may be disposed downstream of the bundle discharge roller  49  as long as it crosses the folded sheet conveying path BP. 
     As illustrated in  FIG. 4 , in the stepwise folding unit  50 , a right side plate  53  disposed at one side of the device, a left side plate  54  disposed facing the right side plate  53 , and a connecting angle  55  connecting the right side plate  53  and the left side plate  54  constitute the entire frame of the device. A pressing roller unit  56  is disposed between the right side plate  53  and left side plate  54  as a moving unit that is reciprocated therebetween. The pressing roller unit  56  configured to be reciprocated is slidably moved along an upper guide rail  57  and a lower guide rail  58  extending between the right side plate  53  and the left side plate  54 . More specifically, the pressing roller unit  56  is supported such that an upper slide block  60  attached to an upper portion of the pressing roller unit  56  is slid along the upper guide rail  57  and a lower slide block  61  attached to a lower portion of the pressing roller unit  56  is slid along the lower guide rail  58 . 
     A moving belt  65  is provided above the pressing roller unit  56  so as to be stretched between the right side plate  53  and the left side plate  54 . A right pulley  63  that winds the moving belt  65  is provided at the right side plate  53  side, and a left pulley  64  that winds the moving belt  65  is provided at the left side plate  54  side. One end of the moving belt  65  is fixed to a belt fixing portion  65   b  at an upper end of the pressing roller unit  56 . Thus, when the moving belt  65  is moved to move the belt fixing portion  65   b  from a device back side (right side) to a device front side (left side), the pressing roller unit  56  is moved from the device back side (right side) to the device front side (left side) along the upper guide rail  57  and the lower guide rail  58 . When the moving belt  65  is moved in the opposite direction, the belt fixing portion  65   b  is also moved in the opposite direction, which moves the pressing roller unit  56  in the opposite direction. 
     The left pulley  64  that winds the moving belt  65  is mounted to a motor gear unit  68  provided on the left side plate  54  and connected to a drive motor  69  configured to be rotatable in normal and reverse directions through the motor gear unit  68 . Rotation drive of the drive motor  69  is transmitted from a motor output gear  67  to a transmission gear  66  provided in the motor gear unit  68 , and then to the left pulley  64  of the moving belt  65 . 
     Thus, selecting a rotating direction of the drive motor  69  allows the pressing roller unit  56  to be moved selectively in the directions from the device back side (right side) to the device front side (left side) and from the device front side (left side) to the device back side (right side). A unit flag  107  is provided near a right side plate  53  side end portion on an upper surface of the pressing roller unit  56 . The unit flag  107  indicates that the pressing roller unit  56  is situated at a home position near the right side plate  53 . When the unit flag  107  is detected by a home position sensor  108 , it can be determined that the pressing roller unit  56  is situated at the home position. 
     When the pressing roller unit  56  is moved to the left in  FIG. 4  from the home position, a position of the pressing roller unit  56  is discriminated by a not illustrated pulse generator incorporated in the drive motor  69 , whereby it is determined that the pressing roller unit  56  is situated at a returning position near the left side plate  54 . When the pressing roller unit  56  is situated at the returning position, the drive motor  69  is rotated in the reverse direction to move the pressing roller unit  56  toward the home position. Thus, the pressing roller unit  56  is a moving member that is moved by means of the moving belt  65  and the like. 
     [Pressing Roller Unit] 
     The following describes the pressing roller unit  56  configured to be moved to the left and right in  FIG. 4 . As illustrated in  FIG. 5 , which is a perspective view as viewed from the folding roller  45  side, the pressing roller unit  56  is surrounded by a unit base plate  62   a  constituting a back surface side of the unit, front upper and lower base plates  62   b  and  62   c  which are separated up and down, a preceding unit side plate  95 , a following unit side plate  96 , a unit top plate  59   a , and a unit bottom plate  59   b . The preceding unit side plate  95  has a preceding side plate opening  97  having a comparatively large size, and the following unit side plate  96  has a following side plate opening  98  having a size smaller than the preceding side plate opening  97 . These openings  97  and  98  allow the pressing roller unit  56  to be moved along the fold of the folded sheet bundle BS with the fold inserted into the pressing roller unit  56 . 
     As illustrated in detail in  FIG. 6 , inside the pressing roller unit  56 , three rows of pressing roller pairs are arranged from the preceding unit side plate  95  side toward the following unit side plate  96 . An interval between the pair of pressing rollers  70  differs among the rows. That is, the pressing rollers  70  of a first row includes a first upper pressing roller  71  and a first lower pressing roller  72  which are disposed opposite to each other and spaced apart from each other by a predetermined interval. The first upper and lower pressing rollers  71  and  72  have a first upper pressing roller shaft  78   a  and a first lower pressing roller shaft  78   b , respectively. The first upper and lower pressing roller shafts  78   a  and  78   b  are supported by a first upper pressing roller bracket  86   a  and a first lower pressing roller bracket  86   b , respectively. The first upper pressing roller bracket  86   a  is vertically movably supported by the unit top plate  59   a , and the first lower pressing roller bracket  86   b  is vertically movably supported by the unit bottom plate  59   b.    
     Further, a first upper pressing roller pressing spring  91   a  is interposed between the first upper pressing roller bracket  86   a  and the unit top plate  59   a  so as to bias the first upper pressing roller bracket  86   a  and the unit top plate  59   a  in a direction separating them from each other; similarly, a first lower pressing roller pressing spring  91   b  having the same function as the first upper pressing roller pressing spring  91   a  is interposed between the first lower pressing roller bracket  86   b  and the unit bottom plate  59   b . With this configuration, the first upper pressing roller  71  and the first lower pressing roller  72  are always biased in a direction approaching each other. On the other hand, a first upper pressing roller shaft elongated hole  82   a  is formed in the unit base plate  62   a  and the front upper base plate  62   b  that support the first upper pressing roller shaft  78   a.    
     Thus, the biasing force of the first upper pressing roller pressing spring  91   a  is regulated by the first upper pressing roller shaft elongated hole  82   a , and downward movement of the first upper pressing roller  71  is also regulated by the first upper pressing roller shaft elongated hole  82   a . Further, the biasing force of the first lower pressing roller pressing spring  91   b  is regulated by a first lower pressing roller shaft elongated hole  82   b , and upward movement of the first lower pressing roller  72  is also regulated by the first lower pressing roller shaft elongated hole  82   b . Thus, as illustrated in detail in  FIG. 7 , an interval L 1  between the first upper pressing roller  71  and the first lower pressing roller  72  is always kept constant. In the present embodiment, the interval L 1  is set to about 14 mm. Further, the first upper pressing roller pressing spring  91   a  and the first lower pressing roller pressing spring  91   b  are set so as to apply a load of 4.0 kg to the first upper and lower pressing rollers  71  and  72  in a state where the rollers  71  and  72  are brought into contact with each other. 
     Further, as can be seen well from  FIGS. 6 and 7 , the pressing roller pair  70  of a second row has the same configuration as that of the pressing roller pair  70  of the first row. 
     That is, the pressing rollers  70  of the second row includes a second upper pressing roller  73  and a second lower pressing roller  74  which are disposed opposite to each other and spaced apart from each other by a predetermined interval. The second upper and lower pressing rollers  73  and  74  have a second upper pressing roller shaft  79   a  and a second lower pressing roller shaft  79   b , respectively. The second upper and lower pressing roller shafts  79   a  and  79   b  are supported by a second upper pressing roller bracket  87   a  and a second lower pressing roller bracket  87   b , respectively. The second upper pressing roller bracket  87   a  is vertically movably supported by the unit top plate  59   a , and the second lower pressing roller bracket  87   b  is vertically movably supported by the unit bottom plate  59   b.    
     Further, a second upper pressing roller pressing spring  92   a  is interposed between the second upper pressing roller bracket  87   a  and the unit top plate  59   a  so as to bias the second upper pressing roller bracket  87   a  and the unit top plate  59   a  in a direction separating them from each other; similarly, a second lower pressing roller pressing spring  92   b  having the same function as the second upper pressing roller pressing spring  92   a  is interposed between the second lower pressing roller bracket  87   b  and the unit bottom plate  59   b.    
