Patent Publication Number: US-6209861-B1

Title: Sheet transport device and automatic document feeder

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a sheet transport device for use in various image reading/forming apparatuses such as copiers, scanners, facsimiles, and printers for transporting sheets such as documents and copy sheets to a specified image reading/forming position successively to read original document images and/or form an image on the copy sheets, successively. 
     In the above image forming apparatus such as copiers, a sheet inverting mechanism for turning the side of a document and a copy sheet (hereinafter, also simply referred to as “sheet,” as a general term for document and copy sheet,) is required in a document feeder/sheet transport device in order to read an image on the opposite sides of the document or form an image on opposite sides of the copy sheet. 
     Heretofore, it has been a general practice to adopt a switchback mechanism in the document feeder/sheet transport device wherein the sheet transport direction is inverted in such a manner that the lead end of the sheet in the sheet entering direction into a switchback route becomes the tail end thereof in the sheet exiting direction in order to turn the side of the sheet. 
     For instance, Japanese Unexamined Patent Publication No. HEI 7-64359 discloses an automatic document feeder for feeding a document to an image reading position for double side image reading. The device of this publication is operated such that: a document is fed from a document tray to the specified image reading position for one side image reading; temporarily discharged onto a document discharge tray after the one side image reading; transported in the opposite direction opposite to the document discharge direction onto the document discharge tray by a switchback mechanism; and then fed to the image reading position again to read an image on the opposite side of the document. 
     Similar to the above automatic document feeder, there has been known a sheet transport device for use in an image forming apparatus such as copiers, that uses a switchback mechanism to transport a copy sheet to a specified image forming position for double side image formation. In such a sheet transport device provided with the switchback mechanism, similar to the document feeder, the sheet transport direction is inverted by the switchback mechanism after one side image formation to flip over the side of the sheet for the opposite side image formation. 
     In the switchback mechanism used in the automatic document feeder/sheet transport device, a sheet inverting roller pair for activating a switchback operation is provided near an entrance of a switchback route (the document discharge tray in the prior art device) to invert the sheet transport direction. Specifically, the sheet inverting roller pair is operated such that rotating the sheet inverting roller pair in the forward direction temporarily retracts the sheet into the switchback route from a main transport route, and rotating the sheet inverting roller pair in the reverse direction at a specified timing when the roller pair nips the tail end of the sheet in the entering direction into the switchback route inverts the sheet transport direction to return the sheet from the switchback route to the main transport route. 
     In the above switchback mechanism, the document discharge tray functioning as the switchback route is not provided with a guide means for correcting a skew discharge of a document. Specifically, the lead end of the document on the way of a switchback operation with respect to the entering direction is left free without the guide means with the result that a skew transport of the document is liable to occur. Thereby, it is highly likely that the document (copy sheet) is transported to the image reading (forming) position in a skew transport state, resulting in a poor image reading (formation) such that an image is read (formed) in the skew transport state. 
     SUMMARY OF THE INVENTION 
     In view thereof, it is an object of the present invention to provide a sheet transport device that securely prevents a skew transport of a sheet during a switchback operation thereof. 
     To fulfill the above object, a sheet transport device, according to an aspect of the invention comprises: a sheet setting portion for setting a sheet; a pair of sheet guides adapted in contact with respective lateral opposite ends of the sheet set on the sheet setting portion to prevent a skew transport of the sheet; a main transport route for transporting the sheet in a sheet transport direction from the sheet setting portion to an image reading position to enable an image reading of the sheet or an image forming position to enable an image formation on the sheet; a switchback route intersecting at a specified position of the main transport route and extending upward therefrom to guide a lead end of the sheet between the pair of sheet guides; and a switchback mechanism for guiding the sheet from the main transport route into the switchback route and returning the sheet into the main transport route by inverting a tail end and a lead end of the sheet along the sheet transport direction. 
     In the above arrangement, the sheet entering the switchback route for the switchback operation is guided inward of the pair of sheet guides provided on the sheet setting portion from the switchback route. The inward guiding of the sheet by the pair of sheet guides securely prevents a skew transport state of the sheet during the switchback operation. 
     These and other objects, features and advantages of the present invention will become more apparent upon reading of the following detailed description and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic diagram showing essential parts of an automatic document feeder embodying the present invention; 
     FIG. 1B is an enlarged view of the essential part of the automatic document feeder shown in FIG. 1A; 
     FIG. 2 is a diagram showing a transport control of a set of documents when the documents, stacked in a manner that the page number increases as going down, are successively fed from the uppermost one for double side image reading; 
     FIG. 3 is a rear view of a document setting portion viewed from rearward of the document feeder; 
     FIG. 4 is a perspective view of essential parts of the document setting portion including a pair of document guides; 
     FIG. 5 is a schematic diagram of the document feeder showing a state that a document is being temporarily discharged on the document setting portion during a switchback operation thereof; 
     FIG. 6 is an enlarged side view of a lower transport roller pair and peripheral parts thereof in the document feeder; 
     FIG. 7 is a perspective view of a driven roller of the lower transport roller viewed from the side of a drive roller (drive roller is omitted); 
     FIG. 8 is an enlarged cross-sectional view of a boundary area between the driven roller of the lower transport roller pair and a transport plane for transporting the document in the document feeder; 
     FIG. 9 is an enlarged cross-sectional view of the boundary area between the driven roller of the lower transport roller pair and the transport plane for transporting the document in the document feeder as an altered arrangement; 
     FIG. 10 is an enlarged cross-sectional view of the boundary area between the driven roller of the lower transport roller pair and the transport plane for transporting the document in the document feeder as another altered arrangement; 
     FIG. 11 is a cross-sectional side view of a roller pair and peripheral parts thereof in a conventional device; 
     FIG. 12 is a partially cross-sectional perspective view showing a junction area between a U-shape transport section (main transport route) and a switchback section; 
     FIG. 13 is a block diagram of a control system as an embodiment in the automatic document feeder; 
     FIGS. 14 and 15 are diagrams showing a sequence of a document transport control by the automatic document feeder for double side image reading; 
     FIGS. 16A to  16 C are diagrams showing a manner of transporting a document in the switchback section for a switchback operation; 
     FIG. 17 is a perspective view of essential parts of the document setting portion when a document during a switchback operation is temporarily discharged above the remaining set of documents on the document setting portion; and 
     FIG. 18 is a diagram showing a state that a lead end of a document in the exiting direction out of the switchback section after a switchback operation is nipped by an upper transport roller pair. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An automatic document feeder for use in a copier is described as an embodiment of a sheet transport device according to this invention. 
     It should be noted that the present invention is not limited to an automatic document feeder for successively feeding a set of documents for image reading, but is also applicable to a sheet transport device provided with a switchback mechanism, e.g., a sheet transport device for use in image forming apparatuses such as copiers, printers, and facsimiles for forming an image on a copy sheet. 
