Patent Publication Number: US-7583927-B2

Title: Image forming apparatus

Description:
This application is based on Japanese Patent Application No. 2006-112719 filed on Apr. 14, 2006, the entire content of which is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to an image forming apparatus such as a copying machine or a printer, and in particular, to an image forming apparatus capable of forming images on both sides of a sheet. 
   The image forming apparatus such as a copying machine or a printer is equipped with an image carrier, an image writing section, a developing unit, a sheet-feeding tray, a transfer section and a fixing section. Now, the constitution of the image forming apparatus will be explained as follows, referring to  FIG. 18  which is a cross-sectional view showing the constitution of the image forming apparatus. 
   Automatic conveyance device  10  is a device to conduct conveyance for reading a document. A plurality of documents d each being under the condition that the front surface of the first page of the document faces upward are loaded on document loading section  11  which is for loading documents. Document d is fed out through roller  12   a  and roller  12   b , and is conveyed to image reading section  20  through roller  13 . Then, the document d whose images have been read in the image reading section  20  is reversed by reversing roller  14 , to be ejected on sheet ejection tray  16  with its front surface facing downward. 
   The image reading section  20  scans the document optically to generate image data. An image surface of the document d is illuminated by light source  23 , and its reflected light forms an image on a light-receiving surface of CCD  28  representing a photoelectric conversion means, through mirror  24 , mirror  25 , mirror  26  and combined optical system  27 . Incidentally, when reading the document d by placing it on platen glass  21  so that the surface of the document d to be read may face downward, the optical system is moved along the platen glass  21  for the reading operation. Further, when reading the document d, while conveying it, the reading operation is conducted under the condition the light source  23  and the mirror  24  are fixed on second platen glass  22 . Image data of the document d that has been read are sent to an image processing section (not shown) from CCD  28 . In the mean time, when the document d is conveyed for its both sides by automatic conveyance device  10 , the document d is reversed and conveyed to roller  13  again through reversing roller  14  after the front surface of the document d has been read, whereby, the rear surface of the document d is read by image reading section  20 , and image data obtained through the reading are sent to an image processing section from CCD  28 . 
   Transfer sheets P are loaded on sheet-feeding tray  30 . Incidentally, though a single step of sheet-feeding tray  30  is provided in the structure in  FIG. 18 , it is normal that a plurality of sheet-feeding trays are provided so that transfer sheets having different sizes may be loaded. 
   Sheet supply section  40  supplies transfer sheets P to image forming section  60  from sheet-feeding tray  30 . Transfer sheet P is fed out of the sheet-feeding tray  30  by conveyance roller  41 , and is caused to hit a nip portion of a registration roller  43  through loop rollers  42  to be stopped temporarily, thereby, a skew of the transfer sheet P is corrected. Then, the transfer sheet P is conveyed to photoreceptor drum  61  of the image forming section  60  at prescribed timing. Further, the transfer sheet P is fed out of manual feed tray  31  by conveyance roller  41 , and is conveyed to photoreceptor drum  61  of the image forming section  60  through the same process flow as in the foregoing. 
   Image writing section  50  is composed of a polygon mirror (not shown) that deflects a laser beam emitted from laser element  51  based on inputted image data. The deflected laser beam is caused by the polygon mirror to scan and is projected on photoreceptor drum  61  through a mirror. Owing to this, an electrostatic latent image is formed on the photoreceptor drum  61 . 
   The image forming section  60  records the electrostatic latent image formed on the photoreceptor drum  61  on transfer sheet P through an electrophotographic system. First, when a laser beam emitted from laser diode  51  of the image writing section  50  is irradiated on the photoreceptor drum  61  charged evenly by charging section  67 , an electrostatic latent image is formed. Then the electrostatic latent image formed on the photoreceptor drum  61  is developed by developing unit  62  to form a toner image on the photoreceptor drum  61 . This toner image is transferred onto transfer sheet P by transfer section  63  that is provided below the photoreceptor drum  61 . Then, transfer sheet P that is in contact with the photoreceptor drum  61  is separated by separating section  64 . The transfer sheet P separated from the photoreceptor drum  61  is conveyed to fixing section  70  by conveyance mechanism  65 . 
   The fixing section  70  fixes a toner image transferred onto transfer sheet P through heat and pressure. The toner image transferred onto transfer sheet P is fixed by heat and pressure exerted from fixing roller  71 . 
   Sheet ejection section  80  ejects transfer sheet P on which the image has been fixed. Transfer sheet P on which the image has been fixed is ejected to sheet ejection tray  82  by sheet ejection roller  81 . When forming images on both sides, transfer sheet P is conveyed downward by guide  83 , after the image formed on the front surface is fixed, and the transfer sheet P is sent to reversing path  84 . The transfer sheet P having entered the reversing path  84  is conveyed to reversing conveyance path  86  by reversing conveyance roller  85 . The transfer sheet P having entered the reversing conveyance path  86  is conveyed again to image forming section  60  through sheet supply section  40 . 
   Transfer sheet P is caused to hit a nip portion of the registration roller  43  through loop rollers  42  to be stopped temporarily, thereby, a skew of the transfer sheet P is corrected. Then, the transfer sheet P is conveyed to photoreceptor drum  61  of the image forming section  60  at prescribed timing. 
   On the image forming section  60 , residual toner sticking to the image forming section  60  is removed by cleaning section  66 , to be ready for the succeeding image forming. Under this condition, the transfer sheet P is conveyed to image forming section  60 , and an image is formed on the other surface (rear surface). Then, the transfer sheet P separated from the photoreceptor drum  61  in the separation section  64  is sent again to fixing section  70  through conveyance mechanism  65  to be fixed. In this way, transfer sheet P on which image fixing on each of the front surface and the rear surface has been terminated is ejected to sheet ejection tray  82  by sheet ejection roller  81 . 
   As stated above, the skew of a transfer sheet for the conveyance direction has been corrected by the registration roller  43  before image forming. With respect to the correction of the skew, there has been proposed a method wherein a pattern for measurement is written on the transfer sheet, then, the position of the pattern is detected to detect the positional shifting and the skew of the transfer sheet, and a position of an image to be formed on the reverse side is determined based on the results of the detection (for example, Patent Document 1). 
   As another method, there has been proposed a method wherein a mark is written on the surface of a transfer sheet, and when forming an image on the reverse side, a position of the mark is detected, then, a position of an image to be formed on the reverse side of the transfer sheet is determined based on the position of the mark detected and the position of the mark at the point of time when the mark was written, and further, the magnification of the image to be formed on the reverse side is changed (for example, Patent Document 2). 
   Further, as another method, there has been proposed a method wherein changes in outer dimensions of the transfer sheet generated when the image formed on the surface is fixed are obtained and thereby, the magnification of an image to be formed on the reverse side is changed (for example, Patent Document 3). 
   (Patent Document 1) JP-A No. 10-319674 (Hereinafter, JP-A refers to Japanese Patent Publication Open to Public Inspection) 
   (Patent Document 2) JP-A No. 2003-156974 
   (Patent Document 3) JP-A No. 2004-271926 
   However, actual transfer sheet P has no orthogonality, and corner angles fluctuate, depending on how sheets are cut. As shown in  FIG. 19  ( a ), for example, an angle of a certain corner of transfer sheet P is different, and it is 89° for a certain corner and is 91° for another corner. If an angle of transfer sheet P fluctuates as stated above, there is sometimes an occasion where an image formed on the surface and an image formed on the reverse side do not agree in terms of position each other, even when a skew of transfer sheet P is corrected by the registration roller  43 . 
   When forming an image on the front surface of transfer sheet P, for example, a side in the conveyance direction (a leading edge) of transfer sheet P is caused to hit a nip portion of the registration roller  43  so that a skew of the transfer sheet P for the conveyance direction is corrected, and after that, an image is formed on the surface of image forming section  30 . After the image is formed on the surface, when an image is formed on a reverse side of the transfer sheet P, the transfer sheet P is reversed by reversing path  84 , reversing and conveying roller  85  and reversing and conveying path  86 , and is sent again to image forming section  6 . Since the transfer sheet P is reversed as stated above, a side of transfer sheet P opposite to the leading edge (a trailing edge) is caused to hit a nip portion of the registration roller  43  as shown in  FIG. 19  ( c ), and a skew of the transfer sheet P for the conveyance direction is corrected. After that, an image is formed on the reverse side by the image forming section  60 . 
   However, since a skew angle of the leading edge is different from that of a side opposite to the leading edge (a trailing edge) in transfer sheet P as shown in  FIG. 19  ( a ), an image formed on the surface is deviated from an image formed on the reverse side by the difference equivalent to the difference of the angle as shown in  FIG. 19  ( c ), and it has been difficult to align an image on the surface with an image on the reverse side highly accurately. In other words, since transfer sheet P has no orthogonality with its each side skewing, even when different sides are caused to hit the registration roller  43  for correcting skews, the skew of transfer sheet P for conveyance direction before transferring images onto the surface is different from that before transferring images onto the reverse side. As a result, a position of the image on the surface is shifted from that of the image on the reverse side, resulting in difficulties of aligning images highly accurately. 
   SUMMARY OF THE INVENTION 
   The problems mentioned above are solved by the present invention, and its object is to provide an image forming apparatus wherein, when forming images on both sides of a transfer sheet, images formed on both sides can be aligned highly accurately. 
   An embodiment of the invention is an image forming apparatus that forms an image on a surface of a transfer sheet, then, reverses the transfer sheet on which the image has been formed on the surface and then, forms an image on the reverse side of the transfer sheet, wherein there is provided a correcting section that changes a relative position between the transfer sheet and the image on the surface in accordance with an outer shape of the transfer sheet and further changes a relative position between the transfer sheet and the image on the reverse side of the transfer sheet in accordance with an outer shape of the transfer sheet. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing a constitution of an image forming apparatus relating to the first embodiment of the invention. 