     With this configuration, the second upper pressing roller  73  and the second lower pressing roller  74  are always biased in a direction approaching each other. On the other hand, a second upper pressing roller shaft elongated hole  83   a  is formed in the unit base plate  62   a  and the front upper base plate  62   b  that support the second upper pressing roller shaft  79   a . Thus, the biasing force of the second upper pressing roller pressing spring  92   a  is regulated by the second upper pressing roller shaft elongated hole  83   a , and downward movement of the second upper pressing roller  73  is also regulated by the second upper pressing roller shaft elongated hole  83   a.    
     Further, the biasing force of the second lower pressing roller pressing spring  92   b  is regulated by a second lower pressing roller shaft elongated hole  83   b , and upward movement of the second lower pressing roller  74  is also regulated by the second lower pressing roller shaft elongated hole  83   b . Thus, as illustrated in detail in  FIG. 7 , an interval L 2  between the second upper pressing roller  73  and the second lower pressing roller  74  is always kept constant. In the present embodiment, the interval L 2  is set to about 7 mm. Further, the second upper pressing roller pressing spring  92   a  and the second lower pressing roller pressing spring  92   b  are set so as to apply a load of 4.0 kg to the second upper and lower pressing rollers  73  and  74  in a state where the rollers  73  and  74  are brought into contact with each other. 
     Further, as can be seen well from  FIGS. 6 and 7 , the pressing roller pair  70  of a third row has the same configuration as those of the pressing roller pairs  70  of the respective first and second rows, so descriptions of the pressing roller pair  70  of the third row will be omitted here, and only a different point will be described. That is, the first upper and lower pressing rollers  71  and  72  of the first row are separated from each other by the predetermined interval L 1  (in the present embodiment, about 14 mm) as illustrated in  FIG. 7 ; similarly, the second upper and lower pressing rollers  73  and  74  of the second row are separated from each other by the predetermined interval L 2  (in the present embodiment, about 7 mm). This is because the first upper pressing roller shaft elongated hole  82   a , the first lower pressing roller shaft elongated hole  82   b , the second upper pressing roller shaft elongated hole  83   a , and the second lower pressing roller shaft elongated hole  83   b  serve as an interval regulating member. Thus, by the above elongated holes, the pair of the pressing rollers are position-regulated so as to prevent the interval therebetween from being made smaller than the predetermined interval. 
     On the other hand, third upper and lower pressing rollers  75  and  76  of the third row are always elastically biased so as to be brought into pressure contact with each other. That is, a third upper pressing roller shaft elongated hole  84   a  and a third lower pressing roller shaft elongated hole  84   b  are formed such that a roller interval L 3  is 0. A third upper pressing roller pressing spring  93   a  and a third lower pressing roller pressing spring  93   b  are set so as to apply a load of 4.0 kg to the third upper and lower pressing rollers  75  and  76  at the roller contact position. With this configuration, the stepwise folding is performed while a load exceeding 4 kg is applied to both sides of the fold of the folded sheet bundle BS. 
     Further, as illustrated in  FIG. 6  and in a circle outlined by a long dashed double-short dashed line in  FIG. 6 , the pair of third upper and lower pressing rollers  75  and  76  have a first region R 1  (hereinafter, referred to as “region R 1 ”) where an upper pressing roller large-diameter portion  75   a  and a lower pressing roller large-diameter portion  76   a  are brought into pressure contact with each other and a second region R 2  (hereinafter, referred to as “region R 2 ”) where an upper pressing roller small-diameter portion  75   b  and a lower pressing roller small-diameter portion  76   b  are opposed to each other with a slight interval. The regions R 1  and R 2  are adjacently arranged from the downstream side to upstream side in this order in the sheet bundle conveying direction. This stepped roller pair is integrally formed as illustrated and is configured to be rotatable. The reason for using the stepped roller pair is to reduce impact between the end portion of the folded sheet bundle BS and the pressing rollers. This point will be described later using  FIG. 13 . 
     As described above, the pressing rollers  70  as a sheet pressing member of the present invention include the first, second, and third upper pressing rollers  71 ,  73 , and  75  and first, second, and third lower pressing rollers  72 ,  74 , and  76  which are disposed opposite respectively to the pressing rollers  71 ,  73 ,  75 . The above pressing rollers  70  of a plurality of rows (in the present embodiment, three rows) are supported by the pressing roller unit  56  unitized as a moving member so as to be movable. Further, the above pressing rollers  70  can be rotated in the moving direction thereof. The pressing rollers  70  are moved along the fold with the first upper pressing roller  71  of the first row and its opposing lower pressing roller  72 , the interval between which is largest, in the lead. The interval between the second upper and lower pressing rollers  73  and  74  of the last row but one (in the present embodiment, second row) is made smaller than that between the first upper and lower pressing rollers  71  and  72 . 
     Then, the third upper and lower pressing rollers  75  and  76  of the third row (last row in the moving direction) are configured to press, at the region (R 1 ) corresponding to the large-diameter portions thereof, the folded sheet bundle BS from both sides with a spring force of 4 kg. That is, in the present embodiment, the pressing rollers of the three rows arranged in and supported by one unit are reduced stepwise in the interval from the first row to the last row. The thus configured pressing rollers  70  are moved along the fold of the folded sheet bundle BS while pressing the fold from both sides of the folded sheet bundle BS. Through this process, the stepwise folding is applied to the folded sheet bundle BS. Further, in the above stepwise folding, the pressing rollers of each row are configured to press the folded sheet bundle BS from both sides of the fold with the pressing position set at a substantial center of each roller pair. 
     [Operation of Stepwise Folding Unit] 
     Hereinafter, carry-in of the folded sheet bundle BS to the pressing roller unit  56  in the stepwise folding unit  50  and stepwise pressing operation of the pressing roller unit  56  will be described with reference to  FIGS. 8 to 12 . 
       FIG. 8  illustrates a state where the pressing roller unit  56  is situated at the home position and waits for carry-in of the folded sheet bundle BS.  FIG. 9  illustrates a state where the pressing roller unit  56  is situated at a substantial center position of the folded sheet bundle BS in the sheet width direction and performs the stepwise folding using roller pairs of three rows.  FIG. 10  illustrates a state where the stepwise folding by the roller pairs of three rows is completed, and the pressing roller unit is situated at a returning position. In the following, basic operation of the stepwise folding will be described. Detailed description of the operation involved with the region R 1  between the upper and lower pressing roller large-diameter portions  75   a  and  76   a  and region R 2  between the upper and lower pressing roller small-diameter portions  75   b  and  76   b  which are opposed to each other the with a slight interval will be made later using  FIG. 13  and omitted here. 
       FIG. 8  illustrates a state where the pressing roller unit  56  is situated at the home position and waits for carry-in of the folded sheet bundle BS.  FIG. 8  is a view as viewed from the bundle discharge roller  49  side, and the unit base plate  62   a  is omitted for descriptive convenience. 
     In  FIG. 8 , the unit flag  107  of the pressing roller unit  56  having the pressing rollers  70  of three rows is detected by the home position sensor  108  attached to the right side plate  53 , that is, the pressing roller unit  56  is situated at the home position. When the “stepwise folding mode” described above is set at this time, the pressing roller unit  56  waits for carry-in of the folded sheet bundle BS that is subjected to the folding by the folding roller  45  and conveyed along the folded sheet conveying path BP. 
     In the pressing roller unit  56  situated at the home position, the intervals between the pair of pressing rollers  70  of the respective rows are reduced from the first row toward the last row in the moving direction. That is, the pair of pressing rollers  70  of the last row are brought into pressure contact with each other. As described above, in the present embodiment, the first upper and lower pressing rollers  71  and  72  of the first row are disposed at an interval of about 14 mm, the second upper and lower pressing rollers  73  and  74  of the second row are disposed at an interval of about 7 mm, and the third upper and lower pressing rollers  75  and  76  of the first row are brought into pressure contact with each other in the region R 1 . A center of the separation and pressure contact between the pair of pressing rollers is set so as to substantially coincide with the folding loop leading end (fold) BL 1  which is a center of the folded sheet bundle BS. 
