Abstract:
An image forming apparatus comprises a photoreceptor having an image carrying surface, an exposing device to imagewise expose the image carrying surface to form a latent image, and a developing device to develop the latent image to form a toner image on the image carrying surface, wherein the exposing device is composed of a light source to emit a light beam, a deflector to deflect the light beam so as to form a scanning line with the light beam on the image carrying surface, a cylindrical lens provided between the deflector and the image carrying surface and to focus the light beam on the image carrying surface, and an adjusting mechanism to adjust an inclination of the scanning line and to adjust variance in magnification of an image on the scanning line.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to an image forming apparatus for forming an image wherein a photoreceptor is scanned to be exposed to light by a light beam, and to an assembling method of an exposure unit in the image forming apparatus.  
         [0002]     In the image forming apparatus for forming an image wherein a photoreceptor is scanned to be exposed to light by a light beam, accuracy of incident position of the light beam on the photoreceptor has an influence on image quality. Therefore, an exposure unit is structured so that the light beam may enter at high accuracy.  
         [0003]     In a color image forming apparatus for forming a color image by conducting a plurality of exposures, forming a plurality of images and thereby by superposing the plural images, in particular, it is requested to keep the position stated above accurately.  
         [0004]     Typical one that conducts scanning exposure with a light beam is an exposure unit employing a laser, and with respect to adjustment of the position stated above in a laser exposure unit, there is disclosed in U.S. Pat. No. 2,570,176 official gazette, for example, an adjustment mechanism for adjusting a position and an angle of the exposure unit having a laser light source and a scanning exposure optical system in X, Y and Z directions. This adjustment mechanism adjusts at two points representing both ends of each side of the square exposure unit for each of X, Y and Z directions, and thereby adjusts the incident position of the light beam on the photoreceptor.  
         [0005]     Each of many color image forming apparatuses is equipped with a plurality of exposure units which form a plurality of color images, and these exposure units are provided in the image forming apparatus at high positional accuracy. In particular, it is required that relative positional accuracy between the exposure units is high. If the positional accuracy between the exposure units is not satisfactory, displacement is caused between color images to cause doubling, resulting in deterioration of image quality. The positional relationship between exposure units needs to be maintained during the operation period of the image forming apparatus.  
         [0006]     To satisfy the conditions about positions of the exposure units, a plurality of exposure units have been combined with UV adhesive agents, for example.  
         [0007]     In the adjustment mechanism disclosed in the aforesaid patent publication, there are provided 6 adjustment means in total, because adjustment is made at two points for each of X, Y and Z directions. Therefore, the adjustment is complicated. Further, there has not been disclosed a concrete adjustment mechanism capable of attaining simple structure and high adjustment accuracy, in the prior art.  
         [0008]     Further, in the fixing means to fix between exposure units by adhesive agents, a great many hours are needed for securing desired strength, which has been a problem in manufacture.  
         [0009]     An object of the invention is to solve the aforesaid problems in conventional image forming apparatuses employing a light beam.  
       SUMMARY OF THE INVENTION  
       [0010]     In order to attain the above-described objects, the structure of this invention has been made as follows:  
         [0011]     An image forming apparatus, comprising: 
        a photoreceptor having an image carrying surface;     an exposing device to imagewise expose the image carrying surface to form a latent image; and     a developing device to develop the latent image to form a toner image on the image carrying surface;     the exposing device comprising: 
            a light source to emit a light beam;     a deflector to deflect the light beam so as to form a scanning line with the light beam on the image carrying surface;     a cylindrical lens provided between the deflector and the image carrying surface and to focus the light beam on the image carrying surface; and     an adjusting mechanism to adjust an inclination of the scanning line and to adjust variance in magnification of an image on the scanning line.    
               
 
         [0020]     Further the above objects may be attained by the following structures.  
         [heading-0021]     (Structure 1)  
         [0022]     An image forming apparatus having therein exposure units each being provided with a laser light source and a scanning optical system that deflects light coming from the light source and emits scanning light beam, the exposure units being supported on at least a first supporting point, a second supporting point and a third supporting point, wherein a first adjustment means that adjusts an inclination of the scanning light beam in the primary scanning direction is provided in a first supporting section where the first supporting point is provided, and the inclination in the primary scanning direction is adjusted only by adjustment of the first adjustment means.  
         [heading-0023]     (Structure 2)  
         [0024]     An image forming apparatus having therein exposure units each being provided with a laser light source and a scanning optical system that deflects light coming from the light source and emits scanning light beam, the exposure units being supported on at least a first supporting point, a second supporting point and a third supporting point, wherein a second adjustment means that adjusts variation in magnification of the scanning light beam in the primary scanning direction is provided in a second supporting section where the second supporting point is provided, and the variation in magnification is adjusted only by adjustment of the second adjustment means.  
         [heading-0025]     (Structure 3)  
         [0026]     The image forming-apparatus according to Structure 1 above having therein exposure units each being provided with a laser light source and a scanning optical system that deflects light coming from the light source and emits scanning light beam, the exposure units being supported on at least a first supporting point, a second supporting point and a third supporting point, wherein a second adjustment means that adjusts variation in magnification of the scanning light beam in the primary scanning direction is provided in a second supporting section where the second supporting point is provided, and the variation in magnification is adjusted only by adjustment of the second adjustment means.  
         [heading-0027]     (Structure 4)  
         [0028]     An image forming apparatus according to Structure 1 above having therein exposure units each being provided with a laser light source and a scanning optical system that deflects light coming from the light source and emits scanning light beam, the exposure units being supported on at least a first supporting point, a second supporting point and a third supporting point, wherein a first supporting section in which at least one supporting point among three supporting points is provided a spherical body bearing that supports the exposure unit rotatably in all directions.  
         [heading-0029]     (Structure 5)  
         [0030]     The image forming apparatus according to Structure 1 or Structure 3 above, wherein the first supporting section is provided with a spherical body bearing that supports the exposure unit rotatably in all directions.  