     FIG. 1 is a diagram showing essential parts of an automatic document feeder embodying this invention. 
     In FIG. 1, the automatic document feeder  1  includes a document setting portion  2  for setting a set of documents and a document discharge portion  3  for discharging the document set after image reading. The document setting portion  2  and the document discharge portion  3  are set in a vertically spaced relation. A document transport unit  4  for transporting a document for image reading is provided at a front end (left side in FIG. 1) of the document setting portion  2  and the document discharge portion  3 . 
     The document transport unit  4  includes a U-shape transport section  41  (or main transport route) for transporting a document from the document setting portion  2  up to the document discharge portion  3  substantially along a U-shape track for one side image reading/single side image reading, a switchback section  42  for inverting the sheet transport direction by a switchback operation for double side image reading, and a feedback section  43  for returning a document after one side image reading toward the switchback section  42  for turning the side of the document (for the opposite side image reading). 
     The automatic document feeder  1  is operated such that combining the U-shape transport section  41 , the switchback section  42 , and the feedback section  43  according to needs enables double side image reading (hereinafter, simply referred to as “DS image reading”) as well as single side image reading (hereinafter, simply referred to as “SS image reading”). 
     The automatic document feeder  1  (hereinafter, also referred to as “ADF  1 ”) adopts a system in which a set of documents stacked on the document setting portion  2  are automatically fed one by one from the uppermost one (hereinafter, referred to as “up-to-down-feed-in system”). 
     FIG. 2 is a diagram schematically showing a procedure of feeding a document for DS image reading by the ADF  1 . In FIG. 2, the side of the bold straight line of the document P corresponds to the opposite side (even number page) of the document P, and the side carrying the mark Δ corresponds to one side (odd number page) of the document P. In this embodiment, a set of documents are stacked on the document setting portion  2  in a state that the first sheet P 1  is placed at the uppermost position, and the last sheet P n  is placed at the lowermost position, as shown in FIG. 2, and are fed according to up-to-down-feed-in system. 
     Referring to FIG. 2, the ADF  1  is operated such that the uppermost document P 1  on the document setting portion  2  is fed to the switchback section  42  for turning the side of the document P 1  and transported to an image reader unit  6  to read image data on the opposite side of the document P 1 . The document P 1 , after the opposite side image reading, is fed to the switchback section  42  again via the feedback section  43 . In the switchback section  42 , the transport direction of the document P 1  is inverted and transported to the image reader unit  6  to read image data on one side of the document P 1 . After the one side image reading, the document P 1  is discharged onto the document discharge portion  3  in a state that the one side thereof faces downward. In this way, the set of documents P are transported according to the above procedure one after another for DS image reading and discharged onto the document discharge portion  3  in a state that the page order is identical to the initially stacked state on the document setting portion  2  except that the document P 1  is placed at the lowermost position and the document P n  is placed at the uppermost position on the document discharge portion  3 . 
     In the case of SS image reading, a set of documents P are transported along the U-shape transport section  41  and discharged onto the document discharge portion  3  in the same page order as set on the document setting portion  2  except that the one side of the documents faces downward on the document discharge portion  3  while facing upward on the document setting portion  2 . Accordingly, similar to the DS image reading, the set of documents P are discharged onto the document discharge portion  3  in the same page order as stacked on the document setting portion  2  after the SS image reading. 
     Referring back to FIG. 1, the arrangement of various parts constituting the ADF  1  is described in the following section. 
     The document setting portion  2  includes a document tray  201  made of a resin. The document tray  201  is detachably mounted on a main body of the ADF  1  through a hollow portion (including a document feed-in port  101  and an invert access port  102 , see FIG. 4) in a state that a tail end thereof in the document feed-in direction (right side in FIG. 1) is tilted upward. A pair of document guides (or sheet guides)  202  are set in an upright posture at a bottom surface of the document tray  201 . The document guides  202  are adapted for preventing a skew transport of the document during a switchback operation on the document tray  201  when feeding the document in and out through the invert access port  102  (see FIG. 4) while aligning the width of the document on the document tray  201 , which is described below in detail. As shown in FIGS. 3 and 4, the pair of document guides  202  are set slidable toward and away from each other in a symmetrical relation relative to a center line L on the document tray  201  to adjust the distance therebetween in accordance with various sizes of document. 
     Each of the document guides  202  has a rectangular plate-like shape and. is formed with a tapered portion  202   a  at an upper end thereof. The tapered portion  202   a  is bent outward at such an angle that the distance between the document guides  202  increases as directing toward the tip thereof. The tapered portion  202   a  is adapted for guiding a lead end of the document P temporarily discharged on the document tray  201  through the invert access port  102  during a switchback operation, inward of the opposing document guides  202  above the remaining set of documents P stacked on the document setting portion  2 , which is described below. 
     Guiding the document P temporarily discharged on the document tray  201  during the switchback operation inward of the pair of document guides  202  enables correcting a skew feed-out (discharge) operation of the document P onto the document tray  201  as well as a skew feed-in operation of the document P into the switchback section  42  while aligning the width of the document P by the document guides  202 . 
     Specifically, in the embodiment, the document guides  202  function as a guide means for correcting a skew feed-in/feed-out operation of the document P onto the document setting portion  2  during the switchback operation, as well as a guide means for preventing a skew feed-in operation of the document P set on the document setting portion  2 . This arrangement eliminates providing an additional member (guide means) for correcting a skew transport of the document P during the switchback operation. Accordingly, the number of guide means projecting upward from the upper surface of the ADF  1  for correcting a skew transport is reduced to avoid an undesired state that the protruding guide means would impair the external appearance of the apparatus (e.g., copier) as a whole. 
     Sliding the opposing document guides  202  toward and away from each other to adjust the distance therebetween in accordance with the size of the document P when the document P is set on the document setting portion  2  automatically secures the distance between the document guides  202  suitable for preventing a skew feed-in/feed-out operation during the switchback operation, as well as aligning the width of the document P to be fed into the document feed-in port  101 . Thereby, the operability of the device (copier) is improved. 
     Projecting pieces  202   b  in the bar shape (see FIGS. 3 and 4) are provided on an inner surface at a front side of the respective document guides  202 . The projecting pieces  202   b , as shown in FIG. 5, securely support the document P from the bottom on the way of switchback operation to keep the lead end of the document P sufficiently away from the document feed-in port  101  on the document tray  201 . 
     Projecting pieces  202   b  function as a partition that divides a lower sheet transport path between the document tray  201  and the document feed-in port  101  and an upper sheet transport path between the document tray  201  and the invert access port  102  so that the two paths do not interfere with each other. 
