Patent Publication Number: US-7900905-B2

Title: Post-processing apparatus and image forming system having the same apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on Japanese Patent Application No. 2007-087183 filed on Mar. 29, 2007 with the Japanese Patent Office, the entire content of which is hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a post-processing apparatus and an image forming system having the same. 
     BACKGROUND OF THE INVENTION 
     In recent years, an image forming system has been required in the market which incorporates a post-processing apparatus to conduct various post-processing operations onto a plurality of sheets. 
     Concerning said post-processing apparatus, Unexamined Japanese Patent Application Publication No. 2005-170,676 discloses a post-processing apparatus which includes an sheet accumulating section which temporarily accumulates sheets, a sheet ejecting section which is positioned downstream of the accumulating section to eject a plurality of sheets accumulated in the sheet accumulating section, and a sheet stacking section which is positioned downstream of the sheet ejecting section to stack the sheets ejected from the sheet ejecting section. 
     The sheet accumulating section accumulates the sheets based on sheet detecting information which is sent from a sheet detecting sensor, in such way that a lower stacked recorded sheet is more advanced in the sheet conveyance direction than an upper stacked sheet, whereby the sheet ejecting section can easily catch the sheet. 
     Further, the sheet ejecting section receives sheets one by one in an order from bottom to top, and ejects the sheets one by one to the sheet stacking section. 
     Further, the sheet stacking section urges an ejected sheet one by one to move in the conveyance direction, so that a plurality of sheets is coordinated all together. 
     The present invention will be structured described below. 
     SUMMARY OF THE INVENTION 
     Structure 1. A post-processing apparatus, including: 
     a stacker which stacks sheets; 
     an accumulating section which is positioned upstream of the stacker and superimposes a plurality of sheets to be supplied to the stacker, and 
     a first sheet detection section which is positioned upstream of the accumulating section to detect the sheet, wherein the stacker includes: 
     a sheet stopping member which coordinates the sheets, and 
     an urging member which urges the sheet to move toward the sheet stopping member, 
     wherein the post-processing apparatus further includes a control section which controls the accumulating section, in such a way that based on a first sheet detection signal sent from the first sheet detection section, when the sheets are to be stacked in the stacker, a leading section of a primary sheet is positioned toward an upstream side for a first predetermined length from a leading section of a secondary sheet which is superimposed on the primary sheet in the accumulating section, and the control section further controls the urging member to rub a leading section of the primary sheet which protrudes from the secondary sheet toward the sheet stopping member, and controls the urging member to move the primary sheet toward the sheet stopping member. 
     Structure 2. A post-processing apparatus, including: 
     a stacker which stacks sheets; 
     an accumulating section which is positioned upstream of the stacker and superimposes a plurality of sheets to be supplied to the stacker, and 
     a second sheet detection section which is positioned downstream of the accumulating section to detect the sheet, wherein the stacker includes: 
     a sheet stopping member which coordinates the sheets, and 
     an urging member which urges the sheet to move toward the sheet stopping member, 
     wherein the post-processing apparatus further includes a control section which controls the accumulating section, in such a way that based on a first sheet detection signal sent from the first sheet detection section, when the sheets are to be stacked in the stacker, a leading section of a primary sheet is positioned toward an upstream side for a first predetermined length from a leading section of a secondary sheet which is superimposed on the primary sheet in the accumulating section, and the control section further controls the stacker in such a way that the urging member rubs a leading section of the primary sheet which protrudes from the secondary sheet toward the sheet stopping member, and that the urging section moves the primary sheet toward the sheet stopping member. 
     Structure 3. 
     An image forming system which includes said post-processing apparatus, and an image forming apparatus which supplies the sheets to said post-processing apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a drawing to explain an image forming system including post-processing apparatus  2  and image forming apparatus  3 . 
         FIG. 2  is a sectional drawing of an example of the image forming apparatus. 
         FIG. 3  is a sectional drawing of an example of the post-processing apparatus. 
         FIG. 4  is block diagram to explain the controls of the accumulating section and the stacker of the post-processing apparatus. 
         FIG. 5  is a drawing to explain the first embodiment of the accumulating section. 
         FIG. 6(   a ) is a drawing to explain the operation of the first embodiment of the accumulating section. 
         FIG. 6(   b ) is a drawing to explain the operation of the first embodiment of the accumulating section. 
         FIG. 7  is a flow chart of the first embodiment of the accumulating section. 
         FIG. 8  is a flow chart of the first embodiment of the accumulating section. 
         FIG. 9  is a flow chart of the stacker operation. 
         FIG. 10  is a drawing to explain the second embodiment of the accumulating section. 
         FIG. 11(   a ) is a drawing to explain the operation of the second embodiment of the accumulating section. 
         FIG. 11(   b ) is a drawing to explain the operation of the second embodiment of the accumulating section. 
         FIG. 11(   c ) is a drawing to explain the operation of the second embodiment of the accumulating section. 
         FIG. 12  is a flow chart of the second embodiment of the accumulating section. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiments of the present invention will now be detailed while referring to the drawings. The descriptions in this specification do not limit the technical scope of the claims nor the meaning of the terms. 
       FIG. 1  is a drawing to explain image forming system  1  including post-processing apparatus  2  and image forming apparatus  3 . 
     The sheets, which can be used in image forming system  1 , include paper sheets, plastic sheets and sheets of other materials. Further, the type of sheet is not limited to one size, that is, a plurality of types of the sheet can be used. 
     In image forming system  1 , which includes image forming apparatus  3  and post-processing apparatus  2 , after image formation is conducted on sheet P by image forming apparatus  3 , post-processing apparatus  2  conducts post-processing operation, such as a stapling operation, onto sheet P carrying said formed image. 
     Image forming apparatus  3  includes document image reading section  10  which reads out a document image, automatic document feeding device  20  which conveys document D, image forming section  30  which forms an image based on document image information read out by document image reading section  10 , sheet supplying section  40  which supplies sheet P to image forming section  30 , fixing section  50  which fixes a toner image, operation panel  90  which incorporates a display section and various operation switches, and control section C 2  which controls the above listed devices and sections. 
     Post-processing apparatus  2  includes hole punching section  110  which punches holes into a sheet, sheet shifting section  120  which shifts the position of the sheet for each job, stapling section  150  which staples a bundle of sheets, folding section  160  which folds said bundle of sheets, accumulating section  130  which temporarily accumulates the sheets, stacker  140  which temporarily stacks the sheets supplied from accumulating section  130 , and control section C 1  which controls the above listed sections. 