     When the folding loop of the folded sheet bundle BS becomes a predetermined size (in the present embodiment, 22 mm in the vertical direction of the loop), the folding roller  45  is stopped, and the drive motor  69  is driven to move the pressing roller unit  56  to the left in  FIG. 8 . When this movement is started, the first upper and lower pressing rollers  71  and  72  of the first row override a right side (one end side) end portion (sheet end portion) of the folded sheet bundle BS and are moved to the left while creating a fold at a position slightly above the folding loop leading end BL 1 . As described above, the size of the loop in the present embodiment is about 22 mm, and the interval between the first upper and lower pressing rollers  71  and  72  is about 14 mm, so that the first upper and lower pressing rollers  71  and  72  overlap the upper and lower portions of the loop, respectively, each by a length of slightly less than about 4 mm to thereby create a first fold line  100  illustrated in  FIG. 11A . Further, since the first upper and lower pressing rollers  71  and  72  are disposed at a large interval, the pressing rollers  71  and  72  can override the end portion of the folded sheet bundle BS without significantly damaging the end portion. Further, the pressing rollers  70  including the first upper and lower pressing rollers  71  and  72  are axially supported in the same direction as the sheet conveying direction so as to be rotatable about this axis, and this rotation makes it easy for the pressing rollers to ride over the end portion of the folded sheet bundle BS. 
     When the pressing roller unit  56  is further moved, the loop of the folded sheet bundle BS pressed between the first upper and lower pressing rollers  71  and  72  is further pressed between the second upper and lower pressing rollers  73  and  74 , the interval between which is slightly smaller than the interval between the first upper and lower pressing rollers  71  and  72 , whereby a second fold is created. In the present embodiment, the interval between the second upper and lower pressing rollers  73  and  74  is set to about 7 mm, so that the second upper and lower pressing rollers  73  and  74  overlap the respective upper and lower portions of the loop which has been pressed by the first upper and lower pressing rollers  71  and  72 , each by a length of about 3.5 mm, whereby a second fold line  101  illustrated in  FIG. 11B  is added. 
     Successively, the folding loop leading end BL 1  is subjected to the stepwise folding between the third upper and lower pressing rollers  75  and  76  of the third row. That is, the third upper and lower pressing rollers  75  and  76  are brought into a substantially pressure contact state (region R 1  between the large-diameter roller portions) with the interval therebetween set to 0, so that the folded sheet bundle BS are subjected to the stepwise folding while being pressed by the third upper and lower pressing roller pressing springs  93   a  and  93   b  by an amount corresponding to a thickness of the sheet bundle at the fold, whereby a last fold line  102  illustrated in  FIG. 11C  is added. 
       FIG. 9  illustrates a state where the pressing roller unit  56  pressing the folded sheet bundle BS stepwise in one unit is situated at a substantial center of the folded sheet bundle BS in the sheet width direction. From this state, the pressing roller unit  56  is further moved to the left in  FIG. 9  while adding stepwise folding to the sheet by the pressing roller pairs each having smaller interval in the thickness direction of the fold of the folded sheet bundle BS. In this movement, the third upper and lower pressing rollers  75  and  76  of the third row pass through the right side (one end side) end portion (sheet end portion) of the folded sheet bundle BS to apply the stepwise folding. After passing through the folded sheet bundle BS, the pressing roller unit  56  reaches the returning position at the left side plate  54  side. This state is illustrated in  FIG. 10 . When the pressing roller unit  56  reaches the returning position, drive of the drive motor  69  is stopped. Thereafter, the pressing roller unit  56  waits for the stepwise-folded sheet bundle BS (folded sheet bundle BS that has pressed by the pressing rollers  70 ) to be discharged by rotation of the folding roller  45  and the bundle discharge roller  49  in the discharge direction. When a state where the stepwise-folded sheet bundle BS is discharged is detected by the bundle discharge sensor (SEN 4 )  131  illustrated in  FIG. 2 , the pressing roller unit  56  is returned from the returning position to home position and made to wait for carry-in of the next folded sheet bundle BS at the position illustrated in  FIG. 8 . 
     In the above description, the pressing roller unit  56  is returned to the home position after the stepwise-folded sheet bundle BS of  FIG. 10  is once discharged; alternatively, however, the following configuration may be adopted. That is, before discharge of the folded sheet bundle BS, the pressing roller unit  56  is moved from the left to right of  FIG. 10  toward the home position to press once again the fold of the folded sheet bundle BS between the third upper and lower pressing rollers  75  and  76 , whereby the folded sheet bundle BS can surely be subjected to the stepwise folding by the pressing rollers of the last row. 
     As described above, in the present embodiment, the pressing roller unit  56  is used to perform the three-step folding for the folded sheet bundle BS. The following describes the folded sheet bundle BS in a state after being subjected to the stepwise folding and discharged with reference to  FIGS. 11A to 11C and 12 . 
     As described hereinbefore, the first upper and lower pressing rollers  71  and  72  as the sheet bundle pressing member of the present invention are moved, along the fold direction, on a part of the folded sheet bundle BS where the fold is created by the folding roller  45  and thus the loop is generated while pressing the folded sheet bundle BS in a thickness direction (vertical direction crossing, at the fold of the folded sheet bundle BS, a conveying direction of the folded sheet bundle BS) of the fold to thereby create a plurality of folds. As described above, the interval between the first upper and lower pressing rollers  71  and  72  of the first step is set to a value (in the present embodiments, about 14 mm relative to the loop size (height) of 22 mm) slightly smaller than the loop size, and the first upper and lower pressing rollers  71  and  72  are moved along the fold created by the folding roller  45  to thereby create the first fold. The first fold is illustrated as the first fold line  100  indicated by a solid arrow in  FIG. 11A . In  FIG. 12 , this first fold line  100  is represented by a light line on the folded sheet bundle BS. That is, as illustrated in  FIG. 11A , a part of the loop that is pressed by the first upper and lower pressing rollers  71  and  72  is applied with a concentrated load, causing buckling in the sheet bundle to generate the fold. Then, by the movement of the first upper and lower pressing rollers  71  and  72  in the width direction, the buckled part appears as the first fold line  100 . 
     In the second step, the second upper and lower pressing rollers  73  and  74  as the sheet bundle pressing member, the interval between which is set to a value (in the present embodiments, about 7 mm) slightly smaller than the size of the loop pressed in the first step, and the second upper and lower pressing rollers  73  and  74  are moved along the fold created by the folding roller  45  to thereby create the second fold. The second fold is illustrated as the second fold line  101  positioned on a back side of the first fold line  100  and indicated by a solid arrow in  FIG. 11B . In  FIG. 12 , this second fold line  101  is represented by a light line on the folded sheet bundle BS. That is, as illustrated in  FIG. 11B , a part of the loop that is pressed by the second upper and lower pressing rollers  73  and  74  is applied with a concentrated load, causing buckling in the sheet bundle to generate the fold. Then, by the movement of the second upper and lower pressing rollers  73  and  74  in the width direction, the buckled part appears as the second fold line  101 . 
     The third upper and lower pressing rollers  75  and  76  as the sheet bundle pressing member of the final step are brought into pressure contact with each other by the elastic force of the third upper and lower pressing roller pressing springs  93   a  and  93   b . In the final step, unlike the first and second steps, no interval is provided between the third upper and lower pressing rollers  75  and  76  (in the present embodiments, interval is set to 0 in the region R 1 ). Thus, in the final step, the third upper and lower pressing rollers  75  and  76  are moved along the fold while pressing the position corresponding to the thickness of the folded sheet bundle BS that has been pressed in the first and second steps. The fold created by the pressing rollers  70  of the last row is illustrated as a last fold line  102  indicated by a slid arrow in  FIG. 11C . In  FIG. 12 , this last fold line  102  is represented by a comparatively dark line on the folded sheet bundle BS. At end portions of the folded sheet bundle BS in the width direction, end portion folds  103  created when the folding roller  45  and pressing rollers  70 , which are brought into a pressure contact state, override the folded sheet bundle BS are formed. A part that is pressed between the third upper and lower pressing rollers  75  and  76 , which are brought into a substantially pressure contact state, appears as an enhanced fold, i.e., the last fold line  102 . 