       FIG. 2  is a top view showing an outer shape of a transfer sheet. 
       FIG. 3  is a top view showing a schematic structure of a registration roller. 
       FIG. 4  is a flow chart for illustrating series of operations of an image forming apparatus relating to the first embodiment of the invention. 
       FIG. 5  is a block diagram showing a structure of an image forming apparatus relating to the second embodiment of the invention. 
       FIG. 6  is a flow chart for illustrating series of operations of an image forming apparatus relating to the second embodiment of the invention. 
       FIG. 7  is a block diagram showing a structure of an image forming apparatus relating to the third embodiment of the invention. 
       FIG. 8  is a pattern diagram for illustrating a position of start writing images. 
       FIG. 9  is a flow chart for illustrating series of operations of an image forming apparatus relating to the third embodiment of the invention. 
       FIG. 10  is a block diagram showing a structure of an image forming apparatus relating to the fourth embodiment of the invention. 
       FIG. 11  is a flow chart for illustrating series of operations of an image forming apparatus relating to the fourth embodiment of the invention. 
       FIG. 12  is a block diagram showing a structure of an image forming apparatus relating to the fifth embodiment of the invention. 
       FIG. 13  is a top view showing an arrangement of a photodetector. 
       FIG. 14  is a diagram showing an output wave form of the photodetector. 
       FIG. 15  is a flow chart for illustrating series of operations of an image forming apparatus relating to the fifth embodiment of the invention. 
       FIG. 16  is a flow chart for illustrating other operations of an image forming apparatus relating to the fifth embodiment of the invention. 
       FIG. 17  is a top view showing a schematic structure of a registration roller. 
       FIG. 18  is a cross-sectional view showing a structure of an image forming apparatus. 
       FIG. 19  is a top view for illustrating operations to correct a skew of a transfer sheet in an image forming apparatus relating to conventional technologies. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   First Embodiment (Structure) 
   A structure of an image forming apparatus relating to the first embodiment of the invention will be explained as follows, referring to  FIG. 1  which is a block diagram showing a constitution of an image forming apparatus relating to the first embodiment of the invention. 
   As shown in  FIG. 18 , an image forming apparatus relating to the present embodiment is composed of image reading section  20 , sheet-feeding tray  30 , sheet supply section  40 , image writing section  50 , image forming section  60 , fixing section  70  and sheet ejection section  80 . 
   In the present embodiment, information representing an outer shape of transfer sheet P is inputted with operation section  1 . As an outer shape, there is given a skew (angle) of transfer sheet P. An operator inputs a skew (angle) of a leading edge of transfer sheet P and a skew (angle) of a trailing edge representing a side that is opposite to the leading edge, with operation section  1 . The skew (angle) of a leading edge of transfer sheet P results in a correction value (angle) for a skew in the case of forming an image on the surface of transfer sheet P, while, the skew (angle) of a trailing edge results in a correction value (angle) for a skew in the case of forming an image on the reverse side. 
   Now, a skew of transfer sheet P will be explained, referring to  FIG. 2  which is a top view showing an outer shape of a transfer sheet. When forming an image on the front surface of transfer sheet P, a skew of the leading edge in the conveyance direction of transfer sheet P is corrected by causing the leading edge in the conveyance direction of transfer sheet P to hit a nip portion of the registration roller  43 . When forming an image on the reverse side of transfer sheet P, a skew of the leading edge in the conveyance direction of transfer sheet P is corrected when transfer sheet P is reversed and a leading edge in the conveyance direction (the trailing edge shown in  FIG. 2 ) is caused to hit the nip portion of the registration roller  43 . 
   When an axis perpendicular to the conveyance direction serves as a reference axis, let it be assumed that a skew of the leading edge of transfer sheet P relative to the reference axis is angle A, and a skew of the trailing edge of transfer sheet P is angle B. When the leading edge of transfer sheet P is skewed to the conveyance direction, the direction of the skew is made to be “−” and a skew of the leading edge in this case is made to be “−A°”. Further, as shown in  FIG. 2 , when the leading edge is skewed to the direction opposite to the conveyance direction, the direction of the skew is made to be “+” and a skew of the leading edge in this case is made to be “+A°”. Even for the trailing edge, when the trailing edge is skewed to the conveyance direction, the direction of the skew is made to be “−” and a skew of the trailing edge in this case is made to be “−B°”. Further, when the trailing edge is skewed to the direction opposite to the conveyance direction, the direction of the skew is made to be “+” and a skew of the trailing edge in this case is made to be “+B°”. 
   When information showing a skew (angle) of transfer sheet P in inputted by an operator, the information showing the skew (angle) is stored in correction data storage section  3 . Since a skew (angle) of transfer sheet P varies depending on a corner of transfer sheet P, a skew (angle) of the leading edge and a skew (angle) of the trailing edge are inputted by the use of operation section  1 , to be stored in correction data storage section  3 . In other words, when forming an image on the surface of transfer sheet P, a leading edge of transfer sheet P is caused to hit the registration roller  43  to correct the skew of transfer sheet P, while, when forming an image on the reverse side of transfer sheet P, transfer sheet P is reversed and a trailing edge of transfer sheet P is caused to hit the registration roller  43  to correct the skew of transfer sheet P. Therefore, the side hitting the registration roller  43  in the case of forming an image on the surface is different from that in the case of forming an image on the reverse side. Accordingly, the skew (angle) of the leading edge and the skew (angle) of the trailing edge are inputted to be stored in correction data storage section  3 . 
   Operation section  1  is composed of an input portion and a display portion. The input portion includes a keyboard equipped with a cursor key, a numeral input key and respective functional keys, and a hold-down signal corresponding to the key pressed on the keyboard is outputted to controller  2 . The display portion is composed of a liquid crystal display and an EL display, and it displays image data and text data on a screen in accordance with an instruction of display signals outputted from controller  2 . 
   Pieces of information showing an outer shape of transfer sheet P inputted by operation section  1 , namely, correction values for correcting a skew of transfer sheet P with the registration roller  43  are stored in correction data storage section  3 . Since the correction values include a correction value for correcting a skew of transfer sheet P when forming an image on the surface of transfer sheet P and a correction value for correcting a skew of transfer sheet P when forming an image on the reverse side, a correction value for the surface and that for the reverse side are stored in the correction data storage section  3 . Specifically, as a correction value for the surface, “+A°” or “−A°” which shows a skew of a leading edge is stored in correction data storage section  3 . Further, as a correction value for the reverse side, “+B°” or “−B°” which shows a skew of a trailing edge is stored in correction data storage section  3 . 
   The registration roller  43  is arranged in the direction perpendicular to the conveyance direction for transfer sheet P, and a skew of the leading edge of transfer sheet P for the conveyance direction of transfer sheet P is corrected when the transfer sheet P is caused to hit the registration roller  43 . Further, the registration roller  43  is slanted in accordance with a correction value (angle) stored in correction data storage section  3 , to correct the skew of transfer sheet P. The control for the slant of the registration roller  43  is made by drive controller  4 . The drive controller  4  causes motor M to rotate in accordance with correction values (angles) stored in correction data storage section  2 . When drive power of the motor M is transmitted to the registration roller  43 , the registration roller  43  is slanted by an angle equivalent to the drive power to correct the skew of transfer sheet P. 
   This registration roller  43  will be explained as follows, referring to  FIG. 3 .  FIG. 3  is a top view showing a schematic structure of a registration roller. As shown in  FIG. 3  ( a ), the registration roller  43  is installed on holding unit  43 B and is arranged in the direction perpendicular to the conveyance direction of transfer sheet P. The registration roller  43  rotates on a rotation axis (roller rotation axis  43 A) which is in the direction perpendicular to the conveyance direction. Then, drive power is inputted from drive source input section A by control of drive controller  4 , and holding unit  43 B is slanted on fulcrum B in the conveyance direction (−direction) or in the direction (+direction) opposite to the conveyance direction. When the holding unit  43 B is slanted, the registration roller  43  installed on the holding unit  43 B is slanted in the conveyance direction (−direction) or in its opposite direction (+direction). A slant of the registration roller  43  is in correspondence to the correction value (angle) stored in correction data storage section  3 . For example, if a correction value stored in correction data storage section  3  is “negative (−)”, the registration roller  43  is slanted in the “−” direction, and if a correction value is “positive (+)”, the registration roller  43  is slanted in the “+” direction. Transfer sheet P is caused to hit the registration roller  43  under the condition that the registration roller  43  is slanted as stated above, whereby a skew of the transfer sheet P is corrected. 
   Next, a mechanism for slanting the registration roller  43  will be explained. In the present embodiment, two examples are given as a mechanism to slant the registration roller  43 . First, a mechanism to slant the registration roller  43  by using a gear will be explained, referring to  FIG. 3  ( b ). On holding unit  43 B, there is provided gear G 2  whose direction is in the conveyance direction. The gear G 2  is engaged with gear G 1  whose direction is in the direction perpendicular to the conveyance direction. The gear G 1  is connected with motor M, and when the gear G 1  is rotated by the motor M, a rotation of the gear G 1  is transmitted to the gear G 2 , and holding unit  43 B is swiveled on the fulcrum B representing an axis to be moved in the conveyance direction (−direction) or its opposite direction (+direction). Owing to this, a slant of the registration roller  43  can be changed. 
   Next, a mechanism for slanting the registration roller  43  by using a cam will be explained, referring to  FIG. 3  ( c ) . The holding unit  43 B is connected with cam G 3 , and when this cam G 3  is rotated, the holding unit  43 B is swiveled on the fulcrum B representing an axis to be moved in the conveyance direction (−direction) or its opposite direction (+direction) . Owing to this, a slant of the registration roller  43  can be changed. 