     Securely supporting the document P on the way of switchback operation by the projecting pieces  202   b  in vertically spaced relation from the document feed-in port  101  prevents a possibility that the lead end of the document P coming onto the document tray  201  (rightward direction in FIG. 5) during the switchback operation droops downward, curls into the document feed-in port  101  and is stuck therein, which may cause a jam of the document P. Also, this arrangement prevents a possibility that the remaining set of documents stacked on the document setting portion  2  may interfere with the feed-in/feed-out operation of the document P under the switchback operation. 
     The projecting piece  202   b  functions as a restricting means for preventing droop of the document P on the way of the switchback operation onto the document tray  201 . The restricting means may have any shape besides the bar-shape, as far as it accomplishes the function as the restricting means. Further, the projecting piece  202   b  may be set at any height level as long as it accomplishes the function as the restricting means. Also, the projecting pieces  202   b  may function as a document number determinator for determining the maximum number of documents settable on the document tray  201  by setting the projecting pieces  202   b  at such a height level. 
     A skew correction of the document P on the way of switchback operation and countermeasures for preventing an interference of the switchback operation of the document P by the remaining set of documents are described below in detail combined with operations of the ADF  1 . 
     Referring back to FIG. 1, a sheet separator/feeder unit  21  is provided at the lead end of the document setting portion  2  in the document feed-in direction. The sheet separator/feeder unit  21  includes a document sensor S 1 , a feed roller  211 , a separation roller  212 , and a forward roller  213 . 
     The document sensor S 1  detects the presence or absence of document P on the document tray  201 , and is provided at a bottom portion at the lead end of the document tray  201 . The forward roller  213  is adapted for feeding the document P (uppermost document) in contact therewith in the forward direction utilizing a frictional force between the forward roller  213  and the document P, and is provided at an upper position at the lead end of the document tray  201 . The forward roller  213  is selectively seftable at an upper position and a lower position by a sheet feed solenoid (not shown). The feed roller  211  and the separation roller (driven roller)  212  feed the uppermost document P in contact with the forward roller  213  forward toward the document trans port unit  4  by driving the feed roller  211 . The feed roller  211  and the separation roller  212  are provided at the lead end of the document tray  201 . 
     The feed roller  211  and the forward roller  213  are designed to rotate in clockwise direction in FIG. 1 in such a manner that a rotating drive force of a feed motor (not shown) is transmitted to the feed roller  211  and the forward roller  213  via a feed clutch (not shown) to feed the uppermost document P on the document tray  201  toward the document transport unit  4 . The separation roller  212  is rendered in sliding contact state with the feed roller  211  and is driven followed by the rotation of the feed roller  211  while causing a rotational friction between the feed roller  211  and the separation roller  212 . Due to the frictional contact, the uppermost document P on the document tray  201  is separated from the remaining set of documents and fed forward toward the document transport unit  4 . 
     The document transport unit  4  includes the U-shape transport section  41  consisting of a first transport route  411 , a second transport route  412 , a third transport route  413 , a fourth transport route  414 , and a fifth transport route  415  which jointly form a substantially U-shape, a feed sensor S 2 , a timing sensor S 3 , a pair of registration rollers  416 , an upper transport roller pair  417 , a lower transport roller pair  418 , an intermediate roller pair  419 , and a discharge roller pair  420 . The U-shape transport section  41  consisting of the first to fifth transport routes  411  to  415  constitutes a main transport route of the document feeder according to this invention. 
     The first transport route  411  and the second transport route  412  extend downstream of the feed roller  211  and the separation roller  212  substantially in a straight shape. The third transport route  413  is curved into C-shape and extends downward from the exit of the second transport route  412 . The fourth transport route  414  is so constructed as to extend from the exit of the third transport route  413  and pass an image reading position P 3  located below the second transport route  412 . The fifth transport route  415  extends from the exit of the fourth transport route  414  toward the document discharge portion  3 . 
     A plurality of roller pairs for transporting the document P according to the transport procedure of the ADF  1  are provided at respective junctions of the first to fifth transport routes  411  to  415 . Specifically, the registration roller pair  416  is provided at the junction between the first transport route  411  and the second transport route  412 , the upper transport roller pair  417  is provided at the junction between the second transport route  412  and the third transport route  413 , the lower transport roller pair  418  is provided at the junction between the third transport route  413  and the fourth transport route  414 , and the intermediate roller pair  419  is provided at the junction between the fourth transport route  414  and the fifth transport route  415 . The discharge roller pair  420  is provided at the exit of the fifth transport route  415 . 
     The feed sensor S 2  is provided at a base end of the first transport route  411  to control operation timings of various parts by detecting the position of the document being transported inside the ADF  1 . The timing sensor S 3  is provided at a base end of the fourth transport route  414 . 
     The feed sensor S 2  detects whether the document P is fed out from the document setting portion  2 . A detection signal outputted from the feed sensor S 2  is used to control drive start timing of the registration roller pair  416 . 
     The registration roller pair  416  is adapted for correcting a skew transport of the document P fed out from the document setting portion  2 , and is driven in such a direction as to transport the document P downstream by drive of the feed motor via a registration clutch (not shown). 
     The upper transport roller pair  417  and the lower transport roller pair  418  are driven in such a direction as to transport the document P to the image reading position P 3 . The upper transport roller pair  417  and the lower transport roller pair  418  are designed to rotate in such a direction as to transport the document P downstream by operation of a transport motor (not shown). 
     The intermediate roller pair  419  is designed to changeably discharge the document P to the document discharge portion  3  or return the document P into the first transport route  411  via the feedback section  43 , and is driven in such a direction as to transport the document P downstream by operation of the transport motor. 
     The discharge roller pair  420  discharges the document P after the image reading onto the document discharge portion  3 , and is designed to rotate in such a direction as to discharge the document P onto the document discharge portion  3  by operation of the transport motor. 
     The upper transport roller pair  417  functions as a registration roller pair (corrector for correcting a skew transport of the document P after a switchback operation) for correcting a skew transport of the document P by driving the upper transport roller pair  417  at a certain timing, which is described below, when the document P exits from the switchback section  42  after the switchback operation. 
     The lower transport roller pair  418  is provided at such a position as to nip the document P near a transport plane constituting the third transport route  413 , on the side where a driven roller  418   b  of the lower transport roller pair  418  is provided, so as to prevent occurrence of a document jam around the lower transport roller pair  418 . 
     Hereinafter, the location of the lower transport roller pair  418  is described with reference to FIGS. 6 to  11 . 
     FIG. 6 is an enlarged side view of the lower transport roller pair  418  consisting of a drive roller  418   a  and the driven roller  418   b  and the vicinity thereof, FIG. 7 is a perspective view of the driven roller  418   b  viewed from the side of the drive roller  418   a  (drive roller  418   a  is omitted in FIG.  7 ), FIGS. 8 to  10  are enlarged cross-sectional views of a boundary area between the driven roller  418   b  and the transport plane  48   a  on the side of the driven roller  418   b , and FIG. 11 is a cross-sectional side view showing an example of the arrangement of a conventional roller pair. 