     As described above, the pressing roller pairs  70  having mutually different intervals among the rows are used to buckle the folded sheet bundle BS to create the folds. As a result, a folding direction is directed to a closing direction (line extending in the conveying direction that passes the fold) of the folded sheet bundle BS at the respective positions of the first fold line  100  (indicated by the light line) of the first step, second fold line  101  (indicated by the light line) of the second step, and last fold line  102  (indicated by the comparatively dark line) of the final step generated in accordance with the thickness of the folded sheet bundle BS. With this configuration, it is possible to prevent the folded sheet bundle BS from being opened after discharge to thereby prevent degradation of aligning property and accumulating property. 
     [Stepped Roller of Last Row] 
     The following describes a configuration of the stepped third upper and lower pressing rollers  75  and  76  opposite to each other in the last row of the pressing roller unit  56 , which has been mentioned in  FIGS. 6 and 7 . 
     In a case where the folded sheet bundle BS is formed by a comparatively large number of sheets and is thus thick, when the third upper and lower pressing rollers  75  and  76  which are brought into pressure contact with each other override the end portion of the folded sheet bundle BS in the width direction, they abut against and collide with the end portion to damage the same. Further, when the impact of the collision is large, a position of the folded sheet bundle BS retained by the folding roller  45  may be deviated from a proper position. 
     In order to cope with this problem, in the present embodiment, the third upper pressing roller  75  of the last row includes the upper pressing roller large-diameter portion  75   a  and the upper pressing roller small-diameter portion  75   b  disposed adjacent to the upper pressing roller large-diameter portion  75   a  upstream thereof in the sheet conveying direction, and the third lower pressing roller  76  includes the lower pressing roller large-diameter portion  76   a  and the lower pressing roller small-diameter portion  76   b , as illustrated in  FIGS. 6, 7, and 13 . As illustrated in  FIG. 6  and in the circle outlined by a long dashed double-short dashed line in  FIG. 6 , the upper pressing roller large-diameter portion  75   a  and the lower pressing roller large-diameter portion  76   a  constitute the region R 1  where they are brought into pressure contact with each other, and the upper pressing roller small-diameter portion  75   b  and the lower pressing roller small-diameter portion  76   b  constitute the region R 2  where they are opposed to each other with a slight interval. The upper pressing roller  75  and the lower pressing roller  76  are each integrally formed as a stepped roller. 
     Thus, as illustrated in  FIG. 7 , in the region R 1  where the upper pressing roller large-diameter portion  75   a  and the lower pressing roller large-diameter portion  76   a  are brought into pressure contact with each other, the interval L 3  is 0, while in the region R 2 , an interval between the upper pressing roller small-diameter portion  75   b  and the lower pressing roller small-diameter portion  76   b  is set to L 4  (about 2 mm, in the present embodiment). The interval L 4  may be set to about 2 mm to about 4 mm, depending upon the number or a thickness of the sheets to be used. When the number of sheets exceeds a comparative large number (in the present embodiment, 15 sheets (30 sheets in a folded state, which corresponds to about 3 mm or more in thickness)), the fold of the folded sheet bundle BS is positioned to the region R 2  between the upper pressing roller small-diameter portion  75   b  and lower pressing roller small-diameter portion  76   b ; on the other hand, when the number of sheets is small (in the present embodiment, 15 sheets or less), the fold of the folded sheet bundle BS is positioned to the region R 1  between the upper pressing roller large-diameter portion  75   a  and the lower pressing roller large-diameter portion  76   a  which are brought into pressure contact with each other. With this configuration, the third upper and lower pressing rollers  75  and  76  can easily override even the end portion of the folded sheet bundle BS having a large thickness, thereby reducing damage to the sheet end portion. 
     [Region Setting for Folded Sheet Bundle] 
     The following describes, with  FIG. 13 , a configuration to position the folded portion leading end (back) of the folded sheet bundle BS to the region R 1  where the upper and lower pressing roller large-diameter portions  75   a  and  76   a  are brought into pressure contact with each other or the region R 2  where the upper and lower pressing roller small-diameter portions  75   b  and  76   b  are opposed to each other with an interval. 
     As described using  FIGS. 3A to 3D , the binding position of the sheet bundle which is positioned at a substantial center thereof in the sheet conveying direction is pushed into between the upper and lower pressure contact rollers  45   a  and  45   b  by the folding blade  46 . The pushed folded sheet bundle BS is conveyed toward the pressing rollers  70  by rotation of the upper and lower pressure contact rollers  45   a  and  45   b . The leading end (back) of the conveyed folded sheet bundle BS is detected by the sheet bundle detection sensor (SEN 3 )  129  provided between the folding roller  45  and the pressing rollers  70 . Upon detection by the sensor SEN 3 , an encoder  127  mounted to a drive shaft of a folding motor  126  that drives the upper pressure contact roller  45   a  into rotation so as to be rotated integrally therewith is counted by an encoder sensor  128 . In a case where the number of the folded sheet bundle BS is small (15 sheets or less, in the present embodiment), the folding motor  126  is stopped at a time point when the count indicates that the fold (back) of the folded sheet bundle BS enters the region R 1 . On the other hand, in a case where the number of the folded sheet bundle BS is large (16 sheets or more, in the present embodiment), the folding motor  126  is stopped at a time point when the count indicates that the fold (back) of the folded sheet bundle BS enters the region R 2 . As described above, the folding roller  45  serves also as a conveying member and is rotated in conjunction with the bundle discharge roller during discharge of the folded sheet bundle BS. 
     Sheet number information of the folded sheet bundle BS can be set as follows: the number of documents set in the document feeder  15  mounted to the image forming device A is counted, and the obtained sheet number information is transmitted to a sheet center-folding controller  122  through the image forming device controller  110  for setting thereof. Alternatively, the number of sheets discharged from the main body sheet discharge port  3  is counted at predetermined intervals in the image forming device A, and the obtained count information is transmitted to the sheet processing device. Further, alternatively, an S 1  sensor provided at the carry-in port  23  of the sheet processing device B is used to count the number of sheet for setting. Further, alternatively, as illustrated in  FIG. 13 , a shaft  45   bx  of the lower pressure contact roller  45   b  is measured by a laser displacement meter  130 , and the obtained thickness information is recognized as the sheet number information so as to determine large number of sheets/small number of sheets. 
     The following describes, using  FIGS. 14A to 14D , operation of the stepwise folding by the pressing roller unit  56  when the folded sheet bundle BS is stopped in the region R 2  for the large number of sheets. 
       FIGS. 14A and 14B  illustrate a case where the number of sheets forming the folded sheet bundle BS is large. As illustrated in  FIG. 14B , the folded sheet bundle BS is stopped in the region R 2  where the upper and lower pressing roller small-diameter portions  75   b  and  76   b  are opposed to each other with an interval. As described using  FIGS. 8 to 10  and as illustrated in  FIG. 14B , the pressing roller unit  56  supporting the pressing roller pairs of three rows having mutually different intervals among the rows is moved in an illustrated arrow direction to apply the stepwise folding to the folded sheet bundle BS stopped in this position. At this time, the third upper and lower pressing rollers  75  and  76  smoothly override the end portion of the folded sheet bundle BS formed by a large number of sheets, which is situated in the region R 2  where the upper and lower pressing roller small-diameter portions  75   b  and  76   b  are opposed to each other with an interval (2 mm, in the present embodiment) with little resistance and impact. Further, the folded sheet bundle BS has a larger thickness than the interval L 4  (see  FIG. 7 ) and, therefore, the folding can be achieved surely. 