   Image processing section  5  is composed of reading controller  5 A and writing controller  5 B. The reading controller  5 A controls image reading section  20 , and the writing controller  5 B controls image writing section  50 . Incidentally, though  FIG. 1  shows only the reading controller  5 A and the writing controller  5 B, the image processing section  5  may also conduct compression, extension or conversion of image data. 
   System controller  2  reads a correction value (angle) about a surface on which an image is formed from correction data storage section  3 , and outputs the correction value to drive controller  4 . For example, when forming an image on the surface of transfer sheet P, the system controller  2  reads a correction value about a surface of the system controller  2  from correction data storage section  3 , and outputs to drive controller  4 . Further, when image forming on the surface of transfer sheet P is completed, and when forming an image on the reverse side, the system controller  2  reads a correction value about the reverse side from correction data storage section  3 , and outputs to drive controller  4 . 
   Meanwhile, the system controller  2  is connected to various sections (image reading section  20 , sheet-feeding section  40 , image writing section  50 , image forming section  60  and fixing section  70 ) of an image forming apparatus, and controls transfer processing, fixing processing and reversing processing. 
   (Operations) 
   Next, operations (image forming method) of an image forming apparatus relating to the first embodiment will be explained.  FIG. 4  is a flow chart for illustrating series of operations of an image forming apparatus relating to the first embodiment of the invention. 
   (step S 01 ) 
   First, in step S 01 , an operator inputs information showing an outer shape of transfer sheet P, namely, information showing a skew (angle) of transfer sheet P, by using operation section  1 . Specifically, an operator inputs “+A°” or “−A°” as a skew (angle) of a leading edge of transfer sheet P, and inputs “+B°” or “−B°” as a skew (angle) of a trailing edge. In this case, it is assumed that “+A°” is inputted as a skew of a leading edge and “+B°” is inputted as a skew of a trailing edge. The skew (angle) of the transfer sheet P inputted at the operation section  1  is stored in correction data storage section  3  as a correction value for the skew. 
   In the meantime, a skew (angle) of each transfer sheet P may also be inputted. By inputting a skew (angle) of each transfer sheet P, it is possible to correct a skew of each transfer sheet P even when each transfer sheet P varies slightly in terms of a shape. 
   (step S 02 ) 
   In step S 02 , the registration roller  43  is slanted based on the correction value (angle) of the leading edge inputted during step S 01 . System controller  2  reads from correction data storage section  3  the correction value for the skew of the surface, and outputs to drive controller  4 . After receiving the correction value from the system controller  2 , the drive controller  4  causes motor M to rotate and slants the registration roller  43 . For example, when a skew of the leading edge of transfer sheet P is “+A°”, the drive controller  4  slants the registration roller  43  by “+A°” by causing motor M to rotate. 
   (step S 03 ) 
   Next, in step S 03 , the leading edge of transfer sheet P hits a nip portion of the registration roller  43 . In this case, a side (leading edge) of transfer sheet P in the conveyance direction hits a nip portion of the registration roller  43 , and thereby, a prescribed loop is formed, and a skew of the transfer sheet P for the conveyance direction is corrected. The registration roller  43  is slanted from the conveyance direction by “+A°”, and the leading edge of the transfer sheet P is skewed by “+A°”, which means that a slant of the registration roller  43  agrees with a skew of the leading edge of the transfer sheet P. Due to this, the transfer sheet P is corrected in terms of a skew for the conveyance direction to be in parallel with the conveyance direction. After that, the transfer sheet P is conveyed to photoreceptor drum  61  of image forming section  60  at prescribed timing. By correcting the skew of transfer sheet P for the conveyance direction by slanting the registration roller  43  as stated above, it is possible to change a relative position between the transfer sheet P and the image formed on the surface, to form an image on the surface. 
   (step S 04 ) 
   After the skew for the front surface has been corrected in step S 03 , a toner image is transferred onto transfer sheet P in image forming section  60 , and the toner image thus transferred is fixed in fixing section  70 . 
   (step S 05 ) 
   Then, the transfer sheet P is conveyed again to image forming section  60  under the condition that the transfer sheet P has been reversed by reversing path  84 , reversing conveyance roller  85  and reversing conveyance path  86 , for forming an image on the reverse side. 
   (step S 06 ) 
   In step S 06 , the registration roller  43  is slanted based on the correction value (angle) inputted during step S 01 . System controller  2  reads from correction data storage section  3  the correction value for the skew of the reverse side, and outputs to drive controller  4 . After receiving the correction value from the system controller  2 , the drive controller  4  causes motor M to rotate and slants the registration roller  43 . For example, when a skew of the trailing edge of transfer sheet P is “+B°”, the drive controller  4  slants the registration roller  43  by “+B°” by causing motor M to rotate. 
   (step S 07 ) 
   Next, in step S 07 , the leading edge of transfer sheet P hits a nip portion of the registration roller  43 . In this case, a side (trailing edge) of transfer sheet P hits a nip portion of the registration roller  43 , and thereby, a prescribed loop is formed, and a skew of the transfer sheet P for the conveyance direction is corrected. The registration roller  43  is slanted from the conveyance direction by “+B°”, and the trailing edge of the transfer sheet P is also skewed by “+B°”, which means that a slant of the registration roller  43  agrees with a skew of the trailing edge of the transfer sheet P. Due to this, the transfer sheet P is corrected in terms of a skew for the conveyance direction to be in parallel with the conveyance direction. After that, the transfer sheet P is conveyed to photoreceptor drum  61  of image forming section  60  at prescribed timing. By correcting the skew of transfer sheet P for the conveyance direction by slanting the registration roller  43  as stated above, it is possible to change a relative position between the transfer sheet P and the image formed on the reverse side, to form an image on the reverse side. 
   (step S 08 ) 
   After the skew about the reverse side is corrected in step S 07 , a toner image is transferred onto transfer sheet P at image forming section  60 , and the transferred toner image is fixed at fixing section  70 . 
   (step S 09 ) 
   The transfer sheet P on which image fixing has been completed on each of the surface and the reverse side, is ejected by sheet-ejection roller  81  onto sheet-ejection tray  82 . 
   In the image forming apparatus relating to the first embodiment, as stated above, a skew of transfer sheet P from the conveyance direction is corrected by changing a slant of the registration roller  43  in accordance with an outer shape of a transfer sheet, specifically with a skew of the leading edge of transfer sheet P and a skew of the trailing edge, thus, images formed on both surfaces can be aligned in terms of position highly accurately, by canceling positional difference between the image on the surface and the image on the reverse side. 
   Further, by correcting a skew of each transfer sheet P by inputting information showing a skew (angle) of each transfer sheet P, it is possible to correct a skew of each transfer sheet P even when each transfer sheet P varies slightly in terms of a shape. 
   Incidentally, it is possible to store information showing a skew (angle) of transfer sheet P inputted at operation section  1  in correction data storage section  3 , and thereby to correct a skew of the succeeding transfer sheet P by using a correction value (angle) stored in the correction data storage section  3 , when forming an image on the succeeding transfer sheet P. In other words, when forming an image on fresh transfer sheet P, it is possible to correct a skew of transfer sheet P without inputting an outer shape of transfer sheet P each time, by correcting a skew of fresh transfer sheet P by using an outer shape of the preceding transfer sheet P. 
   Second Embodiment 
   Structures of the image forming apparatus relating to the second embodiment of the invention will be explained as follows, referring to  FIG. 5  which is a block diagram showing a structure of an image forming apparatus relating to the second embodiment of the invention. 
   In the same way as in the aforesaid first embodiment, the image forming apparatus relating to the second embodiment is composed of image reading section  20 , sheet-feeding tray  30 , sheet supply section  40 , image writing section  50 , image forming section  60 , fixing section  70  and sheet ejection section  80 . Further, a registration roller for correcting a skew of transfer sheet P also has the aforesaid constitution. In the same way as in the first embodiment, the image forming apparatus relating to the second embodiment is composed of system controller  2 , correction data storage section  3 , drive controller  4 , motor M and registration roller  43 . 
   In the second embodiment, image reading section  20  is used to read information showing an outer shape of transfer sheet P, in place of inputting the outer shape of transfer sheet P from operation section  1 . As an outer shape, there are given a skew (angle) of a leading edge and a skew (angle) of a trailing edge of transfer sheet P, in the same way as in the first embodiment. 
   Image reading section  20  generates image data by scanning transfer sheet P optically. The image data represent an outer shape of transfer sheet P. After the outer shape of transfer sheet P is read by the image reading section  20 , information showing the outer shape is outputted to angle calculating section  5 C of image processing section  5 . The angle calculating section  5 C obtains a skew (angle) of a leading edge and a skew (angle) of a trailing edge of transfer sheet P from the outer shape. For example, as shown in a top view in  FIG. 2 , when the direction that is in parallel with a shorter side of transfer sheet P is made to be the conveyance direction, the axis perpendicular to this conveyance direction is made to be a reference axis. Then, the angle calculating section  5 C obtains a skew of a leading edge relative to the reference axis, and its angle is made to be angle A and a skew of a trailing edge is made to be angle B. 
   The information showing a skew (angle) of the leading edge and a skew (angle) of the trailing edge of transfer sheet P obtained by the angle calculating section  5 C are stored in correction data storage section  3  as a correction value for a skew of transfer sheet P. Drive controller  4  slants the registration roller  43  in accordance with the correction value (angle) stored in correction data storage section  3 , in the same way as in the first embodiment. 
   (Operations) 
   Operations (image forming method) of an image forming apparatus relating to the second embodiment will be explained next, referring to  FIG. 6 .  FIG. 6  is a flow chart for illustrating series of operations of an image forming apparatus relating to the second embodiment of the invention. 