     As shown in FIG. 6, the drive roller  418   a  of the lower transport roller pair  418  is constructed such that a part of the drive roller  418   a  projects inward of the third transport route  413  and the fourth transport route  414  from a transport plane  47   a  of a guide plate constituting the third transport route  413  and the fourth transport route  414 . On the other hand, the driven roller  418   b  is constructed such that a part of the outer circumference thereof is set substantially on the same plane as the transport plane  48   a  of the guide plate  48  constituting the third transport route  413  and the fourth transport route  414 . In this arrangement, a nip portion NP defined by the drive roller  418   a  and the driven roller  418   b  for nipping the document P is located on the transport plane  48   a.    
     The above arrangement of the lower transport roller pair  418  is adopted in this embodiment for the following reason. If a transport roller pair consisting of a drive roller  81   a  and a driven roller  81   b , as shown in prior art arrangement of FIG. 11, is arranged in such a manner that the driven roller  81   b  bulges inwardly into a transport route  83  from a transport plane  82   a  on the side of the driven roller  81   b , it is difficult to nip a sheet or a document having less flexibility (e.g., rigid copy sheet) or a sheet or a document with a lead end curled toward the side of the driven roller  81   b  (e.g., an outwardly curled sheet P shown in FIG. 11) between the drive roller  81   a  and the driven roller  81   b  due to abutment of the lead end of the sheet P against a boundary portion between the drive roller  81   a  and the driven roller  81   b , which may likely lead to a sheet transport failure or a sheet transport jam. The less the curvature of radius R of the transport route  83  is, the more the above drawback is likely to happen. 
     In the case where the drive roller  81   a  and the driven roller  81   b  function as a registration roller pair, it is highly likely that the lead end of the sheet P is abutted against the boundary portion due to the property (state) of the surface of the driven roller  81   b  which is made of a slippery material such as polyacetal (POM) and metal. Accordingly, there rises a high risk of occurrence of jam. 
     In this embodiment, the above arrangement is adopted to solve the above problem. The above arrangement is advantageous in the following point. Even if a document with less flexibility or a document with a lead end thereof curled greatly toward the driven roller  418   b  (e.g., an outwardly curled document P in FIG. 6) is transported toward the lower transport roller pair  418 , the document P is securely guided along the nip portion NP set at the substantially same plane as the transport plane  48   a . Thereby, the document P is securely and smoothly transported toward the image reading position P 3 . 
     In this embodiment, the nip portion NP is set at the substantially same plane as the transport plane  48   a  on the side of the driven roller  48   b . Alternatively, the nip portion NP may be set inward of the third transport route  413  and the fourth transport route  414 . As far as the nip portion NP is set in the vicinity of the transport plane  48   a , the same effect as mentioned in the embodiment is obtainable. 
     Specifically, as a result of various experiments conducted by the applicant of this application, the following was confirmed. In the case of transporting a sheet of plain paper copy (PPC) or its equivalent, it was confirmed that the above effect is obtained as far as the bulging amount of the driven roller  418   b  projecting inward the third transport route  413  and the fourth transport route  414  is set within the range of 0 to 0.8 mm from the transport plane  48   a . In other words, when the nip portion NP between the drive roller  418   a  and the driven roller  418   b  is set flush with or in the vicinity of the transport plane  48   a , a sheet with less flexibility or with a curled lead end as well as PPC sheet is securely nipped between the lower transport roller pair  418  and smoothly transported along the third transport route  413  and the fourth transport route  414 . 
     In order to transport the document P in contact with the nipped portion NP along the transport plane  48   a , it is necessary to form an opening  48   b  in the transport plane  48   a  of the guide plate  48  for exposing part of the driven roller  418   b  corresponding to the nip portion NP inward of the third transport route  413  and the fourth transport route  414 . When forming the opening  48   b , it is essential to set the relation between the width WH of the opening  48   b  in the rotating axial direction of the driven roller  418   b  (in the width direction of the document P) and the width WR of the driven roller  418   b  in the axial direction thereof at a proper value. 
     The reason for setting the relation is as follows. It has been a common practice to set the width WH of the opening  48   b  equal to or slightly wider than the width of the driven roller  418   b . According to this conventional arrangement, it is likely that a wrinkle or a shriveling portion is formed on a boundary portion of a sheet between a nipped portion and an un-nipped portion during transport thereof due to the following reason. In case that the clearance is set small and the projection amount of the driven roller  418   b  inward to the transport route is almost none, it is likely that the boundary portion of the sheet is pressed hard against the lateral edge of the opening. 
     To solve the above problem, in this embodiment, as shown in FIGS. 7 and 8, the width WH of the opening  48   b  is set sufficiently larger than the width WR of the driven roller  418   b  such that, e.g., the clearance between one axial end of the driven roller  418   b  and the corresponding rim of the opening  48   b  in the axial direction of the driven roller  418   b  is set as wide as 2 to 3 mm. 
     As an altered arrangement, an end portion  48   c  of the opening  48   b  opposing to the driven roller  418   b  is shaped into a curve (i.e., a sharp edge is rounded off), as shown in FIG. 9, such that the width WH of the opening  48   b  increases as directing inward of the third transport route  413  and the fourth transport route  414  in the length direction (axial direction) of the driven roller  418   b . Further, the end portion  48   c  may be shaped into a taper as shown in FIG.  10 . The se arrangement make it possible to more reliably solve the above problem. 
     In this embodiment, the above arrangement for preventing jam of the document P was described taking the example of the lower transport roller pair  418  in the ADF  1 . However, since the ADF  1  is provided with a plurality of transport roller pairs for transporting the document P while nipping the document P therebetween having the same mechanism as the lower transport roller pair  418 , it is clear that the above arrangement is applicable to any roller pair such as the registration roller pair  416  and the upper transport roller pair  417  and the same effect as mentioned in the embodiment is obtained. 
     In the embodiment, the above arrangement was described for the lower transport roller pair  418  provided on the curved third and fourth transport routes  413 ,  414 . It is true to say that the same effect as mentioned in the embodiment is obtained for the roller pair provided on a straight transport route. 
     Referring back to FIG. 1, a contact glass  7  is provided underneath the image reading position P 3  of the fourth transport route  414 . The image reader unit  6 , provided with an image pick-up sensor such as a CCD line sensor, is provided below the contact glass  7 . In the image reader unit  6 , the image of the document P is read in synchronism with a detection timing of the document P by the timing sensor S 3 . 
     Next, the arrangement of the switchback section  42  is described. 