     In order that entrance section  100  of post-processing apparatus  2  successfully receives sheet P which is conveyed from image forming apparatus  3 , image forming apparatus  3  and post-processing apparatus  2  are combined in such a way that paired sheet ejection rollers  76  of image forming apparatus  3  matches entrance section  100  of post-processing apparatus  2 , with respect to their position and height. 
     Accordingly, sheet P is successfully conveyed by paired sheet ejection rollers  76  of image forming apparatus  3  to entrance section  100  of post-processing apparatus  2 . 
     Further, since image forming apparatus  3  includes communication section T 2 , and post-processing apparatus  2  includes communication section T 1 , both apparatuses  3  and  2  are electrically connected by communication section T 2  and communication section T 1 , which communicate various types of information. 
     For example, post-processing apparatus  2  conducts the post-processing operation, based on post-processing information which is communicated from image forming apparatus  3  through communication section T 2  and communication section T 1 . 
     The operator sets contents of the post-processing operation by operation panel  90 . 
     Image forming apparatus  3  and post-processing apparatus  2  of image forming system  1  will be detailed below. 
       FIG. 2  is a sectional drawing of an example of image forming apparatus  3 . 
     As an example of the image forming apparatus, a tandem-type full-color copier, serving as the image forming apparatus, will be detailed below. 
     Image forming apparatus  3  includes: document image reading section  10 , serving as a document image reading section, which reads out document image data to obtain specific document image information, automatic document feeding device  20 , serving as an automatic document feeding means, which conveys document D to document reading area R, image forming section  30  which forms an image by an electro-photographic method, sheet supplying section  40 , serving as a sheet supplying means, which supplies sheet P to image forming section  30 , image fixing section  50 , serving as a fixing means, which fixes a toner image onto sheet P, and control section C 2 , serving as a control means, which controls the above listed devices and sections. 
     Automatic document feeding device  20  sequentially picks up an original document D, placed on document supplying plate  21 , one by one, and conveys it to document reading area R, after which automatic document feeding device  20  ejects document D onto document ejection plate  29 . 
     To be more precise, after document D, placed on document supplying plate  21 , is conveyed one by one by paired elevating feed-out rollers  22 , document D is guided by rotating conveyance drum  24  exhibiting a large diameter, and is conveyed along the external surface of conveyance drum  24 , whereby document D reaches document reading area R. 
     After document D passes through document reading area R, document D is ejected onto document ejection plate  29  by paired ejection rollers  28 . 
     Document image reading section  10  reads out the image of document D conveyed by automatic document feeding device  20 , or an image of document D placed on glass platen  11 . 
     When a stationary optical system reading function is used for reading out the image of document D conveyed to document reading area R, light source  12  illuminates the image of document D, conveyed to document reading area R, through slit glass  19 , the light reflected from the image of document D is concentrated onto line image sensor CCD through first mirror  13 , second mirror  15 , third mirror  16 , and focusing lens  18 . 
     It is also possible to read out the image of document D, placed on glass platen  11 , by a moving optical system reading function, without using automatic document feeding device  20 . 
     Analog signals of the image of document D, which have been photo-electrically converted by line image sensor CCD, are changed to digital image data of yellow (Y), magenta (M), cyan (C) and black (K), via an analog process, A/D conversion, shading correction, and image compression, which are conducted by an image processing section, which is not illustrated. 
     Photoconductor drums (hereinafter referred to as “photoconductors”)  1 Y,  1 M,  1 C and  1 K, serving as first image carriers for each respective color Y, M, C and K, are electrically charged by charging device  2 Y,  2 M,  2 C and  2 K, respectively. 
     Exposure devices  3 Y,  3 M,  3 C and  3 K, each working for each color, form a latent image on photoconductors  1 Y,  1 M,  1 C and  1 K, based on digital image data of each color. 
     Each colored toner is supplied to developing devices  5 Y,  5 M,  5 C and  5 K by toner supplying devices  4 Y,  4 M,  4 C and  4 K, working for supplying respective new color toner, whereby the latent images of each color, formed on photoconductors  1 Y,  1 M,  1 C and  1 K, are developed by developing devices  5 Y,  5 M,  5 C and  5 K. 
     Developing devices  5 Y,  5 M,  5 C and  5 K, as well as photoconductors  1 Y,  1 M,  1 C and  1 K, are cascade-aligned in the vertical direction. To the left of  FIG. 2  of photoconductors  1 Y,  1 M,  1 C and  1 K, rotatable intermediate transfer body  70 , serving as a second image carrier, being a semi-conductive endless belt, is entrained about rollers  71 ,  72 ,  73  and  74 . 
     Intermediate transfer body  70  is rotated by roller  71  which is driven by a not-illustrated driving device, in the arrowed direction. 
     First transfer rollers  6 Y,  6 M,  6 C and  6 K, serving as the first transfer means, are selected to be driven by control section C 2 , based on the type of the image, and first transfer rollers  6 Y,  6 M,  6 C and  6 K press intermediate transfer body  70  against photoconductors  1 Y,  1 M,  1 C and  1 K, respectively. 
     That is, after the toner images of each color are formed on photoconductors  1 Y,  1 M,  1 C and  1 K, by developing devices  5 Y,  5 M,  5 C and  5 K, respectively, said formed toner images are successively transferred and synchronously superposed onto rotating intermediate transfer body  70  by first transfer rollers  6 Y,  6 M,  6 C and  6 K, respectively, so that a full color image is formed. 
     After first transfer rollers  6 Y,  6 M,  6 C and  6 K transfer the toner image of each color onto intermediate transfer body  70 , photoconductors  1 Y,  1 M,  1 C and  1 K are cleaned by cleaning device  7 Y,  7 M,  7 C and  7 K, so that any remaining toner is removed. 
     Sheet supplying section  40 , serving as a sheet supplying means, includes first sheet supplying cassette  41   a , second sheet supplying cassette  41   b , and third sheet supplying cassette  41   c , all of which are sheet storing members, in which varying sizes of sheets P are stored. 
     After an individual sheet P is picked up by sheet supplying unit  42 , serving as a sheet pick-up means, sheet P is conveyed to secondary transfer area  75   a  via paired intermediate rollers  43 ,  44 ,  45  and  46 , and paired registration rollers  47 , so that the full color toner image, precisely superposed on intermediate transfer body  70 , is transferred onto sheet P by secondary transfer roller  75 . 
     Sheet P, carrying the full color toner image, is permanently fixed by fixing section  50 , which includes heat roller  51  having heat source H, and pressing roller  52 , after which said sheet P is nipped by paired sheet ejection rollers  76 , and is sent through ejection outlet to the post-processing apparatus which serves as a downstream apparatus of the image forming system. 