         [heading-0031]     (Structure 6)  
         [0032]     The image forming apparatus according to Structure 1 or Structure 3 above, wherein a movement of the first supporting point is conducted around a straight line passing through the other two supporting points which serves as the axis, and at least one of the other two supporting points can move in the direction that is mostly perpendicular to the movement direction of the first supporting section.  
         [heading-0033]     (Structure 7)  
         [0034]     The image forming apparatus according to Structure 2, wherein the second supporting section is provided with a spherical body bearing that supports the exposure unit rotatably in all directions.  
         [heading-0035]     (Structure 8)  
         [0036]     The image forming apparatus according to Structure 2 or Structure 3 above, wherein a movement of the second supporting point is a rotation around the first supporting point, and the second supporting section is provided with a slide which makes the exposure unit to move in the direction of a normal line for the direction of the aforesaid rotation.  
         [heading-0037]     (Structure 9)  
         [0038]     The image forming apparatus according to either one of Structure 1-Structure 4 above, wherein the third supporting point is made to be a fixed point.  
         [heading-0039]     (Structure 10)  
         [0040]     The image forming apparatus according to Structure 9, wherein the third supporting point is provided on the plane that mostly includes an optical axis of the scanning optical system and is mostly perpendicular to the scanning plane.  
         [heading-0041]     (Structure 11)  
         [0042]     The image forming apparatus according to Structure 9 or Structure 10, wherein the first supporting point and the second supporting point are provided respectively on both end portions of an optical path formed by the scanning optical system.  
         [heading-0043]     (Structure 12)  
         [0044]     The image forming apparatus according to either one of Structure 9-Structure 11 above, wherein the first supporting point and the second supporting point are provided respectively at positions which are mostly symmetrical about an optical axis of the scanning optical system.  
         [heading-0045]     (Structure 13)  
         [0046]     The image forming apparatus according to Structure 4 above, wherein the spherical body bearing has a separation preventing member that prevents separation of the supporting spherical body.  
         [heading-0047]     (Structure 14)  
         [0048]     The image forming apparatus according to Structure 13 above, wherein the separation preventing member is elastic.  
         [heading-0049]     (Structure 15)  
         [0050]     The image forming apparatus according to Structure 13 or Structure 14 above, wherein the spherical body bearing supports the spherical body at three points thereon, and the separation preventing member makes the spherical body to be in pressure contact with the spherical body bearing by touching at least one point on the spherical body.  
         [heading-0051]     (Structure 16)  
         [0052]     The image forming apparatus according to Structure 13 or Structure 14 above, wherein the spherical body bearing supports the spherical body in the shape of a circular ring, and the separation preventing member makes the spherical body to be in pressure contact with the spherical body bearing by touching at least one point on the spherical body.  
         [heading-0053]     (Structure 17)  
         [0054]     The image forming apparatus according to either one of Structure 2-Structure 4 and Structure 9-Structure 16 above, wherein a distance keeping means that keeps a distance between the exposure unit and the second supporting section and an elastic member that presses the exposure unit against the second supporting section are provided in the vicinity of the second supporting section in which the second supporting point is provided.  
         [heading-0055]     (Structure 18)  
         [0056]     The image forming apparatus according to either one of Structure 1-Structure 17, wherein the first supporting section is composed of a driving screw and an engagement plate that engages with the driving screw, and the first adjustment means is composed of the driving screw, the engagement plate and a driving source that drives the driving screw.  
         [heading-0057]     (Structure 19)  
         [0058]     The image forming apparatus according to either one of Structure 1-Structure 18, wherein the second supporting section is composed of a driving screw and an engagement plate that engages with the driving screw, and the second adjustment means is composed of the driving screw, the engagement plate and a driving source that drives the driving screw.  
         [heading-0059]     (Structure 20)  
         [0060]     The image forming apparatus according to Structure 18 or Structure 19, wherein the engagement plate stated above is composed of two sheets which engage with the driving screw respectively in two directions which are opposite to each other.  
         [heading-0061]     (Structure 21)  
         [0062]     The image forming apparatus according to Structure 20, wherein the two engagement sheets are formed as protruded sections extended from one sheet of common base section.  
         [heading-0063]     (Structure 22)  
         [0064]     The image forming apparatus according to Structure 20 or Structure 21, wherein the two engagement sheets engage with the driving screw at two positions which are deviated in terms of phase by approximately a half of a pitch of the driving screw each other.  
         [heading-0065]     (Structure 23)  
         [0066]     The image forming apparatus according to either one of Structure 1-Structure 17, wherein the first supporting section is composed of a driving male screw and an engagement block which has a female screw that engages with the driving male screw and is fixed on the exposure unit, and the first adjustment means is composed of the driving male screw, the engagement block and a driving source that drives the driving screw.  
         [heading-0067]     (Structure 24)  
         [0068]     The image forming apparatus according to either one of Structure 1-Structure 17, wherein the second supporting section is composed of a driving male screw and an engagement block which has a female screw that engages with the driving male screw and is fixed on the exposure unit, and the second adjustment means is composed of the driving male screw, the engagement block and a driving source that drives the driving screw.  
         [heading-0069]     (Structure 25)  
         [0070]     The image forming apparatus according to Structure 23, wherein an engagement clearance between the driving male screw and the female screw is made to be 1.5 times the maximum position correction amount of the exposure unit or more, so that rotating operations of the driving male screw in the first supporting section may not be interrupted.  
         [heading-0071]     (Structure 26)  
         [0072]     The image forming apparatus according to Structure 24, wherein an engagement clearance between the driving male screw and the female screw is made to be 1.5 times the maximum position correction amount of the exposure unit or more, so that rotating operations of the driving male screw in the second supporting section may not be interrupted.  