     On the other hand,  FIGS. 14C and 14D  illustrate a case where the number of sheets forming the folded sheet bundle BS is small. As illustrated in  FIG. 14D , the folded sheet bundle BS is stopped in the region R 1  where the upper pressing roller large-diameter portion  75   a  and lower pressing roller large-diameter portion  76   a  are brought into pressure contact with each other without an interval. As described using  FIGS. 8 to 10  and as illustrated in  FIG. 14D , the pressing roller unit  56  supporting the pressing roller pairs of three rows having mutually different intervals among the rows is moved in an illustrated arrow direction to apply the stepwise folding to the folded sheet bundle BS stopped in this position. In this case, the folded sheet bundle BS is formed by a small number of sheets, so that the third upper and lower pressing rollers  75  and  76  smoothly override the end portion of the folded sheet bundle BS which is situated in the region R 1  where the upper pressing roller large-diameter portion  75   a  and the lower pressing roller large-diameter portion  76   a  are brought into pressure contact with each other with little resistance and impact. Further, a fold line can surely be given by the upper pressing roller large-diameter portion  75   a  and the lower roller pressing large-diameter portion  76   a  brought into pressure contact with each other without an interval (interval L 3 , see  FIG. 7 ). 
     Hereinafter, with reference to flowcharts of  FIGS. 15 and 16 , the stepwise folding as illustrated in  FIGS. 14A to 14D  will be described in two cases according to the steps (S): where the folded sheet bundle BS is formed by a large number of sheets (exceeding a predetermined number of sheets) and where folded sheet bundle BS is formed by a small number of sheets (equal to or less than a predetermined number of sheets). 
     First, the “stepwise folding mode” is designated through the control panel  18 . When the “stepwise folding mode” is designated, a designated number of sheet bundles each formed by a predetermined number of sheets are created and saddle-stitched by the saddle stitching stapler  40  for each bundle. Then, the sheet bundle is folded in two by the folding roller  45  and the folding blade  46  and then conveyed to the stepwise folding unit  50  through the folded sheet conveying path BP. In this process, it is determined whether the number of sheets is equal to or less than 15 sheets (30 sheets in a folded state) or more than 15 sheets (S 201 ). In place of the number of sheets, the large number of sheets and small number of sheets may be determined based on the thickness. 
     [For Small Number of Sheets] 
     When the folded sheet bundle BS is formed by 15 or less sheets, a drive rotation range of the folding motor  126  that rotates the folding roller  45  is set such that the folded sheet bundle BS is stopped in the region R 1  illustrated in  FIG. 13  and  FIGS. 14A to 14D  where the upper pressing roller large-diameter portion  75   a  and the lower pressing roller large-diameter portion  76   a  are brought into pressure contact with each other without an interval. After the setting, counting is performed using a counter to determine whether or not the fold (back) of the folded sheet bundle BS is situated in the region R 1  between the large-diameter roller portions (S 203 ). When it is determined that the fold of the folded sheet bundle BS is situated in the region R 1 , the drive rotation of the folding motor  126  is stopped (S 204 ). In this state, the fore edge of the folded sheet bundle BS is held between the rollers of the folding roller pair  45 . 
     Then, the unit drive motor  69  is driven in the normal direction so as to move the pressing roller unit  56  in the sheet width direction (S 205 ). The normal direction rotation of the unit drive motor  69  causes the unit flag  107  illustrated in  FIG. 5  to be separated from the home position sensor  108  (i.e., to be turned OFF). Counting is started with the separation, and the pressing roller unit  56  is moved until the count indicates that the pressing roller unit  56  has passed through the folded sheet bundle BS in the width direction (S 206 ). When the count reaches a moving amount corresponding to the width, it is determined that the pressing has been finished, and the drive of the unit drive motor  69  is stopped (S 207 ). With the movement of the pressing roller unit  56  in the width direction, the fold of the folded sheet bundle BS is pressed. That is, the fold of the folded sheet bundle BS is pressed stepwise as illustrated in  FIGS. 11A to 11C  in the stepwise folding mode. At this time, in the third upper and lower pressing rollers  75  and  76  of the last row, the fold of the folded sheet bundle BS is pressed in the region R 1  where the upper pressing roller large-diameter portion  75   a  and the lower pressing roller large-diameter portion  76   a  are brought into pressure contact with each other. As a result, the fold of the folded sheet bundle BS formed by a small number of sheets is surely pressed. 
     In the state where the unit drive motor  69  is stopped, it is determined that the movement of the pressing roller unit  56  has been completed. That is, in this state, the pressing roller unit  56  is situated at an opposite side to the home position. This state is determined as a pressing completion state, and the folding motor  126  and the bundle discharge motor that drives the bundle discharge roller  49  as the discharge member are driven (S 208 ). The rotations of the above motors cause the stepwise-folded folded sheet bundle BS formed by a small number of sheets to be discharged to the second sheet discharge tray  22 . The completion of the discharge is detected by the bundle discharge sensor (SEN 4 )  131  provided near the bundle discharge roller  49 . The detection of the discharge is confirmed when a state of the bundle discharge sensor (SEN 4 )  131  is changed from “ON” to “OFF” (S 209 ). Upon detection of the discharge, the folding motor  126  and the bundle discharge motor that drives the bundle discharge roller  49  are stopped (S 210 ). 
     After the discharge of the folded sheet bundle BS to the second sheet discharge tray  22 , the unit drive motor  69  is driven in the reverse direction (S 211 ). The reverse rotation of the unit drive motor  69  causes the pressing roller unit  56  situated at the opposite side to the home position to be moved toward the home position. The unit flag  107  attached to the pressing roller unit  56  as illustrated in  FIG. 4  is detected by the home position sensor  108  (S 212 ). By the detection, it is determined that the pressing roller unit  56  has returned to the home position, and the unit drive motor  69  is stopped (S 213 ). In this process, presence/absence of a succeeding folded sheet bundle BS is checked (S 214 ). When the succeeding folded sheet bundle BS is present, the processing flow returns to the start point (round mark  1  in  FIG. 15 ), where processing is performed from S 201 . When the succeeding folded sheet bundle BS is absent, this routine is ended. 
     In the above operation flow, when the folded sheet bundle BS is discharged to the second sheet discharge tray  22  (S 210 ), the unit drive motor  69  is driven in the reverse direction (S 211 ) to cause the pressing roller unit  56  to return to the home position (S 213 ). However, when the folded sheet bundle BS is formed by an extremely small number (e.g., two or three sheets) of sheets, the following configuration may be adopted. That is, the pressing roller unit  56  is not moved back to the home position immediately after the discharge of the preceding folded sheet bundle BS, but the pressing roller unit  56  is moved back to the home position after the fold (back) of the succeeding folded sheet bundle BS is positioned in the region R 1 . In this returning process, the pressing roller unit  56  presses the fold of the succeeding folded sheet bundle BS. Thus, discharge of the preceding folded sheet bundle BS and carry-in of the succeeding folded sheet bundle BS to the region R 1  are simultaneously performed for each movement of the pressing roller unit  56  in one direction, whereby a processing speed (productivity) is further improved. 
     [For Large Number of Sheets] 
     The following describes processing to be performed when the folded sheet bundle BS is formed by a large number of sheets. Referring back to  FIG. 15 , when the number of sheets forming the folded sheet bundle BS exceeds 15 (S 201 ), the drive rotation range of the folding motor  126  that rotates the folding roller  45  is set such that the folded sheet bundle BS is stopped in the region R 2  illustrated in  FIG. 13  and  FIGS. 14A to 14D  where the upper and lower pressing roller small-diameter portions  75   b  and  76   b  are opposed to each other with a slight interval, followed by driving of the folding motor  126  (S 221 ). After the setting, it is determined based on the counter value whether or not the fold (back) of the folded sheet bundle BS is situated in the region R 2  between the small-diameter portions (S 222 ). When the fold (back) of the folded sheet bundle BS is situated in the region R 2 , the folding motor  126  is stopped. In this state, the fore edge of the folded sheet bundle BS is held between the rollers of the folding roller pair  45  (S 223 ). 