     After the full-color image is transferred onto sheet P by secondary transfer roller  75 , sheet P is separated from intermediate transfer body  70 , whereby any remaining toner on intermediate transfer body  70  is cleaned by cleaning section  77 . 
     Operation panel  90  is installed on the top surface of document image reading section  10 , which includes a touch-sensitive panel and various operation switches. 
     Control section C 2 , for controlling image forming apparatus  3 , allows the touch-sensitive panel of operation panel  90  to display selection keys of post-processing operations, and also allows communication section T 2  to send post-processing information (such as information of the stapling operation), selected by the selection key toward communication section T 1  of post-processing apparatus  2 , other than the above described image forming operation. In addition, the post-processing operation includes a stapling operation, a folding operation, a hole-punching operation, a book-binding operation, and the like. 
       FIG. 3  is a sectional drawing of an example of post-processing apparatus  2 . 
     In  FIG. 3 , mounted on the top position of post-processing apparatus  2 , are first sheet supplying tray  101 , second sheet supplying tray  102 , and stationary sheet supplying tray  103 , which receives the ejected sheets. 
     In the middle position of post-processing apparatus  2 , hole-punching section  110 , serving as a hole-punching means, sheet-shift section  120 , serving as a sheet-shifting means, and sheet ejection roller  106  are horizontally aligned. 
     At the lower position of post-processing apparatus  2 , accumulating section  130  which shifts succeeding sheet positions in the conveyance direction to superimpose the sheets, stacker  140  which stacks and temporarily stores sheets P, staple section  150  which serves as a stapling means, and folding section  160  which serves as a folding means, are aligned at an angle to the horizontal. 
     Further, on the left side of  FIG. 3 , arranged are elevating sheet-ejection tray  104  which stacks sheets P, on which the shifting process has been conducted, or the sheet bundle on which the stapling process has been conducted, and stationary sheet-ejection tray  105  which stacks the three-folded or two-folded sheet bundles. 
     When the hole-punching process is selected for sheets P which already carry a formed image, formed by image forming apparatus  3 , and which were sent to receiving section  100  of post-processing apparatus  2 , said sheets P are punched by hole-punching section  110 , after which said sheets P pass through the upper route of first switching gate G 1 , and are ejected onto elevating sheet-ejection tray  104  by ejection roller  106 . 
     When the sheet-sorting process is selected as the post-processing operation, after sheets P pass through the upper route of first switching gate G 1 , sheets P which are grouped in one unit are shifted perpendicular to the sheet conveyance direction. Subsequently, sheets P are ejected onto elevating sheet-ejection tray  104  by ejection roller  106 . 
     When the sheet-stapling process is selected as the post-processing operation, after sheets P pass through the lower route of first switching gate G 1 , the conveyance-directional position of the succeeding sheets are shifted at a predetermined length by accumulating section  130 . Then a sheet bundle, including the superimposed sheets whose positions are shifted at the predetermined length, is supplied to stacker  140 . 
     Next, leading section urging roller  147  of stacker  140  rubs the shifted portion, being a protruded section, to move toward movable stopper  151 , whereby the sheets P are stopped by sheet stopping surface  151   a , and stacked in stacker  140 . 
     Subsequently, the sheet bundle, aligned in the conveyance direction, is stapled by stapling section  150 , after which said sheet bundle is ejected onto elevating sheet-ejection tray  104  by ejection roller  106  through stacker  140 . 
     When the sheet-folding process is selected for sheet P as the post-processing operation, after sheets P are stapled, movable stopper  151  is lowered so that the center of the stapled sheet bundle just meets pushing member  162  of folding section  160 . 
     Next, pushing member  162  is driven to push the center of the sheet bundle so that the sheet bundle is sandwiched between rotating paired folding rollers  161 , that is, the sheet bundle is folded by this operation, after which the folded sheet bundle is ejected onto stationary sheet-ejection tray  105  by ejection roller  106  through stacker  140 . 
     In the various cases above, the selected post-processing operation is recognized by control section C 1 , through communication section T 2  of image forming apparatus  3  and communication section T 1  of post-processing apparatus  2 . 
     Control section C 1  of post-processing apparatus  2  controls all the above described operations of post-processing apparatus  2 , and also communicates with control section C 2  of image forming apparatus  3 , through communication section T 2  of image forming apparatus  3  and communication section T 1  of post-processing apparatus  2 . 
     For example, through communication section T 1 , control section C 1  receives post-processing information (for example, the stapling process is going to be conducted), which is selected by the post-processing key, touched by the operator on operation panel  90  of image forming apparatus  3 , and control section C 1  conducts the selected post-processing process. 
     Next, the operations of accumulating section  130  and stacker  140  will be detailed, as an example of the stapling process and the folding process. 
     After sheet P is conveyed to first conveyance route A through first switching gate G 1 , sheet P is conveyed to accumulating section  130 , by paired conveyance rollers  191  and paired registration rollers  192 . 
     Subsequently, sheets P are superimposed, while each of two sheets P is shifted relative to each other for length L in the sheet conveyance direction, which will be detailed later. Sheet bundle P′ including two-superimposed sheets which are shifted from each other for length L in the sheet conveyance direction, are conveyed by paired conveyance rollers  193  to an upper space of slanting stacker  140 , after which sheet bundle P′ is dropped onto slanting chute  141  of stacker  140 , or dropped onto a foregoing sheet bundle stacked on slanting chute  141 . 
     In this case, the sheet conveyance direction on fourth conveyance route D, which passes through accumulating section  130 , is directed upward in  FIG. 3 . However, after the sheet is dropped on slanting chute  141  of stacker  140  or dropped on the foregoing sheet stacked on slanting chute  141 , the sheet is urged by an after-mentioned urging roller, that is, the sheet is conveyed downward in  FIG. 3 , being different from the sheet conveyance direction in fourth conveyance route D. 
     Accordingly, concerning the conveyance direction, the downstream side of the sheet conveyed on fourth route D, is opposite to the downstream side of the sheet to be stacked into stacker  140 . 
     Thus, sheet bundle P′, conveyed to the upper space of stacker  140 , is conveyed obliquely downward by the urging roller on the slanting surface of slanting chute  141 . 
     Stacker  140  includes paired width adjusting members  142 , movable stopper  151  which serves as a stopping member to adjust the position of sheet P in the sheet conveyance direction, leading section urging roller  147 , which serves as a first urging member, to move sheet P toward movable stopper  151 , and trailing section urging roller  148 , serving as a second urging member, which is located farther from movable stopper  151  than leading section urging roller  147  and rubs the substantially-central portion of sheet P to allow it to drop toward movable stopper  151 . 