         [heading-0073]     (Structure 27)  
         [0074]     The image forming apparatus according to either one of Structure 23-Structure 26, wherein the engagement block has two connecting members protruded in the direction perpendicular to a rotary axis of the driving male screw, and the engagement member is combined with the exposure unit by the connecting member.  
         [heading-0075]     (Structure 28)  
         [0076]     The image forming apparatus according to either one of Structure 1-Structure 17, wherein the first supporting section is composed of a driving male screw, an engagement block that engages with the driving male screw and a driving source that drives the driving screw, and there are provided on the engagement block a hole through which the driving male screw enters and an engagement member that has a tip portion engaging with the driving male screw and engages with the engagement block through screw.  
         [heading-0077]     (Structure 29)  
         [0078]     The image forming apparatus according to either one of Structure 1-Structure 17, wherein the second supporting section is composed of a driving male screw, an engagement block that engages with the driving male screw and a driving source that drives the driving screw, and there are provided on the engagement block a hole through which the driving male screw enters and an engagement member that has a tip portion engaging with the driving male screw and engages with the engagement block through screw.  
         [heading-0079]     (Structure 30)  
         [0080]     The image forming apparatus according to Structure 1 or Structure 3, wherein a plurality of the exposure units are provided, and the first adjustment means stated above is provided on each of the plural exposure units.  
         [heading-0081]     (Structure 31)  
         [0082]     The image forming apparatus according to Structure 1 or Structure 3, wherein a plurality of the exposure units are provided, and the second adjustment means stated above is provided on each of the plural exposure units.  
         [heading-0083]     (Structure 32)  
         [0084]     The image forming apparatus according to Structure 3, wherein a plurality of the exposure units are provided, and the first adjustment means and the second adjustment means both stated above are provided on each of the plural exposure units.  
         [heading-0085]     (Structure 33)  
         [0086]     The image forming apparatus according to Structure 1 or Structure 3, wherein the first adjustment mean has a driving source composed of a motor.  
         [heading-0087]     (Structure 34)  
         [0088]     The image forming apparatus according to Structure 2 or Structure 3, wherein the second adjustment mean has a driving source composed of a motor.  
         [heading-0089]     (Structure 35)  
         [0090]     The image forming apparatus according to Structure 1 or Structure 3, wherein the first adjustment mean has a driving source composed of a layered piezoelectric actuator.  
         [heading-0091]     (Structure 36)  
         [0092]     The image forming apparatus according to Structure 2 or Structure 3, wherein the second adjustment mean has a driving source composed of a layered piezoelectric actuator.  
         [heading-0093]     (Structure 37)  
         [0094]     An image forming apparatus having therein an exposure unit composed of a laser light source and a scanning optical system that deflects light coming from the laser light source and emits a scanning beam, a first supporting means that supports the exposure unit rotatably, a second supporting means that is in a form of a circular arc whose center is the first supporting means and supports the exposure unit movably along the circular arc, and a magnification adjustment means that adjusts variation of magnification in the primary scanning direction of the scanning optical system by the rotation around the first supporting means.  
         [heading-0095]     (Structure 38)  
         [0096]     An image forming apparatus having therein an exposure unit composed of a laser light source and a scanning optical system that deflects light coming from the laser light source and emits a scanning beam, and an inclination adjustment means supporting the exposure unit, having a supporting shaft that is almost in parallel with an optical axis of the scanning optical system and adjusting an inclination in the primary scanning direction by rotating the exposure unit on the center of the supporting shaft.  
         [heading-0097]     (Structure 39)  
         [0098]     An assembling method of an exposure unit for assembling the exposure unit for an image forming apparatus having therein a photoreceptor, a charging unit that charges the photoreceptor, a exposure unit that conducts scanning exposure on the photoreceptor with a light beam to form an electrostatic latent image and a developing unit that develops the electrostatic latent image formed on the photoreceptor, and having at least a plurality of the exposure units, wherein each of the exposure units is attached to a supporting means so that a position of the exposure unit is corrected, and then, the clearance formed between the plural supporting means is filled with melted resins so that the exposure units and a distance between the exposure units are fixed.  
         [heading-0099]     (Structure 40)  
         [0100]     The assembling method of an exposure unit according to Structure 39, wherein each of the plural supporting means has a plane crossing in the direction in which the plural exposure units are separated each other, and mutual exfoliation is prevented by resistance of the resin itself filled in the clearance formed by the crossing planes.  
         [heading-0101]     (Structure 41)  
         [0102]     The assembling method of an exposure unit according to. Structure 39 or Structure 40, wherein the supporting means is made of resin, and a space between the supporting means is filled with resin that is the same in terms of a type as the resin of which the supporting means is made, and thereby the supporting means are combined each other through their clearances.  
         [heading-0103]     (Structure 42)  
         [0104]     An assembling method of an exposure unit for assembling the exposure unit for an image forming apparatus having therein a charging unit that charges the photoreceptor, an exposure-unit that conducts scanning exposure on the photoreceptor with a light beam and a plurality of image forming sections each having a developing unit that develops an electrostatic latent image formed on the photoreceptor which are arranged to face the photoreceptor, wherein each of the exposure units is attached to a supporting means so that a position of the exposure unit is corrected, and then, the clearance formed between the plural supporting means is filled with melted resins for fixing the position so that the exposure units and a distance between the exposure units are fixed.  
         [heading-0105]     (Structure 43)  
         [0106]     The assembling method of an exposure unit according to Structure 42, wherein each of the plural supporting means has a plane crossing in the direction in which the plural exposure units are separated each other, and mutual exfoliation is prevented by resistance of the resin itself filled in the clearance formed by the crossing planes.  
         [heading-0107]     (Structure 44)  
         [0108]     The assembling method of an exposure unit according to Structure 42 or Structure 43, wherein the supporting means is made of resin, and a space between the supporting means is filled with resin that is the same in terms of a type as the resin of which the supporting means is made, and thereby the supporting means are combined each other through their clearances. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0109]      FIG. 1  is a diagram showing the whole of an image forming apparatus related to Embodiment 1 of the invention.  