   (step S 10 ) 
   First, in step S 10 , an outer shape of transfer sheet P is read by image reading section  20 . Information showing the outer shape is outputted to angle calculating section  5 C. 
   (step S 11 ) 
   Next, in step S 11 , the angle calculating section  5 C calculates a skew of transfer sheet P based on information showing the outer shape of transfer sheet P. Specifically, the angle calculating section  5 C calculates a skew (angle) of the leading edge and a skew (angle) of the trailing edge of transfer sheet P. In this case, it is assumed that “+A°” represents a skew of the leading edge of transfer sheet P and “+B°” represents a skew of the trailing edge. Information showing these skews is stored in correction data storage section  3 . 
   (step S 12 ) 
   Then, in step S 12 , the registration roller  43  is slanted in accordance with a correction value (angle) of the leading edge of transfer sheet P. System controller  2  reads a correction value for the skew of the surface from correction data storage section  3 , and outputs to drive controller  4 . After receiving the correction value from the system controller  2 , the drive controller  4  causes motor M to rotate to slant the registration roller  43 . For example, when the skew of the leading edge of transfer sheet P is “+A°”, the drive controller  4  slants the registration roller  43  by “+A°” by causing the motor M to rotate. 
   (step S 13 ) 
   Next, in step S 13 , the leading edge of transfer sheet P is caused to hit a nip portion of the registration roller  43 . In this case, a side of transfer sheet P in the conveyance direction (a leading edge) hits the nip portion of the registration roller  43 , and thereby, a prescribed loop is formed and a skew of transfer sheet P for the conveyance direction is corrected. Since the slant of the registration roller  43  for the conveyance direction agrees with the skew of transfer sheet P for the conveyance direction, the skew of transfer sheet P for the conveyance direction is corrected to be in parallel with the conveyance direction. After that, the transfer sheet P is conveyed to photoreceptor drum  61  of image forming section  60  at prescribed timing. By correcting the skew of transfer sheet P for the conveyance direction by slanting the registration roller  43  as stated above, it is possible to form an image on the surface by changing a relative position between the transfer sheet P and the image formed on the surface. 
   (step S 14 ) 
   After the skew concerning the surface is corrected in step S 13 , a toner image is transferred onto transfer sheet P by image forming section  60 , and the toner image thus transferred is fixed by fixing section  70 . 
   (step S 15 ) 
   Then, for the purpose of forming an image on the reverse side, the transfer sheet P is conveyed again to the image forming section  60 , under the condition where the transfer sheet P is reversed by reversing path  84 , reversing conveyance roller  85  and reversing conveyance path  86 . 
   (step S 16 ) 
   In step S 16 , the registration roller  43  is slanted in accordance with a correction value (angle) of a trailing edge of transfer sheet P. System controller  2  reads a correction value for the skew of the reverse side from correction data storage section  3 , and outputs to drive controller  4 . After receiving the correction value from the system controller  2 , the drive controller causes motor M to rotate to slant the registration roller  43 . For example, when a skew of the trailing edge of transfer sheet P is “+B°”, the drive controller  4  slants the registration roller  43  by “+B°” by causing motor M to rotates. 
   (step S 17 ) 
   Next, in step S 17 , the trailing edge of transfer sheet P is caused to hit a nip portion of the registration roller  43 . In this case, a side of transfer sheet P in the conveyance direction (a trailing edge) hits the nip portion of the registration roller  43 , and thereby, a prescribed loop is formed and a skew of transfer sheet P for the conveyance direction is corrected. Since the slant of the registration roller  43  for the conveyance direction agrees with the skew of transfer sheet P for the conveyance direction, the skew of transfer sheet P for the conveyance direction is corrected to be in parallel with the conveyance direction. After that, the transfer sheet P is conveyed to photoreceptor drum  61  of image forming section  60  at prescribed timing. By correcting the skew of transfer sheet P for the conveyance direction by slanting the registration roller  43  as stated above, it is possible to form an image on the reverse side by changing a relative position between the transfer sheet P and the image formed on the reverse side. 
   (step S 18 ) 
   After the skew concerning the reverse side is corrected in step S 18 , a toner image is transferred onto transfer sheet P by image forming section  60 , and the toner image thus transferred is fixed by fixing section  70 . 
   (step S 19 ) 
   The transfer sheet P on which image fixing on each of the surface and the reverse side has been terminated is ejected to sheet ejection tray  82  by sheet ejection roller  81 . 
   In the image forming apparatus relating to the second embodiment, it is possible to make a positional slippage of an image on the surface and that on the reverse side to offset each other, and thereby, to align images formed on both sides highly accurately, by correcting a skew of transfer sheet P for the conveyance direction by changing a slant of the registration roller  43  depending on an outer shape of a transfer sheet, specifically, on a skew of the leading edge and a skew of the trailing edge of transfer sheet P, as stated above. 
   Further, by correcting a skew of each transfer sheet P by reading a skew (angle) of each transfer sheet P, it is possible to correct a skew of each transfer sheet P even when each transfer sheet P fluctuates slightly in terms of a shape. 
   Incidentally, it is possible to store information showing a skew (angle) of transfer sheet P read by image reading section  20  in correction data storage section  3 , and thereby to correct a skew of the succeeding transfer sheet P by using the correction value (angle) stored in the correction data storage section  3 , when forming an image on the succeeding transfer sheet P. In other words, when forming an image on fresh transfer sheet P, it is possible to correct a skew of fresh transfer sheet P by correcting a skew of fresh transfer sheet P by using an outer shape of the preceding transfer sheet P, and thereby to correct the skew of transfer sheet P one after another without obtaining the outer shape of the fresh transfer sheet P. 
   Third Embodiment 
   Structures of the image forming apparatus relating to the third embodiment of the invention will be explained as follows, referring to  FIG. 7  which is a block diagram showing a structure of the image forming apparatus relating to the third embodiment of the invention. 
   In the same way as in the aforesaid first embodiment, the image forming apparatus relating to the third embodiment is composed of image reading section  20 , sheet-feeding tray  30 , sheet supply section  40 , image writing section  50 , image forming section  60 , fixing section  70  and sheet ejection section  80  shown in  FIG. 18 . In the same way as in the first embodiment, the image forming apparatus relating to the third embodiment is further composed of system controller  2  and correction data storage section  3 . 
   In the third embodiment, information showing an outer shape of transfer sheet P, namely, information showing a skew (angle) of the leading edge and a skew (angle) of the trailing edge of transfer sheet P is inputted by using operation section  1 , in the same way as in the first embodiment. The skew (angle) of the leading edge and the skew (angle) of the trailing edge thus inputted are stored in correction data storage section  3 . 
   In the third embodiment, a position of an image on the surface and a position of an image on the reverse side are caused to agree each other by changing an image forming condition such as a position to start writing an image, in place of changing a slant of the registration roller  43  in accordance with a skew of transfer sheet P. By changing a position to start writing an image as stated above, it is possible to change a relative position between transfer sheet P and an image to be formed on the surface and a relative position between transfer sheet P and an image to be formed on the reverse side, and thereby to form an image on the surface or on the reverse side. 
   Start writing position determining section  5 D of image processing section  5  reads information showing a skew (angle) from correction data storage section  3 , and determines a position to start writing corresponding to the angle. The position to start writing determined by the start writing position determining section  5 D corresponds to the position to start writing in the main scanning direction for an electrostatic latent image to be formed on photoreceptor drum  61 . Writing controller  5 B causes an electrostatic latent image to be formed on photoreceptor drum  61  by controlling image writing section  50  in accordance with the position to start writing determined by the start writing position determining section  5 D. 
   Now, a position to start writing an image will be explained as follows, referring to  FIG. 8 .  FIG. 8  is a pattern diagram for illustrating a position to start writing an image. When a laser beam is projected on photoreceptor drum  61  by image writing section  50  in the main scanning direction, and when the laser beam is caused to scan photoreceptor drum  61  by a polygon mirror of image writing section  50  in the sub-scanning direction, an electrostatic latent image is formed on the photoreceptor drum  61 . Specifically, when a laser beam is projected along the first line, second line, third line, . . . which are in parallel with the main scanning direction, an electrostatic latent image is formed on the photoreceptor drum  61 . 
   For example, when the leading edge of transfer sheet P is skewed by “+A°”, start writing position determining section  5 D changes the position to start writing for each line in accordance with that angle “+A°”. To be concrete, the start writing position determining section  5 D changes a position to start writing in the main scanning direction in accordance with angle “+A°” depending on the first, second, third . . . lines. In other words, when the position to start writing in the main scanning direction is changed depending on the first, second, third, . . . lines, an electrostatic latent is formed obliquely on the photoreceptor drum  61 . 
   When the leading edge of transfer sheet P is skewed by “+A°”, the start writing position determining section  5 D makes start writing standard position P 1  to be the position to start writing images for the first line, then, makes position P 2  deviated from the original start writing standard established in advance, by angle “+A°”, to be the position to start writing images for the second line, and makes position P 3  deviated from the original start writing standard by angle “+A°” to be the position to start writing images for the third line. By changing the position to start writing in the main scanning direction for each line in accordance with an angle of the leading edge or the trailing edge of transfer sheet P as stated above, a slanted electrostatic latent image is formed on photoreceptor drum  61 , and a slanted image is transferred onto transfer sheet P accordingly. 
   Specifically, the start writing position determining section  5 D determines a start writing position based on the following expression (1).
 
Start writing position for each line=Position of start writing standard−25.4 (mm)/( I ×tan( A ))  Expression (1)
 
   In this case, the position of start writing standard is an original start writing position established in advance. Further, I represents the resolution which is represented by the number of lines in the sub-scanning direction per one inch. For example, when the resolution is 600 (dpi), I is 600. 