     Both leading section urging roller  147  and trailing section urging roller  148  urge sheet P to have it drop toward sheet stopping surface  151   a  of movable stopper  151 , and said both rollers have plural paddles  147   a  and  148   a  on their rotation shaft. 
     When each urging roller rotates, top sections of paddles  147   a  and  148   a  rub down the surface of sheet P pushing it toward sheet stopping surface  151   a  so that sheet P is urged downward. Paddles  147   a  and  148   a  are formed of a thin and flexible rubber. 
     Sheet bundle P′, including two-ply sheets P which are an upper sheet as the secondary sheet, and a lower sheet as the primary sheet, slides down on the slanting surface of slanting chute  141 , wherein the leading section of the primary sheet protrudes for length L toward movable stopper  151  from the leading section of the secondary sheet in the sheet dropping direction. 
     When the leading section (shown by shift length section Q in  FIG. 3 ) of the primary sheet is rubbed by leading section urging roller  147 , the primary sheet is urged by leading section urging roller  147  to move toward movable stopper  151 , while the secondary sheet does not disturb the movement of primary sheet, the leading section of the primary sheet is rubbed downward so that the primary sheet drops and is stopped by stopping surface  151   a.    
     Then, the substantially-central portion of the second sheet superimposed on the primary sheet is urged to drop by trailing section urging roller  148 , which rotates in the arrowed direction, whereby the secondary sheet superimposed on the primary sheet drops toward movable stopper  151 . 
     Then, the leading section of both the primary sheet and the secondary sheet are rubbed by leading section urging roller  147 , the substantially-central portion of both the primary sheet and the secondary sheet are rubbed by trailing section urging roller  148 , whereby the top (which faces downward in  FIG. 3 ) of each sheet touches stopping surface  151   a  so that each sheet is stopped, and the alignment of the sheets in the conveyance direction is completed. 
     Paired width adjusting members  142 , which are mounted on both sides of slanting chute  141 , move perpendicular to the sheet conveyance direction. When sheet P is conveyed on the slanting surface of slanting chute  141 , said paired width adjusting members  142  are opened wider than the width of sheet P, and softly contact the sides of sheet P to conduct the width alignment of sheets P. 
     Stapling section  150  includes stapling mechanism  150 A and receiving mechanism  150 B, between which sheet path  152  is formed to pass sheet P. 
     After two sheets P are stacked and aligned in stacker  140 , stapling mechanism  150 A and staple receiving mechanism  150 B are driven perpendicular to the sheet conveyance direction by a driving means, which is not illustrated, whereby sheets P are stapled. 
     In this case, movable stopper  151  has been stopped at a position corresponding to a position where stapling mechanism  150 A staples the sheet bundle stacked on movable stopper  151 . 
     Areas having no stacking surface is formed on a portion of the sheet stacking surface of slanting chute  141 , and a plurality of ejection belts  145  are entrained about driving pulley  143  and driven pulley  144 , and said belts  145  rotate in arrowed direction A 1  in  FIG. 3 . 
     Onto some ejection belts  145 , pawl  146  is united with ejection belt  145 . When ejection belts  145  are rotated, the top of pawl  146  tracks in an ellipse. 
     When only the stapling process is conducted, while the other processes are not conducted, a stapled bundle of sheets on ejection belts  145  is supported by pawl  146 , and is conveyed obliquely upward to a nipping position of paired sheet ejecting rollers  106   
     Said stapled bundle of sheets is ejected and stacked on elevating sheet-ejection tray  104  by paired ejection rollers  106 . 
     When a stapled bundle of sheets is folded to form a booklet, said bundle is folded at its center by folding section  160 , and is ejected onto stationary sheet-ejection tray  105 . 
     Folding section  160  includes paired folding rollers  161  which press the stapled sheet bundle to fold it, and pushing member  162  which pushes the center of the stapled sheet bundle between paired folding rollers  161 . 
     Movable stopper  151  moves to a position where pushing member  162  meets the center of the stapled sheet bundle, and stops (in this state, movable stopper  151  is shown by dashed lines). 
     In this stopped position, pushing member  162  pushes the center of stapled sheet bundle between paired folding rollers  161 , whereby the center of the stapled sheet bundle is folded, which becomes a state of a booklet. 
     Further, paired folding rollers  161  rotate to eject the sheet bundle in the state of the booklet onto stationary sheet-ejection tray  105 . 
     Accumulating section  130  will be detailed later. 
       FIG. 4  is block diagram to explain the control of accumulating section  130  and stacker  140  of post-processing apparatus  2 . 
     The control of post-processing apparatus will be detailed while referring to  FIGS. 3 and 4 . 
     The control described below is conducted by control section C 1  of post-processing apparatus  2 . 
     Control section C 1  includes a CPU (central processing unit), RAM (random access memory), ROM (read only memory), I/O controller which controls each input-output device under the control of the CPU, and a bus which connects the above sections (see  FIG. 4 ). 
     Various programs are previously stored in ROM to control post-processing apparatus  2 . The CPU stores these programs into the RAM from the ROM to control each input-output device through the I/O controller. 
     Control section C 1  is connected to first sheet sensor S 1  mounted in first conveyance route A or second sheet sensor S 2  mounted on fourth conveyance route D, communication section T 1  which exchanges information with communication section T 2  of image forming apparatus  3 , and sheet conveyance length measuring section  200  which measures the conveyance length of the sheet, all of which serve as input sections. 
     Further, control section C 1  is connected to registration roller driving motor  192 M to drive registration roller  192 , conveyance roller driving motor  131 M to drive conveyance roller  131  or conveyance roller driving motor  134 M to drive conveyance roller  135 , conveyance roller driving motor  132 M to drive conveyance roller  132  or conveyance roller driving motor  136 M to drive conveyance roller  136 , conveyance roller driving motor  193 M to drive conveyance roller  193 , and solenoid SD which allows conveyance roller  193  to nip the sheet or not, all of which serve as output sections. 
     The above-described driving motors include pulse motors, which are rotated by pulse-motor driving devices (which are not illustrated) of control section C 1 . 
     Sheet conveyance length measuring section  200  counts the driving pulses of each driving motor, and calculates to measure the conveyance length of the sheet. 
     Sheet conveyance length measuring section  200  includes first measuring section  201  which measures the conveyance length of the sheet conveyed through second conveyance route B, and second measuring section  202  which measures the conveyance length of the sheet conveyed through third conveyance route C. 
     To measure the conveyance length of the sheet conveyed through second conveyance route B, first measuring section  201  counts the driving pulses of conveyance roller driving motor  131 M, whereby the conveyance length of the primary sheet conveyed through second conveyance route B is calculated. 