     The subsequent steps (steps S 223  to S 226 ) are substantially the same as those in the processing for the small number of sheets, and descriptions thereof will be omitted. Different points are follows. 
     The first different point is that, in the third upper and lower pressing rollers  75  and  76  of the third row, the fold (back) of the folded sheet bundle BS formed by a large number of sheets is situated in the region R 2  between the upper and lower pressing roller small-diameter portions  75   b  and  76   b  are opposed to each other with a slight interval. Thus, the third upper and lower pressing rollers  75  and  76  (i.e., the upper and lower pressing roller small-diameter portions  75   b  and  76   b ) can easily override even the end portion of the folded sheet bundle BS formed by a large number of sheets and thus having a large thickness. As a result, the third upper and lower pressing rollers  75  and  76  can smoothly override the fold of the folded sheet bundle BS with little resistance and impact, thereby preventing the folded sheet bundle BS from being deviated from a proper position. 
     The second different point is a discharge timing of the folded sheet bundle BS that has been subjected to the stepwise folding to the second sheet discharge tray. That is, for a small number of sheets, the folded sheet bundle BS that has been subjected to the stepwise folding by the pressing roller unit  56  is first discharged to the second sheet discharge tray with the pressing roller unit  56  situated at the opposite side to the home position, and then the pressing roller unit  56  is returned to the home position. On the other hand, for a large number of sheets, the pressing roller unit  56  is first moved back to the home position, and then the folded sheet bundle is discharged. Referring back to  FIGS. 15 and 16 , this point will be described below. 
     When the step-wise folding in one direction is finished by the movement of the pressing roller unit  56 , the unit drive motor  69  is once stopped (S 226 ). In this state, the folded sheet bundle BS is not discharged, but the unit drive motor  69  is driven in the reverse direction at a speed lower than that for the stepwise folding for a small number of sheets (S 227 ). The reverse drive of the unit drive motor  69  causes the pressing roller unit  56  situated at the opposite side to the home position to be moved toward the home position at a low speed. Then, the unit flag  107  attached to the pressing roller unit  56  as illustrated in  FIG. 4  is detected by the home position sensor  108  (S 228 ). By the detection, it is determined that the pressing roller unit  56  has returned to the home position, and the unit drive motor  69  is stopped (S 229 ). 
     Thus, also in this returning of the pressing roller unit  56 , the fold of the folded sheet bundle BS is pressed between the upper and lower pressing roller small-diameter portions  75   b  and  76   b . That is, the fold is pressed twice and, therefore, the folding can be achieved surely. Since it is estimated beforehand that much processing time is taken for the stepwise folding for a large number of sheets, there is no particular problem even when the folded sheet bundle BS is discharged after completion of the reciprocation of the pressing roller unit  56 . Instead, importance is placed on making the folding more secure by reciprocating the pressing roller unit  56  and by making the moving speed lower than that for the stepwise folding for a small number of sheets. 
     On the other hand, in the case of the stepwise folding for a small number of sheets, the folded sheet bundle BS is pressed between the upper and lower pressing roller large-diameter portions  75   a  and  76   a , the fold line can be easily given without performing additional pressing. Further, since the fast processing is required for a small number of sheets, the folded sheet bundle BS is discharged after the stepwise folding in one direction. 
     In the state where the unit drive motor  69  is stopped, the pressing roller unit  56  is situated at the home position. Thereafter, the folding motor  126  and the bundle discharge motor that drives the bundle discharge roller  49  are driven (S 230 ). The rotations of the above motors cause the stepwise-folded folded sheet bundle BS formed by a large number of sheets to be discharged to the second sheet discharge tray  22 . The completion of the discharge is detected by the bundle discharge sensor (SEN 4 )  131  provided near the bundle discharge roller  49  (S 231 ), and the folding motor  126  and the bundle discharge motor that drives the bundle discharge roller  49  are stopped (S 232 ). 
     After the discharge of the folded sheet bundle BS to the second sheet discharge tray  22 , the folding motor  126  and the bundle discharge motor that drives the bundle discharge roller  49  are stopped (S 232 ). Thereafter, the presence/absence of the succeeding folded sheet bundle BS is checked (S 233 ). When the succeeding folded sheet bundle BS is present, the processing flow returns to the start point (round mark  1  in  FIG. 15 ), where processing is performed from S 201 . When the succeeding folded sheet bundle BS is absent, this routine is ended. 
     As described above, in the above operation, in the case of the stepwise folding for the folded sheet bundle formed by a small number of sheets and thus having a small thickness, the folded sheet bundle BS is discharged after the movement of the pressing roller unit  56  in one direction (forward path); while in the case of the stepwise folding for the folded sheet bundle formed by a large number of sheets and thus having a large thickness, the folded sheet bundle BS is discharged after the reciprocation (backward movement) of the pressing roller unit  56 . In addition, in the backward movement, the unit drive motor  69  is driven at a low speed. As a result, the folding line can be easily given to the folded sheet bundle BS having a large thickness. 
     [Control Configuration] 
     Control configuration of the sheet processing device B provided with the thus described stepwise folding unit  50  and the image forming device A including the sheet processing device B will be described based on a block diagram of  FIG. 17 . An image forming device controller  110  having an image forming section  2  inputs desired processing through user&#39;s operation made to an input section  111  provided on the control panel  18 . This input controls a sheet processing device controller  115  of the sheet processing device B based on a mode setting section  112 . 
     As described above, in the sheet processing device B of the present embodiment, the following modes can be designated: (1) “print-out mode” in which the image-formed sheet is stored in a first sheet discharge tray  21 ; (2) “stapling mode” in which the sheets from the main body sheet discharge port  3  are aligned in a bundle, bound by the end face stapler  33 , and stored in the first sheet discharge tray  21 ; (3) “saddle stitching and folding mode” in which the sheets from the main body sheet discharge port  3  are aligned in a bundle in the stacker section  35  which is the second processing tray, bound at a portion near a center thereof using the saddle stitching stapler  40 , folded into a booklet, and stored in the second sheet discharge tray  22 ; and (4) “stepwise folding mode” in which a sheet loop is folded stepwise at a fold of a folded sheet bundle that has been saddle-stitched and folded into a booklet and then stored in the second sheet discharge tray  22 . 
     The sheet processing device B includes the sheet processing device controller  115  that is made operable in one of the above modes, a ROM that stores an operation program, and a RAM that stores control data. The sheet processing device controller  115  includes a sheet conveying controller  116  that controls sheet conveyance in the device, a single-sheet punching controller  117  that applies pressing, one by one, the sheets by means of a single-sheet punching unit  28 , a processing tray controller  118  that performs control of accumulating the sheets in the processing tray  29 , and an end face binding controller  119  that binds the end face side of the sheets accumulated in a bundle in the processing tray  29  and discharges the bound sheet bundle. 
     The saddle stitching or center-folding of the sheet at about a half position thereof in the sheet conveying direction is controlled by a stacker section controller  120  that accumulates the sheet bundle in the sheet stacker section  35 . The stacker section controller  120  uses the stopper  38  or aligning member  39  that regulates the leading end of the sheets carried in, one by one, to the stacker section  35  to generate an aligned sheet bundle. The sheet processing device controller  115  further includes a saddle stitching controller  121  that controls the saddle stitching stapler  40  to drive a staple or the like to a center portion of the sheet bundle and a sheet center-folding controller  122  that controls the folding blade  46  to push the saddle-stitched sheet bundle into the folding roller  45  to center-fold the sheet bundle. The sheet center-folding controller  122  is connected to the sheet bundle detection sensor (SEN 3 )  129  and encoder sensor  128  and uses output signals therefrom to control the folding motor  126  that drives the folding roller  45 . 