   The start writing position determining section  5 D determines the start writing position for each line in accordance with the aforesaid expression (1), and outputs coordinate information showing the aforesaid position to writing controller  5 B. In the example shown in  FIG. 8 , the start writing position determining section  5 D outputs coordinate information of start writing position P 1  for the first line, coordinate information of start writing position P 2  for the second line, coordinate information of start writing position P 3  for the third line . . . to the writing controller  5 B. After receiving coordinate information showing the start writing position for each line, the writing controller  5 B causes image writing section  50  to form an electrostatic latent slanted by angle “+A°” on photoreceptor drum  61 . 
   An electrostatic latent image formed on photoreceptor drum  61  is transferred onto transfer sheet P. Due to this, the image which is slanted by “+A°” from an angle that is originally transferred is formed on transfer sheet P. By changing the start writing position in the main scanning direction as stated above, it is possible to form an image by changing a relative position between transfer sheet P and an image. 
   Even for the images to be formed on the reverse side of transfer sheet P, the position to start writing images is changed in accordance with a skew of the trailing edge, and an electrostatic latent image is formed on photoreceptor drum  61 . Then, when the electrostatic latent image is transferred onto transfer sheet P, the image which is slanted by “+B°” or “−B°” from an angle that is originally transferred is formed on transfer sheet P. 
   (Operations) 
   Next, operations (image forming method) of an image forming apparatus relating to the third embodiment will be explained as follows, referring to  FIG. 9 .  FIG. 9  is a flow chart for illustrating a series of operations of an image forming apparatus relating to the third embodiment of the invention. 
   (step S 30 ) 
   First, in step S 30 , an operator inputs information showing an outer shape of transfer sheet P, namely, information showing a skew (angle) of transfer sheet P by using operation section  1 . Specifically, in the same way as in the aforesaid first embodiment, “+A°” or “−A°” is inputted as a skew (angle) of the leading edge of transfer sheet P and “+B°” or “−B°” is inputted as a skew (angle) of the trailing edge. In this case, it is assumed that “+A°” is inputted as a skew of the leading edge and “+B°” is inputted as a skew of the trailing edge. A skew (angle) of transfer sheet P inputted at operation section  1  is stored in correction data storage section  3  as a correction value for the skew. 
   (step S 31 ) 
   In step S 31 , the start writing position determining section  5 D reads, from correction data storage section  3 , a correction value (angle: +A°) for the surface, and determines a start writing position for each line in the main scanning direction, in accordance with the aforesaid expression (1). Then, the start writing position determining section  5 D outputs information (coordinate information) showing a start writing position for each line to writing controller  5 B. 
   (step S 32 ) 
   Next, in step S 32 , an electrostatic latent image is formed on photoreceptor drum  61 . In this case, the writing controller  5 B controls image writing section  50  based on the start writing position for each line determined by the start writing position determining section  5 D, to form an electrostatic latent on photoreceptor drum  61 . Owing to this, an electrostatic latent image that is slanted from the original start writing standard by an angle “+A°” is formed on photoreceptor drum  61 . 
   (step S 33 ) 
   Then, in step S 33 , a toner image is transferred onto the surface of transfer sheet P by image forming section  60 , and the toner image thus transferred is fixed by fixing section  70 . Owing to this, an image that is slanted from the original transferred angle by an angle “+A°” is formed on transfer sheet P. 
   (step S 34 ) 
   Then, for the purpose of forming an image on the reverse side, transfer sheet P is sent again to image forming section  6  under the condition that the transfer sheet P is reversed by reversing path  84 , reversing and conveying roller  85  and reversing and conveying path  86 . 
   (step S 35 ) 
   In step S 35 , the start writing position determining section  5 D reads, from correction data storage section  3 , a correction value (angle: +B°) for the reverse side and determines a start writing position for each line in the main scanning direction, in accordance with the aforesaid expression (1). Then, the start writing position determining section  5 D outputs information (coordinate information) showing a start writing position for each line to writing controller  5 B. 
   (step S 36 ) 
   Next, in step S 36 , an electrostatic latent image is formed on photoreceptor drum  61 . In this case, the writing controller  5 B controls image writing section  50  based on the start writing position for each line determined by the start writing position determining section  5 D, to form an electrostatic latent on the photoreceptor drum. Owing to this, an electrostatic latent image that is slanted from the original start writing standard by an angle “+B°” is formed on photoreceptor drum  61 . 
   (step S 37 ) 
   Then, in step S 37 , a toner image is formed on the reverse side of transfer sheet P by image forming section  60 , and the toner image thus transferred is fixed by fixing section  70 . Owing to this, an image that is slanted from the original transferred angle by an angle “+B°” is formed on transfer sheet P. 
   (step S 38 ) 
   The transfer sheet P on which image fixing has been completed on each of the surface and the reverse side, is ejected by sheet-ejection roller  81  onto sheet-ejection tray  82 . 
   As stated above, in the image forming apparatus relating to the third embodiment, an image is formed by changing a position to start writing an image, in accordance with an outer shape of a transfer sheet, specifically with a skew of the leading edge of transfer sheet P and a skew of the trailing edge, thus, images formed on both surfaces can be aligned in terms of position highly accurately, by canceling positional difference between the image on the surface and the image on the reverse side. 
   Further, by correcting a skew of each transfer sheet P by inputting information showing a skew (angle) of each transfer sheet P, it is possible to correct a skew of transfer sheet P even when each transfer sheet P fluctuates slightly in terms of a shape. 
   Incidentally, it is possible to store information showing a skew (angle) of transfer sheet P inputted by operation section  1  in correction data storage section  3 , and thereby to correct a skew of the succeeding transfer sheet P by using the correction value (angle) stored in the correction data storage section  3 , when forming an image on the succeeding transfer sheet P. 
   Fourth Embodiment 
   Next, structures of the image forming apparatus relating to the fourth embodiment of the invention will be explained as follows, referring to  FIG. 10  which is a block diagram showing a structure of the image forming apparatus relating to the fourth embodiment of the invention. 
   In the same way as in the aforesaid first embodiment, the image forming apparatus relating to this embodiment is composed of image reading section  20 , sheet-feeding tray  30 , sheet supply section  40 , image writing section  50 , image forming section  60 , fixing section  70  and sheet ejection section  80  shown in  FIG. 18 . In the same way as in the first embodiment, the image forming apparatus relating to the fourth embodiment is further composed of system controller  2  and correction data storage section  3 . 
   In the fourth embodiment, image reading section  20  reads an outer shape of transfer sheet P, and angle calculating section  5 C obtains a skew (angle) of transfer sheet P based on information showing the aforesaid outer shape, in the same way as in the second embodiment. Information showing a skew (angle) of the leading edge of transfer sheet P and information showing a skew (angle) of the trailing edge are stored in correction data storage section  3 . 
   Further, in the fourth embodiment, a position of an image on the surface and a position of an image on the reverse side are caused to agree with each other, by changing a position to start writing an image in accordance with a skew of transfer sheet P, in the same way as in the third embodiment. In other words, the start writing position determining section  5 D reads, from correction data storage section  3 , information showing a skew (angle) of the surface or of the reverse side, and determines a start writing position for each line in the main scanning direction corresponding to each angle. Specifically, as explained in the third embodiment, the start writing position determining section  5 D obtains the start writing position for each line in the main scanning direction, following expression (1). Then, writing controller  5 B controls image writing section  50  in accordance with the position to start writing determined by the start writing position determining section  5 D to form an electrostatic latent image on photoreceptor drum  61 . 
   (Operations) 
   Next, operations (image forming method) of an image forming apparatus relating to the fourth embodiment of the invention will be explained as follows, referring to  FIG. 11 .  FIG. 11  is a flow chart for illustrating a series of operations of an image forming apparatus relating to the fourth embodiment of the invention. 
   (step S 40 ) 
   First, in step S 40 , an outer shape of transfer sheet P is read by image reading section  20 , and information showing that outer shape is outputted to angle calculating section  5 C. 
   (step S 41 ) 
   Next, in step S 41 , the angle calculating section  5 C calculates a skew of transfer sheet P based on information showing the outer shape of transfer sheet P. Specifically, the angle calculating section  5 C calculates a skew (angle) of the leading edge and a skew (angle) of the trailing edge of transfer sheet P. In this case, it is assumed that “+A°” represents a skew of the leading edge of transfer sheet P and “+B°” represents a skew of the trailing edge. Information showing these skews is stored in correction data storage section  3 . 
   (step S 42 ) 
   In step S 42 , the start writing position determining section  5 D reads, from correction data storage section  3 , a correction value (angle: +A°) for the surface, and determines a start writing position for each line in the main scanning direction, in accordance with the aforesaid expression (1). Then, information (coordinate information) showing the start writing position for each line is outputted to writing controller  5 B. 
   (step S 43 ) 
   Next, in step S 43 , an electrostatic latent image is formed on photoreceptor drum  61 . In this case, the writing controller  5 B controls image writing section  50  based on the start writing position for each line determined by the start writing position determining section  5 D, to form an electrostatic latent on photoreceptor drum  61 . Owing to this, an electrostatic latent image that is slanted from the original start writing standard by an angle “+A°” is formed on photoreceptor drum  61 . 
   (step S 44 ) 
   Then, in step S 44 , a toner image is transferred onto the surface of transfer sheet P by image forming section  60 , and the toner image thus transferred is fixed by fixing section  70 . Owing to this, an image that is slanted from the original transferred angle by an angle “+A°” is formed on transfer sheet P. 
   (step S 45 ) 
   Then, for the purpose of forming an image on the reverse side, transfer sheet P is sent again to image forming section  6  under the condition that the transfer sheet P is reversed by reversing path  84 , reversing and conveying roller  85  and reversing and conveying path  86 . 
   (step S 46 ) 
   In step S 46 , the start writing position determining section  5 D reads, from correction data storage section  3 , a correction value (angle: +B°) for the reverse side and determines a start writing position for each line in the main scanning direction, in accordance with the aforesaid expression (1). Then, information (coordinate information) showing a start writing position for each line is outputted to writing controller  5 B. 