     To measure the conveyance length of the sheet conveyed through third conveyance route C, second measuring section  202  counts the driving pulses of conveyance roller driving motor  132 M so that the conveyance length of the secondary sheet conveyed through third conveyance route C is calculated. 
     A plurality of the embodiments of accumulating section  130  will now be detailed. 
       FIG. 5  is a drawing to explain the first embodiment of accumulating section  130 . 
     Accumulating section  130   a  of the first embodiment includes; 
     second conveyance route B (being a first sheet conveyance route) through which primary sheet P 1  is conveyed, 
     third conveyance route C (being a second sheet conveyance route) through which secondary sheet P 2  is conveyed, 
     second switching gate G 2  which selects second conveyance route B or third conveyance route C, 
     paired conveyance rollers  131 , serving as first conveyance members, which are mounted on second conveyance route B to convey primary sheet P 1  in the downstream direction, 
     paired conveyance rollers  132 , serving as second conveyance members, which are mounted on third conveyance route C to convey secondary sheet P 2  in the downstream direction, and 
     paired conveyance rollers  193 , positioned at the downstream confluent section of both second conveyance route B and third conveyance route C, which convey primary sheet P 1  and secondary sheet P 2 , and are capable of nipping or releasing said sheets. 
     Further, arranged are fourth conveyance route D, positioned downstream of paired conveyance rollers  193 , through which primary sheet P 1  and secondary sheet P 2  are conveyed, and 
     first sheet sensor S 1 , positioned upstream of accumulating section  130   a , which detects a leading edge of sheet P conveyed through first conveyance route A. 
     For first sheet sensor S 1 , a non-contact photoelectrical sensor is preferably employed to detect sheet P, which does not adversely affect the sheet conveyance. 
     Paired conveyance rollers  131 , paired conveyance rollers  132  and paired conveyance rollers  193  are driven by pulse motors or the like. Measurement of the conveyance length of the sheet conveyed by the above-described rollers are determined by counting driving pulses given to each pulse motor. 
     First predetermined length L represents the difference (being a shifted length) in the conveying direction between primary sheet P 1  and secondary sheet P 2  which is superimposed on primary sheet P 1 , when both sheets are stacked on stacker  140 . That is, position q of the trailing section (which exists on the opposite side of movable stopper  151 ) of primary sheet P 1  in the conveyance direction is more advanced to movable stopper  151  at length L than position p of the trailing section (which exists on the opposite side of movable stopper  151 ) of secondary sheet P 2 . 
     That is, position s of the leading section (which exists near movable stopper  151 ) of primary sheet P 1  which positions to be lower to contact slanting chute  141  when it is to be stacked on the stacker, is more advanced toward movable stopper  151  by length L than position r of the leading section (which exists near movable stopper  151 ) of secondary sheet P 2  to be superimposed on primary sheet P 1 . 
     Length L is determined in such a way that when primary sheet P 1  is urged to move toward sheet stopping surface  151   a  by urging roller  147 , that is, when the sheets are stacked on stacker  140 , secondary sheet P 2 , superimposed on primary sheet P 1 , does not disturb primary sheet P 1 , and additionally, length L is also determined based on various sections around stacker  140 . 
       FIGS. 6(   a ) and  6 ( b ) are drawings to explain the operation of the first embodiment of accumulating section  130 . 
     The operation of accumulating section  130   a  of the first embodiment will be detailed below. 
     For simpler explanation, sheets exhibiting the same size, are used in this discussion. 
     As shown in  FIG. 6(   a ), after sheet P, passing through first conveyance route A, is detected by first sheet sensor S 1 , second switching gate G 2  is activated to open second conveyance route B through which sheet P (that is, primary sheet P 1 ) is conveyed. 
     After primary sheet P 1  is detected by first sheet sensor S 1 , paired registration rollers  131 , and paired conveyance rollers  193  are normally rotated, so that primary sheet P 1  is conveyed for a second predetermined length, (that is, the leading section of primary sheet P 1  reaches a position which is farther downstream than paired conveyance rollers  193 ). 
     Next, as shown in  FIG. 6(   b ), second switching gate G 2  opens third conveyance route C. 
     After secondary sheet P 2  is detected by first sheet sensor S 1 , paired registration rollers  132 , and paired conveyance rollers  132  are rotated in the normal direction, so that secondary sheet P 2  are conveyed for a predetermined length through third conveyance route C, (for example, the leading section of secondary sheet p 2  is farther advanced at length L than the leading section of primary sheet P 1  in the conveyance direction). Accordingly, in accumulating section  130   a , secondary sheet P 2  is superimposed on primary sheet P 1 , and the trailing section of primary sheet P 1  is protruded at length L from that of secondary sheet P 2  toward the upstream side. 
     In addition, when secondary sheet P 2  is conveyed in the above-described operation, paired conveyance rollers  193  do not nip the sheets, whereby secondary sheet P 2  can be superimposed on primary sheet P 1 . 
     After the conveyance of secondary sheet p 2  is completed, paired conveyance rollers  193  nip both primary sheet P 1  and secondary sheet P 2 . Paired conveyance rollers  131 , paired conveyance rollers  132 , and paired conveyance rollers  193  convey a sheet bundle, including superimposed sheets P 1  and P 2  whose positions are shifted for length L in the conveyance direction, toward slanting chute  141 . Additionally, when the sheet bundle is ejected from conveyance roller  193 , the trailing section of primary sheet P 1  has been superimposed below secondary sheet P 2 , so that the trailing edge of primary sheet P 1  has been shifted for length L toward the upstream direction from the trailing edge of secondary sheet P 2 . 
     In this case, in accumulating section  130   a , primary sheet P 1  represents a sheet layered under secondary sheet P 2 , when they are to be stacked in stacker  140 . Further, secondary sheet P 2  represents a sheet layered on primary sheet P 1 , when they are to be stacked in stacker  140 . 
     Accordingly, in accumulating section  130   a , concerning primary sheet P 1  on which secondary sheet P 2  is superimposed, the trailing edge of primary sheet P 1  is shifted for length L from the trailing edge of secondary sheet P 2 , toward the upstream direction. Due to this length L, primary sheet P 1  can be rubbed by urging roller  147  toward sheet stopping surface  151   a  in stacker  140 , while secondary sheet P 2  does not disturb said rubbing action conducted by urging roller  147 . In more detail, leading section urging roller  147  and trailing section urging roller  148  rub superimposed primary sheet P 1  and secondary sheet P 2  toward sheet stopping surface  151   a . Accordingly, primary sheet P 1  and secondary sheet P 2  can be surely stopped by sheet stopping surface  151   a . By these operations, precise and effective alignment of the sheets can be conducted, and it is possible to provide post-processing apparatus  2 , which can conduct high quality post-processing operations, exhibiting high productivity. 