     Further, the sheet processing device controller  115  includes a folded sheet stepwise folding controller  123  that is connected to the unit drive motor  69  that moves the pressing roller unit  56  according to the “stepwise folding mode” described above toward the folded sheet bundle BS and controls the unit drive motor  69 . The folded sheet stepwise folding controller  123  is also connected to the home position sensor  108  that is used for checking whether or not the stepwise folding unit is situated at the home position. 
     The folded sheet bundle BS that has been subjected to the stepwise folding is discharged to and accumulated in the second sheet discharge tray under control of a center-folded sheet discharge controller  124  connected to a bundle discharge roller drive motor that drives the bundle discharge roller  49 . The center-folded sheet discharge controller  124  is connected to the bundle discharge sensor (SEN 4 ) so as to check the discharge operation of the folded sheet bundle BS. 
     The control of the folded sheet stepwise folding which is related especially to the present invention has been described based on the description of the respective mechanisms and using operation state explanatory views of  FIGS. 8 to 10  and flowcharts of  FIGS. 15 and 16 , so descriptions thereof will be omitted. The stepwise folding unit  50  is controlled so as to execute a stepwise folding method based on the contents described. 
     The following describes modifications of the present invention. The same reference numerals are given to the members having the same functions as those in the above embodiment, and descriptions thereof will be omitted. The following first and second modifications each differ from the above embodiment in a configuration of the pressing roller unit  56  of the stepwise folding unit  50 . 
     [First Modification] 
     First, a first modification will be described using  FIG. 18 . As illustrated in detail in  FIG. 6 , in the above embodiment, the fold of the folded sheet bundle BS is pressed from the thickness direction of the sheet bundle by the pressing roller pairs  70  of three rows having mutually different intervals among the rows. In this case, the pressing roller unit  56  supports the pressing roller pairs  70  of three rows in one frame. On the other hand, in the first modification, as illustrated in  FIG. 18 , the pressing roller pairs of three rows are supported by units  56   a ,  56   b , and  56   c , respectively (a, b, and c are added to the reference numerals of members separated into independent units). Although not particularly illustrated, these units are connected to drive motors that independently drive the units. Thus, the units are individually moved to press stepwise the fold of the folded sheet bundle in the thickness direction of the fold. As in the case of the above embodiment, the third upper and lower pressing rollers  75  and  76  has the region R 1  between the upper and lower pressing roller large-diameter portions  75   a  and  76   a  and region R 2  between the upper and lower pressing roller small-diameter portions  75   b  and  76   b.    
     Thus, also in the first modification, the fold of the folded sheet bundle formed by a large number of sheets is pressed in the region R 2 , so that the rollers can easily override the comparatively thick end portion, thereby reducing damage to be applied to the sheet end portion. 
     [Second Modification] 
     The following describes a second modification using  FIGS. 19 and 20 . In the embodiment and above first modification, the pressing roller pairs  70  of three rows having mutually different intervals among the rows; while in the second modification, a pressing roller  170 , an interval between which is set in three stages, is configured to be moved three times (1.5 times of reciprocation) in the sheet width direction to press the fold of the folded sheet bundle. 
       FIG. 19  is a view viewed from the bundle discharge roller  49  side. A cam member  180  is used to set an interval between upper and lower restriction pins  182  and  183  of respective upper and lower pressing roller support blocks  173  and  174  that support the pressing roller pair  170 . As illustrated in a cross-sectional view of  FIG. 20 , the upper and lower pressing roller support blocks  173  and  174  are biased, in such a direction that they are brought into pressure contact with each other, by pressing roller pressing springs  90  of upper and lower spring holders  175  and  176 , respectively. The cam member  180  restricts the interval against the biasing force of the pressing springs  90 . The cam member  180  is moved by a cam drive motor  184 . The interval between the rollers of the pressing roller pair  170  can be set in three stages by the cam member  180 . 
     Thus, in the first stage (forward movement), the fold of the folded sheet bundle is pressed by the pressing roller pair  170  set at a comparatively large interval. In the second stage (subsequent backward movement), the fold is pressed by the pressing roller pair  170  set at a second smallest interval. In the third stage, in the case of stepwise folding for the folded sheet bundle BS formed by a small number of sheets, the fold thereof is pressed in the region R 1  where upper and lower pressing large-diameter rollers  151  and  152  are brought into pressure contact; in the case of stepwise folding for the folded sheet bundle BS formed by a large number of sheets, the fold is pressed in the region R 2  where upper and lower pressing small-diameter rollers  153  and  154  each having a diameter smaller by a than that of the large-diameter roller are disposed at an interval. Even in this configuration, rollers can easily override the comparatively thick end portion, thereby reducing damage to be applied to the sheet end portion. 
     The present invention in its preferred embodiment provides the following effects. 
     (1) There is provided a sheet pressing device that presses a fold (BL 1 ) of a folded sheet bundle, the device including: a pair of pressing members (pressing rollers  70 ) that press, in a fold thickness direction, the fold of the folded sheet bundle conveyed in a folded state; and a moving member (pressing roller unit  56 ) that moves the pressing member pair in a sheet fold direction while supporting the same, wherein the pressing member pair presses the fold in one of a first region where members of the pressing member pair (upper and lower pressing roller large-diameter portions  75   a  and  76   a ) are brought into pressure contact with each other and a second region adjacent to the first region on an upstream side thereof where members of the pressing member pair (upper and lower pressing roller small-diameter portions  75   b  and  76   b ) are opposed to each other at an interval. 
     With this configuration, the pressing member pair has the first region where members of the pressing member pair are brought into pressure contact with each other and second region where members of the pressing member pair are opposed to each other at an interval, so that it is possible to reduce impact generated when the pressing member pair abuts against the end portion of the sheet bundle and a moving load of the pressing member pair. 
     (2) In the sheet processing device of (1), the pressing member pair (pressing roller pair  70 ) integrally includes a large-diameter roller pair as the first region and a small-diameter roller pair as the second region, the large-diameter roller pair and the small-diameter roller pair being arranged in a sheet conveying direction and supported on a common axis. 
     With this configuration, the pressing roller pair is constituted by the large-diameter roller pair and the small-diameter roller pair which are opposed to each other, so that the stepped pressing member pair can be realized with a simple configuration. 
     (3) There is provided a sheet pressing device that presses a fold (BL 1 ) of a folded sheet bundle, the device including: a conveying member (folding roller  45 ) that conveys a folded sheet bundle; a pair of pressing members (pressing rollers  70 ) that presses the sheet bundle in a fold thickness direction of the sheet bundle conveyed by the conveying member; a moving member (pressing roller unit  56 ) that moves the pressing member pair in a sheet fold direction; and a controller (sheet center-folding controller  122  and folded sheet stepwise folding controller  123 ) that controls the moving member and conveying member, wherein the pressing member pair includes a first region where members (upper and lower pressing roller large-diameter portions  75   a  and  76   a ) of the pressing member pair are brought into pressure contact with each other and a second region adjacent to the first region on an upstream side thereof where members (upper and lower pressing roller small-diameter portions  75   b  and  76   b ) of the pressing member pair are opposed to each other at an interval, and the controller controls the conveying member to set the fold of the folded sheet bundle to the first region for pressing the fold when the number of sheets forming the conveyed folded sheet bundle is equal to or less than a predetermined number and to the second region when the number of sheets forming the conveyed folded sheet bundle is more than a predetermined number. 
     With this configuration, when the number of sheets is equal to or less than a predetermined number, the folded sheet bundle is pressed in the region where the members of the pressing member pair are brought into pressure contact with each other, so that the folded sheet bundle formed by a small number of sheets can surely be pressed; while when the number of sheets is more than a predetermined number, the folded sheet bundle is pressed in the region where the members of the pressing member pair are opposed to each other at an interval, so that it is possible to reduce impact generated when the pressing member pair overrides on the sheet end portion, thereby reducing deformation of or damage on the sheet end portion. 
     (4) In the sheet processing device of (3), the pressing member pair integrally includes a large-diameter roller pair as the first region and a small-diameter roller pair as the second region, the large-diameter roller pair and the small-diameter roller pair being arranged in a sheet conveying direction and rotatably supported on a common axis. 