         [0110]      FIG. 2  is a sectional view of a top surface of an exposure unit of an image forming apparatus related to Embodiment 1 of the invention.  
         [0111]      FIG. 3  is a diagram showing a first and a second supporting sections of the exposure unit.  
         [0112]      FIG. 4  is a diagram showing a third supporting section of the exposure unit.  
         [0113]      FIG. 5  is a sectional view of a top surface of an exposure unit of an image forming apparatus related to Embodiment 2 of the invention.  
         [0114]      FIG. 6  is a sectional view along lines AX-BX-CX in  FIG. 5 .  
         [0115]      FIG. 7  is a diagram showing a supporting section of the exposure unit.  
         [0116]      FIG. 8  is a diagram showing a supporting section of the exposure unit.  
         [0117]      FIG. 9  is a diagram showing a supporting section of the exposure unit.  
         [0118]      FIG. 10  is a diagram showing a concrete example of a supporting structure of a frame work in a third supporting point.  
         [0119]      FIG. 11  is a diagram showing an example of a second adjustment means.  
         [0120]      FIG. 12  is a diagram showing a supporting section of an exposure unit of an image forming apparatus related to Embodiment 3 of the invention.  
         [0121]      FIG. 13 a  diagram showing a supporting section of an exposure unit of an image forming apparatus related to Embodiment 4 of the invention.  
         [0122]      FIG. 14  is a diagram showing the whole of an image forming apparatus related to Embodiment 5 of the invention.  
         [0123]      FIG. 15  is a perspective view showing the combination structure between exposure units in Embodiment 5 of the invention.  
         [0124]      FIG. 16  is an exploded perspective view of an exposure unit.  
         [0125]      FIG. 17  is a diagram showing the process of combination between exposure units.  
         [0126]      FIG. 18  is a diagram showing an example of a combination section between exposure units.  
         [0127]      FIG. 19  is a diagram showing another example of a combination section between exposure units.  
         [0128]      FIG. 20  is a diagram for illustrating adjustment of a position between exposure units.  
         [0129]      FIG. 21  is a sectional structure diagram of a color image forming apparatus.  
         [0130]      FIG. 22  is a structure diagram of a scanning optical device.  
         [0131]      FIG. 23  shows a plan view, an illustration and a timing chart in the example of the invention.  
         [0132]      FIG. 24  shows a plan view, an illustration and a timing chart in the example of the invention.  
         [0133]      FIG. 25  is a perspective view of a scanning optical device, and sectional side views of essential portions showing examples of the invention.  
         [0134]      FIG. 26  shows a side view for essential portions, a plan view for essential portions and a sectional side view for essential portions of the invention.  
         [0135]      FIG. 27  shows sectional side views showing examples of detecting wire positions. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [heading-0136]     Embodiment 1  
         [0137]     Embodiment 1 of the invention will be explained as follows, referring to  FIGS. 1-3 .  
         [0138]      FIG. 1  is a diagram showing the whole of an image forming apparatus related to Embodiment 1,  FIG. 2  is a sectional view of a top surface of an exposure unit of an image forming apparatus related to Embodiment 1, and  FIG. 3  is a diagram showing a first supporting section and a second supporting section of the exposure unit in  FIG. 2 .  
         [0139]     Belt-shaped photoreceptor  1  wherein an organic photoconductive layer is formed on a conductive base body is trained about driving roller  11  and driven rollers  12  and  13  to be given appropriate tension by tension roller  14 , and is rotated as shown with an arrow. Around the photoreceptor  1 , there are arranged image forming sections each forming each of a yellow image, a magenta image, a cyan image and a black image through charging, exposure and developing. The image forming section for the yellow image is composed of charging unit  2 Y composed of a scorotron charger, exposure unit  30 Y having a light source of a semiconductor laser and a scanning optical system, and developing unit  4 Y having developing sleeve  41  to conduct reversal development. In the same way, the magenta image forming section is composed of charging unit  2 M, exposure unit  30 M and developing unit  4 M, the cyan image forming section is composed of charging unit  2 C, exposure unit  30 C and developing unit  4 C, and the black image forming section is composed of charging unit  2 K, exposure unit  30 K and developing unit  4 K.  
         [0140]     Bias voltage whose polarity is the same as that of an electrostatic latent image on the photoreceptor  1  is impressed on the developing sleeve  41  in each of the developing units  4 Y,  4 M,  4 C and  4 K, and each of these developing units conducts non-contact and reversal development to form a toner image on the photoreceptor  1 . Toner in each color is supplied to each of the developing units  4 Y,  4 M,  4 C and  4 K from each of toner containers  5 Y,  5 M,  5 C and  5 K.  
         [0141]     Further, charging unit  2 F, transfer unit  7  composed of a transfer roller on which transfer voltage is impressed and cleaning unit  8  are arranged to face the photoreceptor  1 .  
         [0142]     Sheet feeding cassettes  20 A and  20 B each being loaded with recording sheet P on which an image is formed are provided at the lower portion of an image forming apparatus, and manual sheet feeding stand  20 C is provided on the side of the image forming apparatus, and each of sheet feeding sections  21 A,  21 B and  21 C provided respectively on the sheet feeding cassettes and the manual sheet feeding stand feeds out recording sheet P toward the transfer unit  7 . The numeral  23  represents a registration roller that conveys recording sheet P to the transfer unit  7  in synchronization with image forming on the photoreceptor  1 . The numeral  24  represents a fixing unit having a heating roller and a pressing roller, while,  25 A,  25 B and  25 C represent conveyance rollers which convey recording sheet P, and  26  represents a sheet ejection tray on which recording sheets P ejected are stacked.  