     The switchback section  42  extends upward from the U-shape transport section  41 . The switchback section  42  is adapted for guiding the document P therein from the U-shape transport section  41  for a switchback operation and returning the document P to the U-shape transport section  41  to turn the side of the document P for image reading outward of the first to fifth transport routes  411  to  415 , namely, opposite to the image reader unit  6 . 
     The switchback section  42  includes a sixth transport route  421 , a seventh transport route  422 , an eighth transport route  423 , an inverting switch lever  51 , an inverting sensor S 4 , a transport direction regulating plate  53 , an inverting roller  424  and an inverting driven roller  425 . 
     The sixth transport route  421  intersects the second transport route  412  at a base end (entrance) thereof and extends obliquely upward therefrom to join the seventh transport route  422 . The eighth transport route  423  intersects a junction between the sixth transport route  421  and the seventh transport route  422 , extends obliquely downward therefrom, and joins the exit of the second transport route  412  connected to the third transport route  413 . 
     The seventh transport route  422  is curved toward the document setting portion  2  from the junction between the sixth transport route  421  and the eighth transport route  423  and further extends substantially horizontally toward a certain position above the document setting portion  2 . The seventh transport route  422  is adapted for temporarily retracting the document P transported along the U-shape transport section  41  via the sixth transport route  421 , inverts the transport direction therealong, and returns the document P to the U-shape transport section  41  via the eighth transport route  423 . The seventh transport route  422  is hereinafter also referred to as “switchback route  422 ”. 
     A switchback cover  49  (or cover  49 ) for covering an upper portion of the switchback route  422  is provided on the main body of the ADF  1 . The cover  49  prevents the document P coming into the switchback route  422  for a switchback operation from interfering with an obstacle, such as a book, that is left at the upper portion of the ADF  1 . Provision of the cover  49  ensures the switchback operation without the possibility of such interference. The cover  49  substantially extends from the junction of the main transport route  41  and the switchback section  42  up to the certain position above the document setting portion  2  to define the invert access port  102  together with the switchback route  422  to guidingly discharge the lead end of the document P in the entering direction into the switchback route  422  for the switchback operation onto the document setting portion  2 . 
     As shown in FIG. 12, the inverting switch lever  51  is provided at the junction between the second transport route  412  and the sixth transport route  421  to selectively guide the document P transported along the first transport route  411  to the second transport route  412  and the sixth transport route  421 . The inverting switch lever  51  is constructed, as shown in FIG. 12, such that a plurality of ribs  511  are formed at a certain interval in an axial direction of a lever shaft  514 . Rotating the lever shaft  514  in a certain direction by an unillustrated inverting solenoid changeably sets the position of the ribs  511  between the solid-line position (invert non-activating position) and the imaginary-line position (invert activating position). 
     Specifically, when the ribs  511  are set at the solid-line position, respective lead ends of the ribs  511  are fitted into recesses  512  formed in the junction area between the second transport route  412  and the sixth transport route  421  (specifically in a lower wall constituting the sixth transport route  421 ) to thereby guide the document P to the second transport route  412 . On the other hand, when the ribs  511  are set at the imaginary-line position, the respective lead ends of the ribs  511  are fitted in recesses  513  formed in the junction area (specifically, in an upper wall constituting the second transport route  412 ) to thereby guide the document P to the sixth transport route  421 . 
     The ribs  511  of the inverting switch lever  51  at a substantially intermediate portion in the axial direction (X direction) of the lever shaft  514  are disposed at a relatively wide interval. A detecting piece of the inverting sensor S 4  (junction sensor S 4 ) projects upward through an opening formed in the intermediate portion of the junction area. Specifically, the height level of the detecting piece of the inverting sensor S 4  substantially coincides with the height level of the ribs  511  of the inverting switch lever  51  with respect to the width direction of the document P (X direction). With this arrangement, when the document P is transported to the junction area from the first transport route  411 , the detecting piece of the inverting switch lever  51  is set to the imaginary-line position in FIG. 12 without interference with of the ribs  511 . Then, the inverting sensor S 4  detects that the document P is coming into the sixth transport route  421  for a switchback operation. The detection of the document P by the inverting sensor S 4  combined with the position of the inverting switch lever  51  is described below in detail. 
     In this embodiment, the junction sensor S 4  (inverting sensor S 4 ) provided in the junction area between the second transport route  412  and the sixth transport route  421  functions as a sensor for detecting that the document P is being transported to the second transport route  412  and a sensor for detecting that the document P is being transported to the sixth transport route  421 . Accordingly, the number of sensors is reduced to simplify the arrangement of the device, reduce the size of the device, and lower the production cost of the device including parts cost. 
     In this embodiment, the junction sensor S 4  consists of a contact-type sensor. Alternatively, any known sensor, e. g., a non-contact type sensor such as a photosensor can be used as the sensors S 1  to S 4 . 
     The transport direction regulating plate  53  is formed of a synthetic resin having elasticity, and is so constructed as to transport the document P in and out of the seventh transport route  422  (switchback route  422 ) without difficulty. 
     Specifically, the transport direction regulating plate  53  is provided at the junction joining the sixth transport route  421 , the seventh transport route  422  and the eighth transport route  423  with such a shape that the width thereof in the X direction decreases toward the exit of the sixth transport route  421  connected to the seventh transport route  422  and increases toward the entrance of the eighth transport route  423  from the seventh transport route  422 . In this way, since the transport direction regulating plate  53  has such a shape as to widen the width of the entrance of the eighth transport route  423 , the document P is securely guided from the sixth transport route  421  into the seventh transport route  422  and exits out of the seventh transport route  422  into the eighth transport route  423 . 
     Referring back to FIG. 1, the inverting roller  424  and the inverting driven roller  425  are disposed at the entrance of the switchback route  422 , namely at the junction connecting the sixth transport route  421 , the switchback route  422 , and the eighth transport route  423 . The inverting roller pair  424 ,  425  are adapted for guiding the document P into the switchback route  422  from the U-shape transport section  41  via the sixth transport route  421 , and returning the document P from the switchback route  422  into the U-shape transport section  41  via the eighth transport route  423 , thereby constructing a switchback mechanism for performing a switchback operation of the document P. 
     Specifically, the inverting roller  424  is driven by direct transmission of the rotating force of the feed motor without the feed clutch. With this arrangement, when the rotating direction of the inverting roller  424  is reversed, the transport direction of the document P is inverted. 
     The inverting driven roller  425  is switchingly settable at the imaginary-line position in FIG. 1 (pressing contact position) where the driven roller  425  is rendered into pressing contact with the inverting roller  424  and the solid-line position in FIG. 1 (release position) where the driven roller  425  is set away from the inverting roller  424  with a certain clearance by driving an unillustrated pressing solenoid. 