     With this configuration, the pressing roller pair is constituted by the large-diameter roller pair and the small-diameter roller pair which are opposed to each other, so that the stepped pressing member pair can be realized with a simple configuration. 
     (5) In the sheet processing device of (4), the conveying member serves as both a folding roller for folding a sheet bundle and a conveying roller for conveying the folded sheet bundle. 
     With this configuration, the folding roller for folding the sheet bundle serves also as the conveying member, thereby reducing the number of rollers to be provided. 
     (6) The sheet processing device of (3) further includes a discharge member (bundle discharge roller  49 ) that discharges the folded sheet bundle after completion of the pressing by the pressing member pair, wherein 
     the controller (including the center-folded sheet discharge controller  124 ) controls, depending on the number of sheets forming the folded sheet bundle, whether to discharge the folded sheet bundle using the discharge member after one-way movement or reciprocating movement of the moving member for pressing. 
     With this configuration, when, particularly, the number of sheets forming the folded sheet bundle is more than a predetermined number, the fold of the folded sheet bundle is pressed in the second region where the members of the pressing member pair are separated from each other, so that when the pressing member pair supported by the moving member overrides the sheet end portion, it is possible to reduce damage on the sheet end portion caused due to abutment between the pressing member pair and the sheet end portion. Further, the moving member is reciprocated with the fold set in the second region to press the fold, thereby suppressing the folded sheet bundle from being opened after discharge. 
     (7) In the sheet processing device of (6), when the number of sheets forming the folded sheet bundle to be pressed by the pressing member pair is equal to or less than a predetermined number, the discharge member discharges the folded sheet bundle after completion of the one-way movement of the moving member, while when the number of sheets forming the folded sheet bundle is more than a predetermined number, the discharge member discharges the folded sheet bundle after completion of the reciprocating movement of the moving member. 
     With this configuration, when the number of sheets is equal to or less than a predetermined number, the folded sheet bundle is discharged after the one-way movement of the moving member, so that processing time does not become long; while when the number of sheets is more than a predetermined number, where longer processing time is accepted, it is possible to surely achieve the folding by discharging the folded sheet bundle after completion of the reciprocating movement of the moving member. 
     (8) In the sheet processing device of (7), the conveying member (folding roller  45 ) serves as both a folding roller for folding a sheet bundle and a conveying roller for conveying the folded sheet bundle and operates in conjunction with the discharging member (bundle discharge roller  49 ) when the folded sheet bundle is discharged. 
     With this configuration, the folding roller for folding the sheet bundle serves also as the conveying member and operates in conjunction with the discharging member, thereby reducing the number of rollers to be provided. 
     (9) In the sheet pressing device of (3), a moving speed of the moving member that is moved in the fold direction is set lower when the number of sheets forming the folded sheet bundle is more than a predetermined number than when the number of sheets forming the folded sheet bundle is equal to or less than a predetermined number. 
     With this configuration, when the number of sheets forming the folded sheet bundle is large, the moving member presses the fold at a comparatively low speed to thereby surely create the fold. 
     (10) There is provided a sheet processing device that presses a fold of a folded sheet bundle, the device including: a conveying roller (folding roller  45 ) that conveys a folded sheet bundle; a pair of pressing rollers (pressing rollers  70 ) that press the sheet bundle in a fold thickness direction of the sheet bundle; a moving unit (pressing roller unit  56 ) that moves the pressing roller pair in a sheet fold direction while supporting the same; and a controller (sheet center-folding controller  122  and folded sheet stepwise folding controller  123 ) that controls the sheet conveying operation of the conveying member and moving operation of the moving member, wherein the moving unit supports a plurality of rows of the pressing roller pairs with the intervals therebetween in the respective rows reduced stepwise in a moving direction of the moving member, the pressing roller pair of the last stage of the plurality of rows includes a first region formed by rollers of large-diameter roller pair (upper and lower pressing roller large-diameter portions  75   a  and  76   a ) which are brought into pressure contact with each other and a second region adjacent to the large-diameter roller pair on an upstream side in a sheet conveying direction, formed by rollers of small-diameter roller pair (upper and lower pressing roller small-diameter portions  75   b  and  76   b ) which are opposed to each other at an interval, and the controller controls the conveying member to set the fold of the folded sheet bundle to the first region for pressing the fold when the number of sheets forming the conveyed folded sheet bundle is equal to or less than a predetermined number and to the second region when the number of sheets forming the conveyed folded sheet bundle is more than a predetermined number. 
     With this configuration, when the number of sheets is equal to or less than a predetermined number, the folded sheet bundle is pressed in the region where the members of the pressing member pair are brought into pressure contact with each other, so that the folded sheet bundle formed by a small number of sheets can surely be pressed; while when the number of sheets is more than a predetermined number, the folded sheet bundle is pressed in the region where the members of the pressing member pair are opposed to each other at an interval, so that it is possible to reduce impact generated when the pressing member pair overrides on the sheet end portion, thereby reducing deformation of or damage on the sheet end portion. Further, there are provided the plurality of rows of the pressing member pairs whose intervals in the respective rows are reduced stepwise, whereby the fold of the folded sheet bundle is directed inward stepwise. Thus, the sheet bundle itself is directed to its binding direction, thereby suppressing the folded sheet bundle from being opened after the folding, which allows more sheet bundles to be accumulated. 
     (11) In the sheet processing device of (10), the plurality of rows of the pressing roller pairs supported in the moving unit so as to be arranged in the moving direction are configured such that two rollers constituting the sheet pressing member pair of each row are disposed opposite to each other, with a sheet fold position as a center, at positions substantially equally distant from the sheet fold position. 
     With this configuration, two rollers constituting the sheet pressing member pair of each row are disposed opposite to each other, with a sheet fold position as a center, at positions substantially equally distant from the sheet fold position, so that the pair of pressing rollers equally approach each other in the sheet thickness direction. Thus, the fold lines created by the pair of pressing rollers are formed at positions substantially equally distant from the sheet fold position, whereby a folded booklet having a good appearance can be created. 
     (12) In the sheet processing device of (10), the pressing roller pair integrally includes the large-diameter roller pair as the first region where the rollers are brought into pressure contact and the small-diameter roller pair as the second region disposed on the upstream side of the large-diameter roller pair, the large-diameter roller pair and the small-diameter roller pair being rotatably supported on a common axis. 
     With this configuration, the pressing roller pair is constituted by the large-diameter roller pair and the small-diameter roller pair which are opposed to each other, so that the stepped pressing member pair can be realized with a simple configuration. 
     (13) In the sheet processing device of (10), there are arranged at least three rows of the pressing roller pairs having mutually different intervals among the rows. 
     With this configuration, there are arranged three rows of the pressing roller pairs having mutually different intervals among the rows, so that it is possible to surely press the fold of the folded sheet bundle irrespective of whether the number of sheets is large or small. 
     (14) There is provide an image forming apparatus including: an image forming section that forms an image on a sheet; and a sheet processing device that applies predetermined sheet processing to the image-formed sheet from the image forming section, wherein the sheet processing device is the sheet processing device as claimed in claim  1 . 
     With this configuration, there can be provided an image forming apparatus having the sheet processing device capable of providing above-described working effects. 
     In the description of the embodiment and the effects thereof, reference numerals are given to constituent elements recited in the claims so as to clarify a correspondence relationship between the description of “Detailed Description” and the description of “What is Claimed is”. 
     Further, it should be appreciated that the present invention is not limited to the present embodiment, and various modifications may be made thereto. Further, all technical matters included in the technical ideas set forth in the claims should be covered by the present invention. While the invention has been described based on a preferred embodiment, those skilled in the art can realize various substitutions, corrections, modifications, or improvements may be made from the content disclosed in the specification by a person skilled in the art, which are included in the scope defined by the appended claims. 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Applications No. 2014-241045 and No. 2014-241046, both filed Nov. 28, 2014, the entire contents of which are incorporated herein by reference.