     The mounting angle of slanting chute  141  looks substantially vertical in  FIGS. 6(   a ) and  6 ( b ), but it is also possible to mount chute  141  substantially horizontally, because urging roller  147  urges the trailing section of sheet P toward stopping surface  151   a , so that two sheets can be stopped by stopping surface  151   a , though chute  141  is substantially horizontal. 
       FIGS. 7 and 8  show the flow charts of the first embodiment of accumulating section  130 . 
       FIG. 9  is a flow chart of stacker  140 . 
     The flow chart relating to the first embodiment of accumulating section  130  will be detailed while referring to  FIGS. 4 ,  5 ,  7  and  8 . 
     The operational flow described below is conducted by control section C 1 . 
     1. Judgment of the necessity of superimposing the sheets (step S 101 ). 
     In  FIG. 7 , control section C 1  determines whether sheets P are to be superimposed or not, based on post-processing content information sent from image forming apparatus  3  through communication section T 1 . 
     In detail, information is previously stored in a memory section, such as a ROM, whether the superimposing operation is necessary or not, for each of the plural post-processing operations. For example, post-processing information showing that the superimposing operation is necessary for the stapling process is sent (Yes in step S 101 ), control section C 1  determines that the superimposing operation is necessary, and advances to the next step. That is, in step S 101 , if the post-processing content for the previously processed sheet bundle needs predetermined time like the stapling process, and if the post-processing content for the succeeding sheet bundle uses stacker  140  like the stapling process, control section C 1  determines that the succeeding sheet bundle requires the superimposing operation. 
     If the post-processing content which does not require the superimposing process, such as a sorting process, is sent, or if information is sent showing that third sheet P or a later sheet is to be processed, though the stapling process is shown in said information, (which is No in step S 101 ), the operation flow jumps to END. 
     The operation flow to be detailed, includes that the superimposing operation is determined to be conducted, and that information of the stapling process has been sent for the post-processing operation. 
     2. Monitoring the first sheet sensor (step S 102 ) 
     Control section C 1 , monitoring first sheet sensor S 1  which is a first sheet detection section, detects the leading edge of sheet P (Yes in step S 102 ), and goes to step S 103 , but if the leading edge is not detected (No in step S 102 ), control section C 1  repeats step S 102 , until the leading edge of sheet P is detected. 
     3. Determining Whether Sheet P is a Primary Sheet or not (Step S 103 ) 
     Control section C 1  determines whether detected sheet P is primary sheet P 1  or not, based on an interrupting point of the job or the group. If the detected sheet is primary sheet P 1  (Yes in step S 103 ), operation flow goes to step S 104 . If the detected sheet is not primary sheet P 1 , control section C 1  determines that the detected sheet is secondary sheet P 2 , and jumps to step S 150 . In addition, instead of the interrupting point of the job or the group, if the post-processing operation is changed to another post-processing operation while the job or the group is being processed, it is possible to prepare a processing flow which determines whether it is primary sheet P 1  at said changing point. 
     4. Switching operation of second switching gate G 2  (step S 104 ) 
     Control section C 1  selects second switching gate G 2  so that primary sheet P 1  is conveyed through second conveyance route B, and the operation flow goes to step S 105 . 
     5. Start of measurement of the conveyance length of primary sheet P 1  (step S 105 ) 
     In order to measure the conveyance length of primary sheet P 1  from first sheet sensor S 1 , control section C 1  clears the count stored in first measuring section  201  to start a new count, and the operation flow goes to step S 106 . 
     6. Continuity of the conveyance of primary sheet P 1  (step S 106 ) 
     Control section C 1  activates registration roller driving motor  192 M, conveyance roller driving motor  131 M, and conveyance roller driving motor  193  to rotate in the normal direction, so that primary sheet P 1  is conveyed further, and the operation flow goes to step S 107 . 
     7. Determination whether a predetermined conveyance length has been completed for primary sheet P 1  (step S 107 ) 
     Control section C 1  monitors the count conducted by first measuring section  201 , and checks whether predetermined numbers have been counted, and if the predetermined numbers have been counted (Yes in step S 107 ), control section C 1  determines that primary sheet P 1  has been conveyed to a second predetermined length, and the operation goes to step S 108 . If the count does not reach the predetermined numbers (No in step S 107 ), step S 107  is repeated until the count reaches the predetermined numbers. 
     Said second predetermined length means the distance from first sheet sensor S 1  to a substantially-central section between slanting chute  141  of stacker  140  and paired conveyance rollers  193  on second conveyance route B, through which primary sheet P 1  is conveyed, which distance has been previously set. 
     8. Stop of conveyance of primary sheet P 1  (step S 108 ) 
     Control section C 1  deactivates registration roller driving motor  192 M, conveyance roller driving motor  131 M, and conveyance roller driving motor  193 , so that conveyance of primary sheet P 1  is stopped. 
     Further, control section C 1  clears the count in first measuring section  201 , and the operation flow goes to step S 109 . 
     9. Switching operation of second switching gate G 2  (step S 109 ) 
     Control section C 1  changes second switching gate G 2  so that secondary sheet P 2  is conveyed to third conveyance route C, and the operation flow goes to a next step. 
     10. Monitoring sheet sensor S 1  in step S 102 , and detecting the passage of sheet P 
     If sheet P is detected (Yes in step S 102 ), the operation flow goes to the next step, but if it has not yet been detected (No in step S 1029 ), control section C 1  repeats step S 102  until said sheet P is detected. 
     11. Control section C 1  determines whether sheet P detected by step S 103  is a primary sheet, based on the interrupting point of the job or the group. If it is primary sheet P 1  (Yes in step S 103 ), the operation flow goes to S 104 , and if it is not primary sheet P 1 , control section C 1  determines that it is secondary sheet P 2 , and the operation flow jumps to step S 150 . 
     12. Starting the measurement of the conveyance length of secondary sheet P 2  (step S 150  in  FIG. 8 ) 
     In order to measure the conveyance length of secondary sheet P 2  from first sheet sensor S 1 , control section C 1  clears the count in second measuring section  202 , so that a new count is started, and the operational flow goes to S 151 . 
     13. Releasing the sheet nip of paired conveyance rollers  193  (step S 151 ) 
     In order to superimpose secondary sheet P 2  on primary sheet P 1  which has been conveyed between paired conveyance rollers  193 , control section C 1  deactivates solenoid SD, so that the sheet nip conducted by paired conveyance rollers  193  is cancelled, and the operation flow goes to S 152 . 