     When the driven roller  425  is set at the pressing contact position, the inverting roller pair  424 ,  425  are enabled to nip the document P to transport the document P downstream followed by rotation of the inverting roller  424 . On the other hand, when the driven roller  425  is set at the release position, the nipped state of the document P is released. As a result, a skew correction of the document P during the switchback operation, which is described below, is enabled. Further, even when the switchback operation is performed for a document with the longer side thereof transported in the transport direction, the tail end of the document exiting out of the switchback route  422  and the lead end of a next document coming into the switchback route  422  can pass through the inverting roller pair  424 ,  425  in an overlapped manner. 
     In this embodiment, the switchback route  422  is provided above the U-shape transport section  41 . Accordingly, the upper space of the ADF  1  is used as a space for temporarily holding the document P for switchback operation. Thereby, production of the device  1  of a smaller size is enabled. 
     The feedback section  43  is adapted for returning the document P after one side image reading to the junction between the first transport route  411  and the second transport route  412  via the fifth transport route  415  of the U shape transport section  41  for DS image reading, and includes a ninth transport route  431  and a discharge switch lever  52 . 
     The ninth transport route  431  intersects at a certain position of the fifth transport route  415 , extends upward in a curved shape from the intersecting position and joins the junction of the first transport route  411  and the second transport route  412 . 
     The discharge switch lever  52  is provided at the junction between the fifth transport route  415  and the ninth transport route  431 , and is selectively set at the solid-line position in FIG. 1 (feedback non-activating position) to open the transport route for the document P toward the discharge roller pair  420  and the imaginary-line position in FIG. 1 (feedback activating position) to open the transport route toward the ninth transport route  431  by activating an unillustrated discharge solenoid. 
     Next, a control system of the ADF  1  is described. 
     FIG. 13 is a block diagram showing an embodiment of the control system of the automatic document feeder  1 . In FIG. 13, elements of the control system corresponding to the parts shown in FIG. 1 are denoted at the same reference numerals. 
     A controller  8  centrally controls a document transport operation for reading an image of the document P, and includes, e.g., a microcomputer. The controller  8  controls operations of various drive elements such as the feed motor  9 , the transport motor  10 , the feed clutch  11 , the registration clutch  12 , the sheet feed solenoid  13 , the pressing solenoid  14 , the inverting solenoid  15 , and the discharge solenoid  16  based on designation from a user inputted through entry means  17  such as a switch and ten keys provided on an unillustrated operation panel and detection signals from various sensors such as the document sensor S 1 , the feed sensor S 2 , the timing sensor S 3 , and the inverting sensor S 4 . 
     The feed motor  9  is a drive source for driving the feed roller  211 , the forward roller  213 , the registration roller pair  416 , and the inverting roller  424 . Specifically, the feed motor  9  is a drive source for performing a switchback operation of the document P transported into the switchback route  422  as well as a drive source for feeding the document P stacked on the document setting portion  2  inside the main body of the ADF 1  toward the first transport route  411 . 
     The feed clutch  11  includes a one-way clutch which is turned on and off to transmit rotating drive force of the feed motor  9  to the feed roller  211  and the forward roller  213 . The feed clutch  11  is controlled to turn on while the feed motor  9  rotates in the forward direction, which is described below. With this arrangement, the feed roller  211  and the forward roller  213  rotate in the forward direction by the operation of the feed clutch  11 . 
     The registration clutch  12  also includes a one-way clutch to transmit rotating drive force of the feed motor  9  to the registration roller pair  416 . The registration clutch  12  is controlled to turn off while the feed motor  9  rotates in the reverse direction, which is described below. With this arrangement, the registration roller  416  rotates in the reverse direction by the operation of the feed clutch  11 . 
     The inverting roller  424  is driven by direct transmission of the rotating drive force of the feed motor  9  without a clutch or its equivalent. Accordingly, the inverting roller  424  is driven in the forward/reverse direction followed by the forward/reverse rotation of the feed motor  9 . 
     The transport motor  11  is a drive source for driving the upper transport roller pair  417 , the lower transport roller pair  418 , the intermediate roller pair  419 , and the discharge roller pair  420 . The upper transport roller pair  417 , the lower transport roller pair  418 , the intermediate roller pair  419 , and the discharge roller pair  420  are driven by direct transmission of the rotating drive force of the transport motor  10  without a clutch or its equivalent. The transport motor  10  is controlled to rotate in a predetermined direction. Accordingly, the upper transport roller pair  417 , the lower transport roller pair  418 , the intermediate roller pair  419 , and the discharge roller pair  420  either suspend its rotation or rotate in the predetermined direction. 
     The sheet feed solenoid  13  is a switch member for switching the position of the forward roller  213  between the upper position and the lower position (contact position). When the sheet feed solenoid  13  is, e.g., turned ON, the for ward roller  213  is set at the lower position, whereas when the sheet feed solenoid  13  is turned OFF, the forward roller  213  is set at the upper position. 
     The pressing solenoid  14  is a switch member for switching the position of the inverting driven roller  425  between the pressing contact position and the release position. When the pressing solenoid  14  is, e.g., turned ON, the driven roller  425  is set at the pressing contact position, whereas when the pressing solenoid  14  is turned OFF, the driven roller  425  is set at the release position. 
     The inverting solenoid  15  is a switch member for switching the position of the inverting switch lever  51  between the invert activating position and the invert non-activating position. When the inverting solenoid  15  is, e.g., turned ON, the inverting switch lever  51  is set at the invert activating position to open the transport route from the first transport route  411  to the sixth transport route  421  (i.e., close the transport route to the second transport route  412 ). On the other hand, when the inverting solenoid  15  is, e.g., turned OFF, the inverting switch lever  51  is set at the invert non-activating position to close the transport route from the first transport route  411  to the sixth transport route  421  (i.e., open the transport route to the second transport route  412 ). 
     The discharge solenoid  16  is a switch member for switching the position of the discharge switch lever  52  between the feedback activating position and the feedback non-activating position. When the discharge solenoid  16  is, e.g., turned ON, the discharge switch lever  52  is set at the feedback activating position to open the transport route from the fifth transport route  415  to the feedback section  43  (ninth transport route  431 ) (namely, close the transport route toward the discharge roller pair  420 ). On the other hand, when the discharge solenoid  16  is turned OFF, the discharge switch lever  52  is set at the feedback non-activating position to close the transport route to the feedback section  43  (namely, open the transport route toward the discharge roller pair  420 ). 
     The controller  8  stores data concerning switching of the position of the inverting switch lever  51  by the inverting solenoid  15 . Specifically, when the inverting sensor S 4  detects the presence of the document P while the inverting switch lever  51  is set at the invert activating position to open the transport route for the sixth transport route  421 , the detection result by the sensor S 4  indicates that the document P is being transported along the sixth transport route  421 . On the other hand, when the inverting sensor S 4  detects the presence of the document P while the inverting switch lever  51  is set at the invert non-activating position to open the transport route for the second transport route  412 , the detection result by the sensor S 4  indicates that the document P is being transported along the second transport route  412 . In other words, the controller  8  constitutes a detection mechanism that detects the location of the document P as to whether the document P is transported along the sixth transport route  421  or the second transport route  412  based on the data concerning the position of the inverting switch lever  51 . 