         [0143]     In image forming, the photoreceptor  1  rotates as shown with an arrow, and in accordance with rotation of the photoreceptor  1 , each of the yellow image forming section, the magenta image forming section, the cyan image forming section and the black image forming section forms a toner image of each color on the photoreceptor  1  through charging, exposure and developing under the controlled timing, thus, a full color toner image composed of the superposed toner images of various colors is formed on the photoreceptor  1 . For the full color toner image thus formed, voltage is adjusted by charging unit  2 F.  
         [0144]     In synchronization with forming of a full color toner image on the photoreceptor  1 , recording sheet P is conveyed from the registration roller  23  to be brought into close contact with the photoreceptor  1  at transfer unit  7 , and the full color toner image on the photoreceptor  1  is transferred onto the recording sheet P by transfer voltage of the transfer unit  7 .  
         [0145]     The full color toner image transferred onto the recording sheet P is fixed by fixing unit  24 , thus, the full color toner image is on the recording sheet P. The recording sheet P which has passed through the fixing unit  24  is conveyed by conveyance rollers  25 A,  25 B and  25 C to be ejected to sheet ejection tray  26 . The photoreceptor  1  which has passed through the transfer position is subjected to removing of remaining toner by cleaning unit  8 , and is subjected to neutralizing by neutralizing lamp  9  so that conditions of the surface of the photoreceptor  1  are uniformed.  
         [0146]     Each of exposure units  30 Y,  30 M,  30 C and  30 K in the present embodiment conducts scanning exposure through scanning by a laser beam in the primary scanning direction and through scanning in the sub-scanning direction by movement of photoreceptor  1  to form an electrostatic latent image on the photoreceptor  1 , and each exposure unit is provided with an adjustment mechanism for scanning exposure which conducts adjustment for variation of a magnification in the primary scanning direction, namely for lateral magnification and adjustment for an inclination in the primary scanning direction.  
         [0147]     An adjustment mechanism used for exposure units  30 M,  30 C and  30 K in common will be explained as follows, wherein these exposure units will be called exposure unit  30  in the explanation. Incidentally, it is not necessary that all of the exposure units  30 Y,  30 M,  30 C and  30 K are provided with the aforesaid adjustment mechanism, and either one of the exposure units may be provided with no adjustment mechanism to be the standard for conducting at least one of the adjustment of an inclination of the exposure unit in the primary scanning direction and the adjustment of variation of magnification in the primary scanning direction of the other exposure units.  
         [0148]     Exposure unit  30  has therein a laser light source composed of semiconductor laser  35  and a scanning optical system composed of first cylindrical lens  37 , polygon mirror unit  31 , fθ lens  32  and second cylindrical lens  33 . These laser light source and scanning optical system are fixed on box-shaped frame object  307 . A light beam emitted from the semiconductor laser  35  is deflected by a polygon rotating at high speed in polygon mirror unit  31  to scan the photoreceptor  1  at high speed in the primary scanning direction, as shown with arrow W.  
         [0149]     Frame object  307  is supported by supporting plate  301  that is fixed on an image forming apparatus at three points of T 1 , T 2  and T 3  representing intersecting points of one-dot chain lines A-A, B-B and C-C. In this way, the exposure unit  30  is supported on the image forming apparatus at the first supporting point T 1 , the second supporting point T 2  and the third supporting point T 3 .  
         [0150]     The structure of a first supporting section for the frame object  307  at the first supporting point is shown in  FIG. 3 ( a ).  
         [0151]     On foot section  304  of the frame object  307 , there is fixed connecting member  302  that is connected by screw section  302   b . The connecting member  302  has the screw portion  302   b  and spherical body  302   a , and the spherical body  302   a  is supported by spherical body bearing  303  to be rotatable in all directions, including vertical and horizontal directions. The connecting member  302  and the spherical body bearing  303  constitute the first supporting section. The spherical body bearing  303  is guided to be movable vertically as shown with arrow X by intermediate member  305  and guiding portion  301   a  provided on supporting plate  301 . The intermediate member  305  is driven vertically by eccentric cam  306  that is driven by motor M 1  to rotate.  
         [0152]     The motor M 1  and eccentric cam  306  constitute a first adjustment means that adjusts an inclination of a light beam in the primary scanning direction. Incidentally, it is also possible to use another driving means such as a layered piezoelectric actuator for the first adjustment means, in place of the motor M 1  and eccentric cam.  
         [0153]     Frame object  307  is driven by motor M 1  to move up and down vertically in the direction of arrow X, and this vertical movement makes exposure unit  30  to rotate around an axis of one-dot chain line B-B, and thereby, an angle of the primary scanning direction shown with W is changed and an inclination of the primary scanning direction is adjusted.  
         [0154]     Structure of the second supporting section for frame object  307  at the second supporting point T 2  is shown in  FIG. 2  and  FIG. 3 .  
         [0155]     On a pair of lifted portions  317  provided on the frame object  307 , there is supported connecting member  316 . The connecting member  316  is constructed to be solid with the lifted portions  317 . Bar-shaped connecting member  316  shown in  FIG. 3 ( c ) is inserted through a round hole provided on the lifted portions  317 . On the connecting member  316 , there is arranged partial spherical body  314   a  capable of sliding on the connecting member  316  in the direction of arrow Z. This partial spherical body  314   a  is supported on spherical body bearing  314  to be rotatable in all directions. The spherical body bearing  314  is supported by intermediate member  315 , and the intermediate member  315  is guided by holding section  321  to be capable of sliding in the direction of arrow Y. The holding section  321  is fixed on supporting plate  301 . The intermediate member  315  is driven to move from side to side by eccentric cam  322  that is driven by motor M 2 .  
         [0156]     The motor M 2  and eccentric cam  317  constitute a second adjustment means that adjusts variation in magnification of a light beam in the primary scanning direction. Incidentally, it is also possible to use another driving means such as a layered piezoelectric actuator for the second adjustment means, in place of the motor M 2  and eccentric cam.  