     14. Continuity of the conveyance of secondary sheet P 2  (step S 152 ) 
     Control section C 1  activates registration roller driving motor  192 M and conveyance roller driving motor  132 M, to rotate in the normal direction, so that secondary sheet P 2  is conveyed further, and the operation flow goes to step S 153 . 
     15. Determination whether the predetermined conveyance length has been completed for secondary sheet P 2  (step S 153 ) 
     Control section C 1  monitors the count conducted by second measuring section  202 , and checks whether predetermined numbers have been counted, and if the predetermined numbers have been counted (Yes in step S 153 ), control section C 1  determines that secondary sheet P 2  has been conveyed for the predetermined length, and the operation flow goes to step S 154 . If the count has not reached the predetermined numbers (No in step S 153 ), step S 153  is repeated until the count reaches the predetermined numbers. 
     Said predetermined length in step S 153  means the distance, which is previously set, from first sheet sensor S 1  to a position where secondary sheet P 2  is advanced at length L in the conveyance direction (that is, in the downstream direction) from a position at which the leading edge of primary sheet P 1  is stopped in step S 108 , through which secondary sheet P 2  is conveyed via third conveyance route C. 
     16. Nipping operation conducted by paired conveyance rollers  193  (step S 154 ) 
     In order to convey primary sheet P 1  and secondary sheet P 2 , both of which are superimposed between paired conveyance rollers  193 , control section C 1  activates solenoid SD so that paired conveyance rollers  193  nip both primary sheet P 1  and secondary sheet P 2 , and the operation flow goes to S 154 . 
     In addition, it is possible for the operation flow to stop both registration roller driving motor  192 M and conveyance roller driving motor  132 M, so that the conveyance of secondary sheet P 2  is temporarily stopped. 
     17. Ejection of the sheet bundle (step S 155 ) 
     Control section C 1  controls conveyance roller driving motor  131 M, conveyance roller driving motor  132 M, and conveyance roller driving motor  193 M, to rotate in the normal direction at the same outer circumferential velocity, so that sheet bundle P′, which includes superimposed primary sheet P 1  and secondary sheet P 2 , is ejected to an upper space of slanting stacker  140 . 
     After both sheets P 1  and P 2  are nipped in step S 154 , each driving motor rotates in the normal direction so that the sheet is conveyed for a length which is 1.2-1.3 times the length of a sheet measured in the conveyance direction. 
     Subsequently, control section C 1  repeats steps  101 - 155  until operation for the group or the job is completed. 
     The operation flow of stacker  140  will be detailed while referring to  FIGS. 4 ,  5  and  9 . 
     The operational flow described below is conducted by control section C 1 . 
     As described above, sheet bundle P′, which was ejected onto the upper space of slanting stacker  140 , drops onto slanting chute  141  of stacker  140 , or drops onto the sheet already stacked on slanting chute  141 . 
     1. Urging action for primary sheet P 1  (step S 1001 ) 
     Control section C 1  activates leading section urging roller driving motor  147 M in the forward direction so that leading section urging roller  147  rotates, and rubs the leading section (which is shifted length L existing in a lower section in  FIG. 5 ) of primary sheet P 1  which slides down positioned on the slanting surface of slanting chute  141 . 
     In more detail, plural paddles  147   a  of leading section urging roller  147  rub the leading section of primary sheet P 1  (which is shifted length L existing in a lower section in  FIG. 5 ), so that primary sheet P 1  is urged toward stopping surface  151   a  of movable stopper  151 . 
     Due to this action, primary sheet P 1  can be stopped by sheet stopping surface  151   a , while secondary sheet P 2  does not disturb said rubbing action of urging roller  147 . 
     2. Urging action for secondary sheet P 2  (step S 1002 ) 
     Control section C 1  activates urging roller driving motor  148 M which drives trailing section urging roller  148 , whereby urging roller  148  continuously urges secondary sheet P 2 , which is superimposed on primary sheet P 1 , toward stopping surface  151   a.    
     In this action, primary sheet P 1  is urged by leading section urging roller  147 , and at the same time, primary sheet P 1  is urged by trailing section urging roller  148  through secondary sheet P 2 . Further, when the leading section of secondary sheet P 2  reaches leading section urging roller  147 , both primary sheet P 1  and secondary sheet P 2  are urged by trailing section urging roller  148  and leading section urging roller  147 . 
     In more detail, plural paddles  148   a  of leading section urging roller  148 , and plural paddles  147   a  of leading section urging roller  147  rub the surface of secondary sheet P 2 , whereby secondary sheet P 2  is urged toward stopping surface  151   a  of movable stopper  151 , and primary sheet P 1  is also urged through secondary sheet P 2  toward stopping surface  151   a  of movable stopper  151 . 
     3. Alignment of primary sheet P 1  and secondary sheet P 2  (step S 1003 ) 
     Control section C 1  allows motor  148 M and motor  147 M to continue to rotate, so that primary sheet P 1  and secondary sheet P 2 , having touched stopping surface  151   a , are further urged, whereby primary sheet P 1  and secondary sheet P 2  are completely aligned. 
     Control section C 1  repeats steps S 1001 -S 1003 , until operation for the group or the job is completed. 
     Subsequently, after sheet bundle P′ is completely aligned, sheet bundle P′ is stapled by stapling section  150 , or sheet bundle P′ is folded by folding section  160 , if necessary. 
     As detailed above, when primary sheet P 1  and secondary sheet P 2  are stacked in stacker  140 , secondary sheet P 2  is positioned on primary sheet P 1 . 
     Accumulating section  130   c  of the second embodiment will now be detailed. 
       FIG. 10  is a drawing to explain accumulating section  130   c  of the second embodiment. 
     In order to avoid repeating explanation, the same matters as for the first embodiment are not detailed again. 
     Accumulating section  130   c  of the second embodiment includes: 
     paired conveyance rollers  135 , serving as third conveyance members, which are mounted in second conveyance route B, and can convey primary sheet P 1  both forward and backward (that is, primary sheet P 1  can be switch-backed in the conveyance direction), and 
     paired conveyance rollers  136 , serving as fourth conveyance members, which are mounted in third conveyance route C, and convey secondary sheet P 2 . 
     Further, second sheet sensor S 2 , serving as a second sheet detecting section to detect sheet P which is conveyed through fourth conveyance route D, is mounted downstream of accumulating section  130   c.    
     First sheet sensor S 1  is not provided in the second embodiment. 
       FIGS. 11(   a ) and  11 ( b ) are drawings to explain the operation of accumulating section  130   c  of the second embodiment. 
     The operation of accumulating section  130   c  of the second embodiment will be detailed below. 
     As shown in  FIG. 11(   a ), sheet P, which has been conveyed through first conveyance route A, is conveyed toward second conveyance route B by second switching gate G 2 . 