     Next, a document transport sequence of the ADF  1  is described with reference to FIGS. 14 and 15 when the ADF  1  is operated to transport the document P for DS image reading. 
     When the document sensor S 1  detects that the document P is set on the document setting portion  2  (YES in Step S 1 ), the count value n indicating the number of document transport is set at “0” (in Step S 3 ), and the feed clutch  11 , the sheet feed solenoid  13 , and the discharge solenoid  15  are turned ON (in Step S 5 ). Then, the feedback section  43  is connected to the U-shape transport section  41 , the forward roller  213  is set at the lower position to be rendered into contact with the document P, and the drive force of the feed motor  9  is transmitted to the feed roller  211  and the forward roller  213 . 
     Subsequently, the feed motor  9  rotates in the forward direction to initiate feeding of the uppermost document P of the document on the document setting portion  2  toward the document transport unit  4  (in Step S 7 ). When the feed sensor S 2  detects that the document P is transported to the document transport unit  4  (YES in Step S 9 ), the feed motor  9  suspends its rotation upon lapse of a predetermined time T 1  from the detection of the feed sensor S 2 , and the feed clutch  11  is turned OFF (in Steps S 11 , S 13 ). The predetermined time T 1  is a time duration necessary for temporarily forming a non-flat portion of the document P around the lead end thereof required for correcting a skew transport of the document P from a timing when the lead end of the document P is nipped by the registration roller pair  416 . 
     Transport of the document P is suspended for a predetermined time T 2  from the operation in Step S 13  in a state that the lead end of the document P is kept in a nipped state by the registration roller pair  416  (in Step S 15 ). The predetermined time T 2  is a time duration required for the registration roller pair  416  to securely hold nipping of the lead end of the document P to initiate a skew feed correction of the document P. 
     Subsequently, upon lapse of the predetermined time T 2 , the pressing solenoid  14  and the inverting solenoid  15  are turned ON (in Step S 17 ). At this time, the U-shape transport section  41  is connected to the switchback section  42 , and the inverting driven roller  425  is pressed against the inverting roller  424 . Then, the feed motor  9  rotates in the reverse direction (in Step S 19 ) to restart transport of the document P by the registration roller pair  416 . In this way, the document P having passed the registration roller pair  416  is transported to the switchback section  42  via the sixth transport route  421  by the inverting switch lever  51 . 
     Next, when the inverting sensor S 4  detects that the document P is transported to the sixth transport route  421  (YES in Step S 21 ), the feed motor  9  is turned ON upon lapse of a predetermined time T 3  from the detection timing of the document P by the inverting sensor S 4 , and the pressing solenoid  14  is turned ON (in Steps S 23 , S 25 ). The predetermined time T 3  is a time duration necessary for the document P to be transported to the switchback route  422  by the inverting roller pair  424 ,  425  until a certain position of a tail end of the document P in the entering direction into the switchback route  422  for a switchback operation is nipped by the inverting roller pair  424 ,  425 . 
     FIGS. 16A to  17  are diagrams showing a manner as to how the document P is transported into the switchback route  422 . As shown in FIGS. 16A to  16 C, the lead end of the document P in the entering direction into the switchback route  422  for a switchback operation is temporarily discharged onto the document setting portion  2  through the invert access port  102  formed at the exit of the switchback route  422 . At this time, as shown in FIG. 17, the lead end of the document P is securely guided inward of the document guides  202  while guided by the tapered portions  202   a  of the document guides  202 . 
     At this time, the lead end of the document P is securely guided on the projecting pieces  202   b  formed on the inner surface of the document guides  202 . Thereby, prevented is a possibility that the lead end of the document P is intruded inside the main body of the ADF 1  through the document feed-in port  101  of the document setting portion  2  and that the switchback operation of the document P is interfered by the remaining set of documents stacked on the document tray  201 . 
     At the timing when the tail end of the document P in the entering direction into the switchback route  422  reaches a certain position before the inverting roller pair  424 ,  425 , the pressing contact of the driven roller  425  against the inverting roller  424  is released. Then, nipping of the tail end of the document P by the inverting roller pair  424 ,  425  is released in a state that the lead end thereof is freely guided inward of the document guides  202 . Thus, the document P is rendered in a free state on the document tray  201 . Accordingly, even if the lead end of the document P is temporarily discharged in a skew transport state through the document discharge port  102  (invert access port  102 ) onto the document setting portion  2 , the pair of document guides  202  correct the skew transport state by guiding the lead end of the document P inward along the tapered portions  202   a . As the lead end of the document P being guided along the tapered portions  202   a , and eventually guided within a pair of vertical portions  202   c  that have substantially the same width as the width of the document P, a skew, if any in the lateral direction, of the document P in its entiety will be securely corrected because the tail end of the document P is not held between the inverted roller pair  424 ,  423 . 
     The pair of document guides  202  also function as a skew feed-in preventing member by guiding the tail end of the document P when the document P is fed into the switchback section  42  through the invert access port  102 . 
     Referring back to FIGS. 14 and 15, when the pressing solenoid  14  is turned ON after lapse of a predetermined time T 4  after correcting the skew transport of the document P coming into the switchback route  422 , the inverting driven roller  425  is pressed against the inverting roller  424  again (in Steps S 27 , S 29 ). Then, the feed motor  9  starts rotating in the forward direction to start feeding the document P temporarily staying in the switchback route  422  in such a direction as to return the document P out of the switchback route  422  toward the main transport route  41 . In this way, the switchback operation of the document P is terminated. 
     Subsequently, when a predetermined time T 5  lapses from start of the forward rotation of the feed motor  9 , the feed motor  9  temporarily suspends its rotation (in Steps S 33 , S 35 ). The rotation suspend state of the feed motor  9  continues for a predetermined time T 6  from the motor rotation suspend timing in Step S 35  (in Step S 37 ). 
     Driving of the feed motor  9  is temporarily suspended to function the upper transport roller pair  417  as a registration roller pair. Specifically, even if the skew transport of the document P is corrected by the registration roller pair  416 , there still remains a possibility that a skew transport of the document P occurs during the switchback operation or the skew transport is not completely corrected. Using the upper transport roller pair  417  as the registration roller pair at the junction between the switchback section  42  and the U-shape transport section  41  to correct the skew transport of the document P after the switchback operation eliminates such drawback. 