         [0157]     Frame object  307  is driven by motor M 2  to move from side to side in the direction of arrow Y, and this movement from side to side makes exposure unit  30  to rotate around the first supporting point as a rotary center, and thereby, an optical path length from a reflection point on polygon mirror unit  31  to the surface to be scanned is changed to be different at each position on the scanning line, and thus, variation of magnification in the primary scanning direction is adjusted.  
         [0158]     Structure of the third supporting section for frame object  307  at the third supporting point T 3  is shown in  FIG. 4 .  
         [0159]     On a pair of lifted portions  308  provided on the frame object  307 , there is supported connecting member  309 . The connecting member  309  is inserted through elongated hole provided on the lifted portions  308 , and it can move in the direction arrow Y in  FIG. 4 ( a ). On the connecting member  309 , there is arranged partial spherical body  312   a  capable of sliding on the connecting member  309  in the direction of arrow Z. This partial spherical body  312   a  is supported on spherical body bearing  312  to be rotatable in all directions. The spherical body bearing  314  is supported by intermediate member  315 , and the intermediate member  315  is guided by holding section  321  to be capable of sliding in the direction of arrow Y. The holding section  321  is fixed on supporting plate  301 . The intermediate member  315  is driven to move from side to side by eccentric cam  322  that is driven by motor M 2 .  
         [0160]     Owing to the supporting structure of the frame object  307 , the frame object  307  can move at the third supporting point T 3  in directions of Y and Z both are perpendicular to each other, and this movement is a rotary movement around the first supporting point T 1 .  
         [0161]     Being driven by motor M 1  to rotate, the frame object  307  moves up and down at the first supporting point and rotates around an axis of one-dot chain line B. Owing to this rotation, an inclination of laser beam L in the primary scanning direction W and an inclination of the scanning line on the surface to be scanned are adjusted. The adjustment is conducted in a way wherein, for example, a plurality of patterns which are in parallel with the primary scanning direction are written by a plurality of scanning optical devices to form images, and then, frame object  307  is moved in the direction X ( FIG. 3 ( a )) so that a difference in inclination of the formed line images may be reduced to zero between exposure units.  
         [0162]     Being driven by motor M 2  to rotate, the frame object  307  rotates around the first supporting point. Owing to this rotation, a difference of an optical path length from a reflection point on polygon mirror unit  31  to the surface to be scanned is adjusted and an optical path length at the position of an image height is made to be appropriate, and thereby, variation of lateral magnification in the primary scanning direction, namely, a difference of magnifications at both ends in the primary scanning direction is adjusted. This adjustment is conducted by measuring variation of lateral magnification, namely, unevenness of magnification in the primary scanning direction, from an image formed by conducting scanning exposure.  
         [0163]     With regard to variation in magnification in the primary scanning direction, it is possible to detect it by measuring at least three points of an image formed through scanning exposure in each exposure unit. For example, an image of a line that is perpendicular to the optical scanning direction is formed by each exposure unit at both ends and a central portion, and a difference of positions in the primary scanning direction of the line image formed by each exposure unit is detected at each measurement point, and thereby, it is possible to detect a difference between magnification of the entire scanning area and magnification at the upstream side and the downstream side of the scanning line, with an optical axis serving as a boundary. Furthermore, when image position detecting patterns ML, MC and MR each being in a shape of “inverted L” are made by a line that is in parallel with the primary scanning direction and a line inclined from the primary scanning line by 45° in place of the line perpendicular to the optical scanning direction as shown in  FIG. 20 , the position of the exposure unit can be detected at less cost by LED SS 1  and light-receiving element SS 2  in place of conventional line CCD.  
         [0164]      FIG. 20  shows a detecting means to detect an inclination of a scanning direction in  FIG. 2 , namely, for example, a detecting means to detect that scanning line SL in  FIG. 20  has inclined to be SL′. Marks shown with ML and MR are formed respectively on both end portions of photoreceptor  1  and a mark shown with MC is formed on the central portion. Each of these marks is a mark in a shape of “inverted L” that is formed by a segment that is in parallel with the scanning line and a segment that intersects the scanning line SL to be tilted from it by 45°. These marks are detected by sensors SSL, SSC and SSR, then, an inclination of the scanning line is detected by the timing of detection signal for a segment of each of marks ML and MR, and a magnification in the primary scanning direction is detected by a difference between detection timing for the parallel segment and detection timing for the intersecting segment (a distance between both segments) for each mark obtained by detecting marks ML, MC and MR.  
         [heading-0165]     Embodiment 2  
         [0166]     Embodiment 2 of the invention will be explained as follows, referring to  FIGS. 5-13 .  FIG. 5  is a sectional view of a top surface of an exposure unit in Embodiment 2 of the invention. Parts identical to those in  FIG. 2  are given the same symbols.  FIG. 6  is a sectional view along lines AX-BX-CX in  FIG. 5 .  
         [0167]     Frame object  401  that supports a light source and a scanning optical system is supported by supporting plate  417  fixed on an image forming apparatus main body at three points of first supporting point T 10 , second supporting point T 20  and third supporting point T 30 . The structure of the third supporting section at the third supporting point T 30  is as follows.  
         [0168]     Connecting member  447  that connects frame object  401  with supporting plate  417  has spherical body  447   a . On the supporting plate  417 , on the other hand, there is fixed spherical body bearing  446  that supports the spherical body  447   a  rotatably in all directions including vertical and horizontal directions. Due to this supporting structure, the frame object  401  is supported rotatably in all directions including vertical and horizontal directions, on the center of the third supporting point T 30 . In the present embodiment, as stated above, the third supporting point T 30  that is a fixed point supporting section among three supporting sections is provided on a plane that includes optical axis center AX of the scanning optical system and is perpendicular to the scanning plane. When the third supporting point T 30  is arranged on the plane stated above, movement adjustment for an exposure unit in the plane that is in parallel with the scanning plane can be conducted almost symmetrically for the scanning area. It is therefore possible to conduct adjustment of magnification in the primary scanning direction uniformly on both sides of the scanning area about its center in the primary scanning direction.  