   (step S 47 ) 
   Next, in step S 47 , an electrostatic latent image is formed on photoreceptor drum  61 . In this case, the writing controller  5 B controls image writing section  50  based on the start writing position for each line determined by the start writing position determining section  5 D, to form an electrostatic latent on the photoreceptor drum. Owing to this, an electrostatic latent image that is slanted from the original start writing standard by an angle “+B°” is formed on photoreceptor drum  61 . 
   (step S 48 ) 
   Then, in step S 48 , a toner image is formed on the reverse side of transfer sheet P by image forming section  60 , and the toner image thus transferred is fixed by fixing section  70 . Owing to this, an image that is slanted from the original transferred angle by an angle “+B°” is formed on transfer sheet P. 
   (step S 49 ) 
   The transfer sheet P on which image fixing has been completed on each of the surface and the reverse side, is ejected by sheet-ejection roller  81  onto sheet-ejection tray  82 . 
   As stated above, in the image forming apparatus relating to the fourth embodiment, an image is formed by changing a position to start writing an image, in accordance with an outer shape of a transfer sheet, specifically with a skew of the leading edge of transfer sheet P and a skew of the trailing edge, thus, images formed on both surfaces can be aligned in terms of position highly accurately, by canceling positional difference between the image on the surface and the image on the reverse side. 
   Further, by correcting a skew of each transfer sheet P by reading a skew (angle) for each transfer sheet P, it is possible to correct a skew of each transfer sheet P even when each transfer sheet P fluctuates slightly in terms of a shape. 
   Incidentally, it is possible to store information showing a skew (angle) of transfer sheet P read by image reading section  20  in correction data storage section  3 , and thereby to correct a skew of the succeeding transfer sheet P by using the correction value (angle) stored in the correction data storage section  3 , when forming an image on the succeeding transfer sheet P. 
   Fifth Embodiment 
   Next, structures of the image forming apparatus relating to the fifth embodiment of the invention will be explained as follows, referring to  FIG. 12-FIG .  14 .  FIG. 12  is a block diagram showing a structure of the image forming apparatus relating to the fifth embodiment of the invention.  FIG. 13  is a top view showing an arrangement of a photodetector.  FIG. 14  is a diagram showing an output wave form of the photodetector. 
   In the same way as in the aforesaid first embodiment, the image forming apparatus relating to the fifth embodiment is composed of image reading section  20 , sheet-feeding tray  30 , sheet supply section  40 , image writing section  50 , image forming section  60 , fixing section  70  and sheet ejection section  80  shown in  FIG. 18 . In the same way as in the first and second embodiments, the image forming apparatus relating to the fifth embodiment is further composed of system controller  2 , correction data storage section  3 , drive controller  4 , motor M and registration roller  43 . 
   An image forming apparatus relating to the fifth embodiment is equipped with detector PS 1  as a first detecting section and detector PS 2  as a second detecting section which detect a skew of transfer sheet P, then, it detects a skew of transfer sheet P by detector PS 1  and detector PS 2  before an image is formed on transfer sheet P, and corrects a skew of transfer sheet P for the conveyance direction based on results of the detection. 
   Now, an example of arrangement of detectors detecting a skew of transfer sheet P will be explained as follows, referring to  FIG. 13 . Each of detector PS 1  and detector PS 2  is composed, for example, of a photo-sensor, and detects transfer sheet P. The detector PS 1  and the detector PS 2  are arranged, for example, between the registration roller  43  and photoreceptor drum  61 . Further, the detector PS 1  and the detector PS 2  are arranged side by side in the main scanning direction, and a distance between them is distance L. Detector PS 3  is composed of an image sensor that detects an edge face of transfer sheet P. 
   Now, an output waveform of each detector will be explained with reference to  FIG. 14 . In  FIG. 14 , counter CLK is a standard clock for detecting a length of transfer sheet P. An output level of each of the detector PS 1  and the detector PS 2  is ranked into level “L” and level “H” depending on presence of transfer sheet P. For example, when neither detector PS 1  nor detector PS 2  detects transfer sheet P, the output level is “H”, and when they detect transfer sheet P, the output level is “L”. 
   Outputs from the detector PS 1  and the detector PS 2  make is possible to judge a skew of transfer sheet P from the conveyance direction and a skew of the leading edge or of the trailing edge. For example, when the leading edge of transfer sheet P is perpendicular to the conveyance direction as shown in  FIG. 13 , namely, when an angle of the skew is “0°”, the detector PS 1  and the detector PS 2  start detecting transfer sheet P simultaneously at time t 1 . In  FIG. 14 , an output of the detector PS 1  and that of the detector PS 2  are changed from level “H” to level “L” simultaneously at time t 1 . 
   On the other hand, when the trailing edge of transfer sheet P is skewed from the direction perpendicular to the conveyance direction, as shown in  FIG. 13 , the detector PS 1  and the detector PS 2  are different each other in terms of a length of a time period for detecting transfer sheet P. In  FIG. 14 , the detector PS 1  detects transfer sheet P for a time period up to the moment of time t 2 , while, the detector PS 2  detects transfer sheet P for a time period up to the moment of time t 3  which is longer than the time period up to time t 2 . In other words, since the trailing edge is skewed, the detector PS 1  and the detector PS 2  which are arranged to be away from each other by distance L are different each other in terms of a time period for detecting transfer sheet P. Then, a length of transfer sheet P in the conveyance direction at the position where the detector PS 1  is arranged is obtained from a conveyance speed for transfer sheet P and from a length of a time period for the detector PS 1  to detect transfer sheet P, and a length of transfer sheet P in the conveyance direction at the position where the detector PS 2  is arranged is obtained from a conveyance speed for transfer sheet P and from a length of a time period for the detector PS 2  to detect transfer sheet P. 
   Skew direction signal represents compounded output of an output of the detector PS 1  and an output of the detector PS 2 . When the leading edge or the trailing edge of transfer sheet P is skewed, an output level is “L”. In examples shown in  FIG. 13  and  FIG. 14 , there is generated a difference between an output of the detector PS 1  and that of the detector PS 2  because the trailing edge of transfer sheet P is skewed, and the skew direction signal is on level “L” at the trailing edge. 
   Results of the detections by the detector PS 1  and the detector PS 2  are outputted to a calculating section of image processing section  5 . The calculation section is composed of length calculating section  5 E, skew calculating section  5 F and correction amount calculating section  5 G, then, a skew of transfer sheet P is obtained based on the results of the detections by the detector PS 1  and the detector PS 2 , and a correction value for correcting the skew is obtained. 
   After receiving outputs of the detectors PS 1  and PS 2 , the length calculating section  5 E obtains a length of transfer sheet P in the conveyance direction at the position where the detector PS 1  is installed, from the conveyance speed for transfer sheet P that is set and from a length of a time period through which the transfer sheet P is detected by the detector PS 1 . Further, the length calculation section  5 E obtains a length of transfer sheet P in the conveyance direction at the position where the detector PS 2  is installed, from the conveyance speed for transfer sheet P that is set and from a length of a time period through which the transfer sheet P is detected by the detector PS 2 . For example, when length a represents a length of transfer sheet P in the conveyance direction at the position where the detector PS 1  is installed, and length b represents a length of transfer sheet P in the conveyance direction at the position where the detector PS 2  is installed, as shown in  FIG. 13 , the length calculation section  5 E obtains length a and length b. 
   After receiving outputs of the detectors PS 1  and PS 2 , the skew calculating section  5 F obtains a skew of transfer sheet P based on the aforesaid outputs. Specifically, the skew calculating section  5 F compounds outputs from the detectors PS 1  and PS 2 , and obtains a skew of transfer sheet P from the compounded signals. For example, an output of the detector PS 1  and an output of the detector PS 2  are compounded as shown in  FIG. 14  to obtain signals in the direction of a skew, and when the signal of the skew is on level “L”, transfer sheet P is judged to be skewed. In the example, shown in  FIG. 14 , the trailing edge of transfer sheet P is judged to be skewed. An occasion where the trailing edge of transfer sheet P is skewed will be explained as follows. 
   Further, the skew calculating section  5 F calculates a difference between length a and length b, and that difference is assumed to be skew amount c. Then, the skew calculating section  5 F obtains angle α of the trailing edge by using distance L between the detectors and the skew amount c. 
   Since the relation of tan (α)=skew amount c/distance L holds, angle α=tan −1  (skew amount c/distance L) holds. 
   Further, the skew calculating section  5 F judges the direction of skew of the trailing edge depending on the relation in terms of a size between length a and length b. 
   Further, when detector PS 1  and detector PS 2  are arranged between the registration roller  43  and photoreceptor drum  61 , it is also possible to judge whether the skew of the leading edge of transfer sheet P has been corrected properly by the registration roller  43  or not. In other words, after the leading edge of transfer sheet P hits a nip portion of the registration roller  43 , the transfer sheet P is conveyed to photoreceptor drum  61  at prescribed timing. By arranging the detectors PS 1  and PS 2  between the registration roller  43  and photoreceptor drum  61 , it is possible to detect how the transfer sheet P after being subjected to registration processing by the registration roller  43  is skewed. 
   For example, a difference between the moment when detector PS 1  starts detecting transfer sheet P and the moment when detector PS 2  starts detecting transfer sheet P corresponds to the skew of the leading edge, and the skew calculating section  5 F obtains skew amount d of the leading edge from the conveyance speed for transfer sheet P and from the time difference. On the other hand, when the detector PS 1  and the detector PS 2  start detecting transfer sheet P simultaneously, the trailing edge of transfer sheet P is judged to be perpendicular to the conveyance direction. For example, as shown in  FIG. 14 , when the detector PS 1  and the detector PS 2  start detecting transfer sheet P simultaneously at the moment of time t 1 , the leading edge of transfer sheet P is judged to be perpendicular to the conveyance direction. 