     Primary sheet P 1  is conveyed in second conveyance route B, after which it is conveyed by paired conveyance rollers  193 , whereby the leading edge of primary sheet P 1  is detected by second sheet sensor S 2 . 
     After second sheet sensor S 2  detects the leading section of primary sheet P 1 , second switching gate G 2  is directed to third conveyance route C. 
     When primary sheet P 1  is detected by second sheet sensor S 2 , primary sheet P 1  is temporarily stopped. 
     As shown in  FIG. 11(   b ), said temporarily stopped primary sheet P 1  is conveyed backward for length L (being the first predetermined length), and is stopped. 
     In this state, after the conveyance length from second sheet sensor S 2  is measured by the counter, the above-described backward conveyance of length L is conducted for the counts by the counter about the conveyance length from second sheet sensor S 2 , which is the same way as shown in steps S 104 -S 108 . 
     As shown in  FIG. 11(   c ), secondary sheet P 2  enters third conveyance route C, and is conveyed by paired conveyance rollers  136 , whereby the leading edge of secondary sheet P 2  is detected by second sheet sensor S 2 , and secondary sheet P 2  stops. 
     In the state shown by  FIG. 11(   c ), paired conveyance rollers  193  do not nip the sheet so that secondary sheet P 2  is superimposed on primary sheet P 1  between conveyance rollers  193 . 
     After the conveyance of secondary sheet P 2  is completed, paired conveyance rollers  193  nip both primary sheet P 1  and secondary sheet P 2 . Then, paired conveyance rollers  193 , paired conveyance rollers  135 , and paired conveyance rollers  136  convey the sheet bundle, including superimposed primary sheet P 1  and secondary sheet P 2 , toward slanting chute  141 . 
     In addition, it is possible to structure the embodiment in such a way that when secondary sheet P 2  is detected by second sheet sensor S 2 , conveyance of secondary sheet P 2  is not instructed to stop, and superimposed primary sheet P 1  and secondary sheet P 2  are nipped again by paired conveyance rollers  193 , and they are conveyed together in the superimposed state. 
     In this case, in accumulating section  130   a , primary sheet P 1  represents a sheet under secondary sheet P 2 , when they are to be stacked in stacker  140 . Further, secondary sheet P 2  represents a sheet on primary sheet P 1 , when they are to be stacked in stacker  140 . 
     In addition to the effect shown in the first embodiment, in the second embodiment, since second sheet sensor S 2 , which is closer to stacker  140  than first sheet sensor S 1  is to, is structured as the standard of position determination, primary sheet P 1  and secondary sheet P 2  can be more accurately shifted than in the first embodiment. 
       FIG. 12  is a flow chart of the second embodiment of accumulating section  130 . 
     1. Step S 301  is conducted in the same manner as above-described step S 101 , accordingly explanation is omitted. 
     2. Step S 302  is conducted in the same manner as above-described step S 103 , so that explanation can be omitted. 
     3. Step S 303  is conducted in the same manner as above-described step S 104 , so that explanation can be omitted. 
     4. Step S 304  is conducted in the same manner as above-described step S 106 , so that explanation can be omitted. 
     5. Monitoring of second sheet sensor S 2  (step S 305 ) 
     Control section C 1  monitors second sheet sensor S 2  which detects the leading edge of primary sheet P 1 . If second sheet sensor S 2  detects the leading edge of primary sheet P 1  (Yes in step S 305 ), the operation flow goes to step S 306 , while if not (No in step S 305 ), control section C 1  repeats step S 305 . 
     6. Step S 306  is conducted in the same manner as above-described step S 108 , so that explanation can be omitted. 
     7. Backward conveyance of primary sheet P 1  for a predetermined length (step S 307 ) 
     Control section C 1  activates registration roller driving motor  192 M to drive paired registration rollers  192 , switch-back roller driving motor  135 M to drive paired conveyance rollers  135  which can move both forward and backward, and conveyance roller driving motor  193 M to drive paired conveyance rollers  193 , in a backward direction, whereby primary sheet P 1  is conveyed in the backward direction (that is, primary sheet P 1  is switch-backed). After primary sheet P 1  is conveyed in the backward direction for a predetermined first length (being shifted amount L), primary sheet P 1  is stopped. 
     The above-described backward conveyance for length L of primary sheet P 1  is conducted based on the counts counted by the counter about the conveyance length from second sheet sensor S 2 . 
     8. Step S 308  is conducted in the same manner as above-described step S 109 , so that explanation can be omitted. 
     9. Steps S 350 -S 351  are conducted in the same manner as above-described steps S 151  and S 152 , so that explanation can be omitted. 
     10. Monitoring of second sheet sensor S 2  (step S 352 ) 
     Control section C 1  monitors second sheet sensor S 2  which detects the leading edge of secondary sheet P 2 . If second sheet sensor S 2  detects the leading edge of secondary sheet P 2  (Yes in step S 352 ), the operational flow goes to step S 353 , while if not (No in step S 352 ), control section C 1  repeats step S 352 . 
     11. Stopping conveyance of secondary sheet P 2  (step S 353 ) 
     Control section C 1  deactivates registration roller driving motor  192 M, and conveyance roller driving motor  136 M which drives paired conveyance rollers  136 , whereby conveyance of secondary sheet P 2  is stopped, and the operation flow goes to step S 354 . 
     12. Ejection of the sheet bundle (step  354 ) 
     The sheet bundle, which includes superimposed primary sheet P 1  and secondary sheet P 2 , is ejected to the upper space of slanting stacker  140 , which is conducted in the same way as described in step S 155 . 
     In addition, it is also possible to structure the operation in such a way that, the conveyance of secondary sheet P 2  is not stopped in step S 353 , and after the sheet bundle, including superimposed primary sheet P 1  and secondary sheet P 2 , is nipped between paired conveyance rollers  193  in step S 354 , said sheet bundle is then ejected toward the upper space of slanting stacker  140 . In this structure, since the conveyance of secondary sheet P 2  is not stopped, operational time is decreased, which is preferable. 
     Further, in the above-described first and second embodiments, two sheets are superimposed. However, for example, if the number of sheet conveyance routes in accumulating section  130  is increased, three or more sheets can be superimposed. In this occasion, control section C 1  controls the accumulating section  130  and the stacker  140  in such a way that the secondary sheet superimpose on the primary sheet and a third sheet superimpose on the secondary sheet, when said three sheets are to be stacked in the stacker  140 , the leading section of the secondary sheet is more advanced than the leading section of the primary sheet, and a leading section of the third sheet is more advanced than the leading section of the secondary sheet, in the accumulating section  130 .