         [0169]      FIG. 6  shows the structure of the first supporting section at the first supporting point T 10  where the first adjustment means for adjusting inclination of the primary scanning direction of a light beam is provided. Next, the structure of the first supporting section will be explained as follows.  
         [0170]     Each of two engagement flakes  426  and  427  made respectively of a thin and elastic metal plate is fixed at its one end on the frame object  401 . On the other hand, driving screw  425  is provided on supporting plate  417  in a way that the screw can rotate but does not move vertically. First supporting point T 10  is a point of intersection between a plane formed by the engagement flakes  426  and  427  and the driving screw  425 . A screw portion of the driving screw  425  held by an unillustrated holding member engages with the engagement flakes  426  and  427 . Further, on the driving screw  425 , there is fixed gear  425   a  which is rotated by motor M 10  through gear  434 . There is further arranged spring  471  that presses frame object  401  downward on a supplemental basis, between the frame object  401  and the supporting plate  417 . The motor M 10 , the driving screw  425  and the engagement flakes  426  and  427  constitute the first adjustment means.  
         [0171]     The engagement flakes  426  and  427  receive a load downward in  FIG. 6  and they also serve as an enduring member for the supporting plate  417  of exposure unit  30 . When the driving screw  425  rotates, its point of engagement with the engagement flakes  426  and  427  moves vertically, and due to this, the exposure unit  30  rotates around a straight line connecting the third supporting point T 30  and the second supporting point T 20 . Between the engagement flakes  426  and  427  and the driving screw  425 , therefore, a clearance is naturally needed in the direction perpendicular to the axial direction of the driving screw  425 . The engagement flakes  426  and  427  also serve as an enduring member for the supporting plate  417  of exposure unit  30 , and even when another regulating means is provided for the positional regulation in the direction-perpendicular to the axial direction of the driving screw  425 , the first adjustment means is not affected adversely.  
         [0172]     As another regulating means stated above, the second adjustment means described later may also be used, and another positional regulating means, for example, may also be provided by the means shown in the first embodiment or a means similar to that.  
         [0173]     When motor M 10  rotates, frame object  401  is rotated by engagement between the engagement flakes  426  and  427  and the driving screw  425 , as shown with arrow X around the center of the third supporting point T 30 . By vertical movement of the frame object  401  in the direction of X, an inclination of a scanning line of light beam L is adjusted.  
         [0174]     The structure of the second supporting section at the second supporting point T 20  where the second adjustment means is provided is as follows.  
         [0175]     Frame object  403  of the second supporting section is structured to be solid with frame object  401 , and engagement flakes  466  and  467  are further fixed on the frame object  403 . On supporting plate  417 , on the other hand, there are formed lifted portions  468  and  469 , and driving screw  431  is supported rotatably. On the driving screw  431 , there is fixed gear  432  which is connected rotatably at high speed reduction ratio through rotation of motor M 20  and worm gear  465  that is fixed on the rotary shaft of the motor M 20 . An extended portion of the frame object  403  comes in contact with a non-screw portion of the driving screw  431 , and thereby, vertical positions of the frame object  401  and supporting plate  417  are regulated. The second supporting point T 20  is a point of intersection between a plane formed by the engagement flakes  466  and  467  and the driving screw  431 . There is further arranged spring  472  that presses on a supplemental basis. Further, the engagement flakes  466  and  467  engage with the driving screw  431 , and spring  474  is provided to prevent engagement play between the engagement flakes  466  and  467  and the driving screw  431 . Rotation of motor M 20  is transmitted to the driving screw  431 , and the engagement flakes  466  and  467  are moved from side to side by that rotation. The motor M 20 , the driving screw  431  and the engagement flakes  466  and  467  constitute the second adjustment means.  
         [0176]     As stated above, the engagement flakes  466  and  467  are moved from side to side for the supporting plate  417  by the rotation of the motor M 20 . Namely, frame object  401  conducts a swiveling action on the third supporting point T 30  that serves as a rotation center in a plane including a contact point between driving screw  425  of the first supporting point T 10  and engagement flakes  426  and  427 , the third supporting point T 30  and a contact point between extended portion  403   a  of frame object  403  of the second supporting point T 20  and a non-screw portion of the driving screw  431 . This swiveling action adjusts variation of lateral magnification in the primary scanning direction.  
         [0177]      FIG. 10  shows a concrete example of the supporting structure of frame object  401  in the third supporting point T 30 .  
         [0178]     In  FIG. 10 ( a ), three balls  446   a ,  446   b  and  446   c  which support at three points are used as a spherical body bearing that supports spherical body  447   a  of connecting member  447 . Holding member  446   d  made of an elastic material makes the spherical body  447   a  to be in pressure contact with balls  446   a ,  446   b  and  446   c . In  FIG. 10 ( b ), as a spherical body bearing that supports spherical body  447   a  of connecting member  447 , a circular hole whose diameter is smaller than a diameter of spherical body  447   a  is made on supporting plate  417  to insert the spherical body  447   a  in the hole, and the spherical body  447   a  is made to be in pressure contact with supporting plate  417  by holding member  446   d  made of an elastic material in the same way as in  FIG. 10 ( a ). In  FIG. 10 ( c ), a spherical body of connecting member  447  is held by drawing of spherical body bearing  446 .  