   Based on length a, length b and angle α, the skew calculating section  5 F judges transfer sheet P to be in any one of the state 1-the state 3 shown below. 
   (State 1) 
   When length a is equal to length b, and angle α is equal to 0[°], the skew calculating section  5 F judges that a shape of transfer sheet P is a rectangle, and the transfer sheet P is conveyed to be in parallel with the conveyance direction. In other words, a judgment is formed that the transfer sheet P itself has no distortion and is conveyed to be in parallel to the conveyance direction. 
   (State 2) 
   When length a is equal to length b, and angle α is not equal to 0[°], the skew calculating section  5 F judges that transfer sheet P is conveyed obliquely relative to the conveyance direction. 
   (State 3) 
   When length a is not equal to length b, and angle α is not equal to 0[°], the skew calculating section  5 F judges that transfer sheet P itself has a distortion and transfer sheet P is skewed. In this case, the trailing edge of transfer sheet P is judged to be skewed, because angle α of the trailing edge is not equal to 0. 
   By using length a and length b of transfer sheet P, distance L between detector PS 1  and detector PS 2  and constant M determined by the mechanism of the registration roller  43 , correction amount calculating section  5 G obtains a distance for the registration roller  43  to be moved when fulcrum B serves as an axis. In this case, constant M corresponds, for example, to the distance from fulcrum B to drive source input section A shown in  FIG. 3  ( a ). An amount of correction obtained by the correction amount calculating section  5 G will be explained later. 
   Incidentally, though a skew of transfer sheet P is corrected by changing a slant of the registration roller  43  in the fifth embodiment, it is also possible to cause positions of images for the surface and the reverse side to agree each other by changing a position to start writing an image as in the third and fourth embodiments. 
   (Operations) 
   Next, operations (image forming method) of an image forming apparatus relating to the fifth embodiment of the invention will be explained as follows, referring to  FIGS. 15 and 16 . Each of  FIGS. 15 and 16  is a flow chart for illustrating a series of operations of an image forming apparatus relating to the fifth embodiment of the invention. Processing in the fifth embodiment is divided into an occasion where the registration processing for the leading edge of transfer sheet P has been carried out normally and an occasion where the registration processing for the leading edge of transfer sheet P is insufficient. First, the occasion where the registration processing for the leading edge has been carried out normally will be explained with reference to  FIG. 15 , and next, the occasion where the registration processing was insufficient will be explained, referring to  FIG. 16 . 
   First, the processing where the registration processing for the leading edge has been carried out normally will be explained with reference to  FIG. 15 . In this case, detector PS 1  and detector PS 2  may either be arranged between the registration roller  43  and photoreceptor drum  61 , or be arranged on this side of the registration roller  43 . In this case, there will be explained an occasion where the detector PS 1  and the detector PS 2  are arranged between the registration roller  43  and photoreceptor drum  61 . 
   (step S 50 ) 
   First, for forming an image on the surface of transfer sheet P, a skew of transfer sheet P relative to the conveyance direction is corrected by causing the leading edge of transfer sheet P to hit a nip portion of the registration roller  43 . After that, the transfer sheet P is conveyed to photoreceptor drum  61  of image forming section  60  at prescribed timing. 
   (step S 51 ) 
   After that, detector PS 1  and detector PS 2  arranged between the registration roller  43  and photoreceptor drum  61  detect transfer sheet P, and a calculation section obtains a skew of transfer sheet P based on the results of the detection. 
   The results of the detections by the detectors PS 1  and PS 2  are outputted to length calculating section  5 E. The length calculating section  5 E obtains length a of transfer sheet P in the conveyance direction at the position where the detector PS 1  is installed, from the conveyance speed of transfer sheet P and from a length of detection time of detector PS 1 , and further obtains length b of transfer sheet P in the conveyance direction at the position where the detector PS 2  is installed, from the conveyance speed of transfer sheet P and from a length of detection time of detector PS 2 . 
   After receiving outputs of the detectors PS 1  and PS 2 , skew calculating section  5 F compounds the outputs of the detectors PS 1  and PS 2 , and judges a skew of transfer sheet P from the compounded signals. In the examples shown in  FIG. 13  and  FIG. 14 , the leading edge of transfer sheet P is judged to have no skew, and the trailing edge is judged to have a skew. 
   Further, the skew calculating section  5 F calculates a difference between length a and length b, and that difference is made to be skew amount c of the trailing edge. Then, the skew calculating section  5 F obtains angle α of the trailing edge by using distance L between detectors and skew amount c. Further, the skew calculating section  5 F judges a direction of a skew of the trailing edge by comparing length a with length b. 
   After receiving skew amount c (=length a−length b of the trailing edge from the skew calculating section  5 F, correction amount calculating section  5 G uses distance L and constant M to obtain a distance (correction amount X 1 ) through which the registration roller  43  is moved. This correction amount X 1  corresponds to the value for correcting a skew (angle α) of the trailing edge. This correction amount X 1  is obtained from the following expression (2).
 
Correction amount X 1:(Length  a −length  b )×constant  M /distance  L   Expression (2)
 
   Incidentally, correction amount X 1  is expressed by the expression (2) above, because the proportional relation of correction amount X 1 : (length a−length b)=constant M: distance L exists. 
   (step S 52 ) 
   Then, in step S 52 , image forming section  60  forms a toner image on the surface of transfer sheet P, and fixing section  70  fixes the transferred toner image. 
   (step S 53 ) 
   Then, for forming an image on the reverse side, the transfer sheet P is conveyed again to image forming section  60  under the condition that the transfer sheet P is reversed by reversing path  84 , reversing conveyance roller  85  and reversing conveyance path  86 . 
   (step S 54 , step S 56 ) 
   When length a is judged by skew calculating section  5 F to be equal to length b (step S 54 , Yes), a skew of transfer sheet P relative to the conveyance direction is corrected (step S 56 ) by causing transfer sheet P to hit a nip portion of the registration roller  43  without slanting the registration roller  43 . Because of the relation of length a=length b, the state of transfer sheet P corresponds to state 1 or state 2. Accordingly, if the state of transfer sheet P is judged by skew calculating section  5 F to correspond to state 1 or state 2, a skew of the transfer sheet P relative to the conveyance direction is corrected, without slanting the registration roller  43 . 
   (step S 54 , step S 55 , step S 56 ) 
   On the other hand, when length a is judged by skew calculating section  5 F to be different from length b (step S 54 , No), drive controller  4  causes motor M to rotate to slant the registration roller  43  to angle α (step S 55 ). In this case, drive controller  4  causes motor M to rotate in accordance with correction amount X 1  obtained by correction amount calculating section  5 G, to slant the registration roller  43  to angle α by swiveling the registration roller  43  by correction amount X 1  around fulcrum B representing an axis. After that, transfer sheet P is caused to hit a nip portion of the registration roller  43 , to correct a skew of transfer sheet P relative to the conveyance direction (step S 56 ). Because of the relation of length a≠length b, the state of transfer sheet P corresponds to the state 3. Therefore, if the state of transfer sheet P is judged by skew calculating section  5 F to correspond to the state 3, a skew of transfer sheet P relative to the conveyance direction is corrected by slanting the registration roller  43  to angle α. 
   (step S 57 ) 
   Then, in step S 57 , image forming section  60  forms a toner image on the reverse side of transfer sheet P, and fixing section  70  fixes the transferred toner image. 
   (step S 58 ) 
   The transfer sheet P on which image fixing has been completed on each of the surface and the reverse side, is ejected by sheet-ejection roller  81  onto sheet-ejection tray  82 . 
   In the image forming apparatus relating to the fifth embodiment, as stated above, a skew of the leading edge or the trailing edge of transfer sheet P is detected by using detector PS 1  and detector PS 2 , and an slant of the registration roller  43  is changed based on the aforesaid detected skew to correct a skew of transfer sheet P relative to the conveyance direction, thus, images formed on both surfaces can be aligned in terms of position highly accurately, by canceling positional difference between the image on the surface and the image on the reverse side. 
   Further, a skew (angle) of transfer sheet P is detected by detector PS 1  and detector PS 2 , and a skew of transfer sheet P is corrected based on the results of the detection, and thereby, positions of images on the surface and on the reverse side can be caused to agree, by correcting a skew of transfer sheet P on a real time basis. 
   Further, by correcting a skew of each transfer sheet P by detecting a skew (angle) of each transfer sheet P, it is possible to correct a skew of each transfer sheet P even when each transfer sheet P varies slightly in terms of a shape. 
   Incidentally, it is possible to store information showing a skew (angle) of transfer sheet P detected by detector PS 1  and detector PS 2  in correction data storage section  3 , and thereby to correct a skew of the succeeding transfer sheet P by using a correction value (angle) stored in the correction data storage section  3 , when forming an image on the succeeding transfer sheet P. 
   Next, an occasion where registration processing for the leading edge of transfer sheet P is insufficient will be explained, referring to  FIG. 16 . In this case, detector PS 1  and detector PS 2  are arranged between the registration roller  43  and photoreceptor drum  61 . 
   (step S 60 ) 
   For forming an image on the surface of transfer sheet P, a skew of transfer sheet P relative to the conveyance direction is corrected first, by causing the leading edge of transfer sheet P to hit a nip portion of the registration roller  43 . After that, the transfer sheet P is conveyed to photoreceptor drum  61  of image forming section  60  at prescribed timing. 
   (step S 61 ) 
   Then, detector PS 1  and detector PS 2  arranged between the registration roller  43  and photoreceptor drum  61  detect transfer sheet P, and based on the results of this detection, a calculation section obtains a skew of transfer sheet P. 