     It should be noted that hereinafter “use something as a registration roller” is meant to be the following unless otherwise indicated clearly: 
     Assuming the registration roller is provided in a manner it extends in a direction orthogonal to the transport direction of the document. When the lead end of the document P being transported, thus having an inertia, makes contact with the registration roller oriented in orthogonal to the transport direction, then if there exists skew in the transported document P, say a lead end portion of the document is not exactly orthogonal to the transport direction, the orientation of the document P is corrected such that the lead end of the document P is aligned parallel with the registration roller due to the contact. Thus when we say “using something as a registration roller” in this specification that is meant to, unless otherwise indicated, be correcting the skew of the document with using a contact inertia of the document and an orientation of the registration with respect to the transport direction. 
     In order to perform a skew correction after the switchback operation, the predetermined time T 5  is set to a time duration required for the document P to form a non-flat portion (redundant portion) around the lead end of the document in the exiting direction from the switchback section  42  after the switchback operation sufficient for performing a skew correction after the lead end comes to a space between the upper transport roller pair  417 . The predetermined time T 6  is a time duration necessary for securely nipping the lead end of the document P to enable a skew correction of the document P. 
     In this embodiment, the upper transport roller pair  417  for transporting the document P along the U-shape transport section  41  is used as the registration roller pair for correcting a skew transport of the document P after a switchback operation. Alternatively, a registration roller pair for correcting a skew transport of the document P after a switchback operation may be provided on the switchback route (seventh transport route)  422 . However, using the upper transport roller pair  417  as the registration roller pair for correcting a skew transport of the document P after a switchback operation, as mentioned in the embodiment, simplifies the construction of the device and reduces the size of the device. 
     When the predetermined time T 6  lapses from suspending of the rotation of the feed motor  9 , and the skew correction by the upper transport roller pair  317  is completed, the feed motor  9  starts rotating in the forward direction and simultaneously, the transport motor  10  starts rotating in the reverse direction (in Step S 39 ). Thereby, the document P starts to be transported by the inverting roller pair  424 ,  425  and the upper transport roller pair  417 . 
     Subsequently, upon lapse of a predetermined time T 7  (in Step S 41 ), the pressing solenoid  14  is turned OFF, and the pressing contact of the inverting driven roller  425  against the inverting roller  424  is released. Then, the feed motor  9  is turned OFF, transmission of the drive force thereof to the inverting roller pair  424 , 425  is suspended, and the driving of the driven roller  424  is suspended (in Step S 43 ). The predetermined time T 7  is a time duration necessary for securely nipping the lead end of the document P by the upper transport roller pair  417  or the lower transport roller pair  418 . 
     In this way, the document P is transported by the upper transport roller pair  417 , and the inverting roller pair  424 ,  425  until the predetermined time T 7  lapses after a skew correction by the upper transport roller pair  417  for the following reason. When the pressing contact of the inverting driven roller  425  against the inverting roller  424  is released when the lead end of the document P comes to a space between the upper transport roller pair  417 , the document P is transported solely by the driving force of the upper transport roller pair  417  in a state that the tail end thereof is left free. Accordingly, there rises a possibility that a skew transport occurs, which deteriorates the skew correction performance of the ADF  1  as a whole. In this embodiment, however, the skew correction performance of the ADF  1  is secured by transporting the document P by the upper transport roller pair  417  and the inverting roller pair  424 ,  425  by a certain distance corresponding to the predetermined time T after the skew correction by the upper transport roller pair  417 . 
     After Step S 43 , when the document P is transported to the image reading position P 3  by the upper transport roller pair  417 , the lower transport roller pair  418 , and the intermediate roller pair  419 , and the timing sensor S 3  detects the lead end of the document P (YES in Step S 45 ), the count value n indicating the number of document transport is incremented by  1  (in Step S 47 ). Then, it is judged whether the count value n is set at “2” (in Step S 49 ). 
     When it is judged that n=2 (YES in Step S 49 ), the discharge solenoid  16  is turned OFF (in Step S 51 ) to discharge the document P after the image reading since the judgement result indicates that the document transport has been performed twice (i.e., the document transport this time is for reading the image on one side of the document P). Then, the feedback section  43  is isolated from the U-shape transport section  41 . 
     On the other hand, when n&lt;2 (NO in Step S 49 ), Step S 51  is skipped to return the document P after the image reading to the switchback section  42  since the document transport this time is for reading the image on the opposite side of the document P, and the discharge solenoid  16  is kept in an ON-state. 
     Subsequently, the image reader unit  6  starts reading the image on the opposite side of the document P at a timing upon lapse of a predetermined time T 8  based on a detection signal from the timing sensor S 3  (in Steps S 53 , S 55 ). 
     In this way, when the image reading of the document P is finished, it is judged whether the count value n equals to “2” (in Step S 57 ). If it is judged that n&lt;2 (NO in Step S 57 ), the sequence returns to Step S 17  to return the document P to the switchback section  42  via the feedback section  43  so as to read the image on one side of the document P. On the other hand, when it is judged that n=2 (YES in Step S 57 ), then it is judged whether there remains a document P on the document setting portion  2  since the judgement result indicates that both side image reading of the document P is finished and the document P is discharged onto the document discharge portion  3  (in Step S 59 ). If it is judged that there remains a document P on the document setting portion  2  (NO in Step S 59 ), this sequence returns to Step S 3  to perform DS image reading of the next document P. 
     If it is judged that no document is left on the document setting portion  2  (YES in Step S 59 ), DS image reading of all the documents stacked on the document setting portion  2  is finished since the judgement result indicates that image reading of all the documents P is completed. 
     It should be noted that in the foregoing description, there are basically two types of skew transport correctors: one is a pair of guide plates  202  each having a tapered portion  202   a  (a first skew corrector) and the other one is a registration roller pair (such as  417 ) using an inertia of document P when it makes contact with the roller pair (a second skew corrector). In addition, the roller pair is also referred to as a skew correcting means. The first skew corrector makes contact with the side of the document P and the second skew corrector makes contact with the lead end of the document P to correct skew transport of the document P. Thus the reader should not be confused with the term “skew corrector” as this term per see contains two types. 
     The sheet transport device (document feeder) according to this invention has been described for the case where the document feeder is incorporated in a copier. This invention is not limited to the above embodiment and applicable to, e.g., an image forming apparatus for transporting a copy sheet to form an image on the copy sheet. In other words, this invention is applicable to any type of sheet transport device as long as the sheet transport device has a mechanism for transporting a sheet such as a document and a copy sheet along a predetermined transport route and the transport route is branched into plural paths at respective junctions. 
     In the foregoing embodiment, described was the document feeder of upto-down-feed-in type in which a set of documents are stacked on the document setting portion in a state that one side of the document faces upward and the documents are successively fed from the uppermost one. Alternatively, this invention is applied to any type of document feeder regardless of the increasing manner of the page number of the document set placed on the document setting portion (upward-increasing or downward-increasing) and feed-in order of document (up-to-down-feed-in system or down-to-up-feed-in system). 
     Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.