   After receiving outputs from detector PS 1  and detector PS 2 , skew calculating section  5 F judges whether the leading edge of transfer sheet P is skewed or not, and when it is skewed, its skew amount d is obtained. For example, when the detectors PS 1  and PS 2  started detecting at the same time, the leading edge of transfer sheet P is judged to be perpendicular to the conveyance direction to be free from a skew. On the other hand, when the time for detector PS 1  to start detecting transfer sheet P is different from that for detector PS 2  to start detecting transfer sheet P, skew amount d of the leading edge is obtained from a difference of the time and from the conveyance speed for transfer sheet P. 
   After receiving skew amount d of the leading edge from the skew calculating section  5 F, correction amount calculating section  5 G uses distance L and constant M to obtain a distance (correction amount X 2 ) through which the registration roller  43  is moved. This correction amount X 2  corresponds to the skew of the leading edge. This correction amount X 2  is obtained from the following expression (3).
 
Correction amount  X 2=skew amount  d ×constant  M /distance  L   Expression (3)
 
   Incidentally, correction amount X 2  is expressed by the expression (3) above, because the proportional relation of correction amount X 2 : skew amount d=constant M: distance L exists. 
   (step S 62 ) 
   Since the leading edge of transfer sheet P is skewed, the skew of the leading edge is corrected by slanting the registration roller  43  while the transfer sheet P is passing through the registration roller  43 , for correcting the skew of the leading edge. Drive controller  4  causes motor M to rotate to slant the registration roller  43  (step S 62 ). In this case, the drive controller  4  causes motor M to rotate in accordance with correction amount X 2  obtained by correction amount calculating section  5 G, to slant the registration roller  43  by moving it. 
   Further, detector PS 1  and detector PS 2  keep detecting transfer sheet P while skewing transfer sheet P with the registration roller  43 . Results of the detections by the detectors PS 1  and PS 2  are outputted to length calculating section  5 E. The length calculating section  5 E obtains length a of transfer sheet P in the conveyance direction at the position where detector PS 1  is installed, from the conveyance speed for transfer sheet P and from a length of a time period for detector PS 1  to detect, and further obtains length b of transfer sheet P in the conveyance direction at the position where detector PS 2  is installed, from the conveyance speed for transfer sheet P and from a length of a time period for detector PS 2  to detect. 
   After receiving outputs of the detectors PS 1  and PS 2 , skew calculating section  5 F compounds outputs from the detectors PS 1  and PS 2 , and obtains a skew of the trailing edge of transfer sheet P from the compounded signals. 
   Correction amount calculating section  5 G obtains a correction amount for correcting a skew of the trailing edge. Incidentally, the correction amount for correcting a skew of the trailing edge varies depending on the occasion where length a is equal to length b and the occasion where length a is different from length b. 
   (step S 63 ) 
   Then, in step S 63 , image forming section  60  forms a toner image on the surface of transfer sheet P, and fixing section  70  fixes the transferred toner image. 
   (step S 64 ) 
   Then, for forming an image on the reverse side, the transfer sheet P is conveyed again to image forming section  60  under the condition that the transfer sheet P is reversed by reversing path  84 , reversing conveyance roller  85  and reversing conveyance path  86 . 
   (step S 65 , step S 66 , step S 68 ) 
   When length a is judged by skew calculating section  5 F to be equal to length b (step S 65 , Yes), drive controller  4  causes motor M to rotate to tilt the registration roller  43  (step S 66 ). In this case, the drive controller  4  causes motor M to rotate in accordance with correction amount X 2  obtained by correction amount calculating section  5 G, to tilt the registration roller  43  by moving it. In other words, the registration roller  43  is tilted in accordance with an correction amount that is the same as correction amount X 2  for correcting the skew of the leading edge. After that, a skew of transfer sheet P relative to the conveyance direction is corrected (step S 68 ) by causing transfer sheet P to hit a nip portion of the registration roller  43 . 
   In the meantime, because of the relation of length a=length b, the state of transfer sheet P corresponds to state 1 or state 2. Accordingly, if the state of transfer sheet P is judged by skew calculating section  5 F to correspond to state 1 or state 2, a skew of the transfer sheet P relative to the conveyance direction is corrected, by tilting the registration roller  43  in accordance with correction amount X 2 . 
   (step S 65 , step S 67 , step S 68 ) 
   On the other hand, when length a is judged by skew calculating section  5 F to be different from length b (step S 65 , No), the registration roller  43  is tilted in accordance with correction amount X 3  obtained by the following expression (4)(step S 67 ).
 
Correction amount  X 3=(length  a −length  b +skew amount  d )×constant  M /distance  L   Expression (4)
 
   This correction amount X 3  is obtained by correction amount calculating section  5 G. 
   When forming an image on the reverse side of transfer sheet P, it is possible to make a positional slippage of an image on the surface and that on the reverse side to offset each other, and thereby, to align images formed on both sides highly accurately, by combining a skew amount of the trailing edge of transfer sheet P and a correction amount for correcting a skew of the leading edge to be correction amount X 3  for correcting a skew of the trailing edge. 
   Drive controller  4  causes motor M to rotate in accordance with correction amount X 3  obtained by correction amount calculating section  5 G, to tilt the registration roller  43  by moving it. After that, a skew of transfer sheet P relative to the conveyance direction is corrected (step S 68 ) when transfer sheet P is caused to hit a nip portion of the registration roller  43 . 
   Meanwhile, because of the relation of length a≠length b, the state of transfer sheet P corresponds to state 3. Accordingly, if the state of transfer sheet P is judged by skew calculating section  5 F to correspond to state 3, a skew of the transfer sheet P relative to the conveyance direction is corrected, by tilting the registration roller  43  to angle α. 
   (step S 69 ) 
   Then, in step S 69 , image forming section  60  forms a toner image on the reverse side of transfer sheet P, and fixing section  70  fixes the transferred toner image. 
   (step S 70 ) 
   The transfer sheet P on which image fixing has been completed on each of the surface and the reverse side, is ejected by sheet-ejection roller  81  onto sheet-ejection tray  82 . 
   In the image forming apparatus relating to the fifth embodiment, as stated above, a skew of the leading edge or the trailing edge of transfer sheet P is detected by using detector PS 1  and detector PS 2 , and a slant of the registration roller  43  is changed based on the aforesaid detected skew to correct a skew of transfer sheet P relative to the conveyance direction, thus, images formed on both surfaces can be aligned in terms of position highly accurately, by canceling positional difference between the image on the surface and the image on the reverse side. 
   Further, a skew (angle) of transfer sheet P is detected by detector PS 1  and detector PS 2 , and a skew of transfer sheet P is corrected based on the results of the detection, and thereby, positions of images on the surface and on the reverse side can be caused to agree, by correcting a skew of transfer sheet P on a real time basis. 
   Further, by correcting a skew of each transfer sheet P by detecting a skew (angle) of each transfer sheet P, it is possible to correct a skew of each transfer sheet P even when each transfer sheet P varies slightly in terms of a shape. 
   Further, it is possible to store information showing a skew (angle) of transfer sheet P detected by detector PS 1  and detector PS 2  in correction data storage section  3 , and thereby to correct a skew of the succeeding transfer sheet P by using a correction value (angle) stored in the correction data storage section  3 , when forming an image on the succeeding transfer sheet P. 
   Though a skew of transfer sheet P relative to the conveyance direction was corrected by changing a tilt of the registration roller  43  in the fifth embodiment, it is also possible to change a position to start writing an electrostatic latent image to be formed on photoreceptor drum  61  in accordance with a skew of the leading edge or trailing edge, in the same way as in the third and fourth embodiments. Even when a position to start writing an image is changed in accordance with a skew of the leading edge or the trailing edge as stated above, it is still possible to align positions of images formed on both sides with high-precision. 
   (Variations) 
   Next, a variation of a registration roller for correcting a skew of transfer sheet P will be explained as follows, referring to  FIG. 17 .  FIG. 17  is a top view showing a schematic structure of a registration roller. In the first-fifth embodiments stated above, the registration roller  43  is tilted or a position to start writing an image is changed to align positions of images formed on the surface and the reverse side. However, in the present invention, it is also possible to align positions of images on the surface and the reverse side, by another means. 
   For example, registration roller  45  equipped with roller  45 A and roller  45 B is used as shown in  FIG. 17 . By making rotation rate r 1  of roller  45 A to be different from rotation rate r 2  of roller  45 B, transfer sheet P conveyed by registration roller  45  can be skewed to either direction. 
   Specifically, when rotation rate r 1  of roller  45 A is made to be greater than rotation rate r 2  of roller  45 B, a portion passing through roller  45 A on transfer sheet P is conveyed faster than a portion passing through roller  45 B, whereby, transfer sheet P is skewed to one direction in the course of passing through registration roller  45 . Therefore, it is possible to correct a skew of the leading edge or the trailing edge by skewing transfer sheet P in one direction by changing the rotation rate of roller  45 A or roller  45 B in accordance with a skew (angle) of the leading edge and that of the trailing edge of transfer sheet P, which has been used in the aforesaid first-fifth embodiments. The control of rotation rates of roller  45 A and roller  45 B is carried out by drive controller  4 . The drive controller  4  makes rotation rate r 1  of roller  45 A to be different from rotation rate r 2  of roller  45 B in accordance with a skew (angle) of the leading edge or the trailing edge. Due to this, transfer sheet P is skewed in either one direction while it is conveyed by registration roller  45 , resulting in correction of the skew of the transfer sheet P. 
   Further, as another variation, it is also possible to arrange so that a skew of the leading edge or the trailing edge of transfer sheet P is corrected when drive controller  4  changes pressure of a conveyance roller other than the registration roller  43  in accordance with a skew of the leading edge or the trailing edge of transfer sheet P.