         [0179]      FIG. 10 ( d ) and  FIG. 10 ( e ) show respectively a variation of engagement flakes  426  and  427  in  FIG. 6  and a variation of engagement flakes  466  and  467  in  FIG. 7 . In the example shown in  FIG. 10 ( d ), protrusions BSA and BSB are provided on a sheet of flat plate BS, and the protrusions BSA and BSB form engagement flakes  426  and  427  or engagement flakes  466  and  467 . In the example shown in  FIG. 10 ( e ), protrusions CSA and CSB are provided on plate CS on which step portion CSC is provided, and these protrusions form engagement flakes  426  and  427  or protrusions  466  and  467 . In the example in  FIG. 10 ( e ), the step portion CSC makes the protrusions CSA and CSB to be formed with their positions shifted each other. As a result, the engagement flakes  426  and  427  engage with driving screw  425  to be shifted each other in terms of a phase by a pitch of the driving screw  425 , while, the engagement flakes  466  and  467  engage with driving screw  431  to be shifted each other in terms of a phase by a pitch of the driving screw  431 . In the example shown in  FIG. 10 ( d ), it is preferable that an elastic plate is used for flat plate GS, and it deforms elastically when it engages with a driving screw so that an appropriate engagement may be obtained. In the example shown in  FIG. 10  (e), an engagement is appropriate and accurate adjustment is possible, because step portion CSC makes positions of protrusions CSA and CSB to be deviated each other by a pitch of a driving screw.  
         [0180]      FIG. 11  shows examples of the first adjustment means for adjusting an inclination of the primary scanning direction of a scanning optical system and the second adjustment means for adjusting variation of magnification in the primary scanning direction of the scanning optical system.  
         [0181]     In FIGS.  11 ( a ) and  11 ( b ), a driving screw (driving male screw) driven by a motor (not shown) is engaged with female screw  481   c  of engagement block  481 , and the engagement block  481  is fixed in a recessed portion provided on frame object  401 . Rotation of driving screw  425  makes the frame object  401  to move up and down relatively to supporting plate  417 , and this movement represents rotating motion of exposure unit  30  around an axis passing through second supporting point T 20  and third supporting point T 30 . In the adjustment wherein rotating motion of exposure unit  30  is caused by linear motion as stated above, there is caused mismatching theoretically between rotating motion and linear motion. However, when a distance between first supporting point T 10  representing a center of rotation and second supporting point T 20  at which linear motion is carried out is long enough, compared with a distance for movement by adjustment, the mismatching is absorbed by a clearance in the screw portion, and adjustment is not affected adversely by the mismatching. Incidentally, it has been confirmed that a preferable condition for the adjustment mechanism capable of absorbing the mismatching is that the screw portion has a clearance which makes an amount of adjustment that is 1.5 times the maximum amount of adjustment of exposure unit  30  (maximum value of a distance of movement by adjustment) or more to be possible.  
         [0182]     In FIGS.  11 ( c ) and  11 ( d ), engagement block  482  having female screw  482   c  is connected with frame object  401  through bar-shaped connecting members  482   a  and  482   b  which are fitted in the engagement block  482 . Driving screw  425  driven by a motor (not shown) is engaged with female screw  482   c  of engagement block  482 . When gear  425   a  that fixes thereon driving screw  425  on which a male screw is formed is driven by a motor, the engagement block  482  moves up and down, and relative position of frame object  410  for supporting plate  417  is changed. In the example in FIGS.  11 ( c ) and  11 ( d ), the engagement block  482  is rotatable around the connecting members  482   a  and  482   b . Therefore, errors for attachment of driving screw  425  are absorbed by this rotatable structure for attachment, and the engagement member  482  is self-adjusted even when angularity between driving shaft  425  and supporting plate  417  is not controlled accurately when the driving shaft is attached, which is effective for reducing assembling man-hour.  
         [0183]     In FIGS.  11 ( e ) and  11 ( f ), driving screw  425  driven by a motor (not shown) is inserted through engagement block  483 . The engagement block  483  is fixed in a recessed portion provided on frame object  401 .  FIG. 11 ( f ) is an enlarged section along lines DX-DX in  FIG. 11 ( e ), and a tip portion of engagement member  484  inserted in engagement block  483  through screw engagement is engaged with a screw portion of driving screw  425 . The first supporting point T 10  is a point of intersection between a tip portion of engagement member  484  and driving screw  425 . Rotation of driving screw  425  makes exposure unit  30  to make a rotating motion around an axis passing through second supporting point T 20  and third supporting point T 30 . Movement of engagement block  483  caused by the driving screw  425  is linear motion, while, exposure unit  30  makes a rotating motion as stated above. The aforesaid adjustment mechanism that employs engagement member  484  absorbs mismatching between linear motion and rotating motion, and it reduces assembling man-hour in the assembly process such as accurate management of angularity, when attaching driving screw  425  on supporting plate  417 .  
         [0184]     The adjustment means shown in each of FIGS.  11 ( a )- 11 ( f ) is an example of a first adjustment means that adjusts an inclination in the primary scanning direction, while, an example shown in  FIG. 11 ( g ) is a concrete example of a second adjustment means that adjusts variation of magnification in the primary scanning direction.  
         [0185]     In  FIG. 11 ( g ), lifted portions  468  and  469  both are fixed on supporting plate  417  are made to support driving screw  431 , and engagement block  491  having a female screw as that shown in  FIG. 11 ( a ) is made to act on driving screw  431  (driving male screw) on which a male screw is formed. The engagement block  491  is connected with frame object  403  or  401  by means of connecting members  491   a  and  491   b  both provided on the engagement block  491 .  
         [0186]     When gear  432  on which driving screw  431  is fixed is driven by an un-illustrated motor, frame object  401  is moved horizontally, and thereby, variation of magnification of a scanning optical system in the primary scanning direction is adjusted. Incidentally, spring  474  is provided for absorbing a play in the adjustment mechanism. Further, protrusion  492  is provided on frame object  401 , and stopper portion  493  corresponding to the protrusion  492  is provided on supporting plate  417 . The protrusion  492  and stopper portion  493  constitute a distance keeping means which keeps the distance between exposure unit  30  and supporting plate  417  to be constant. The numeral  472  is an elastic member which-makes the protrusion  492  to be in pressure contact with the stopper  493 .  
         [heading-0187]     Embodiment 3  
         [0188]      FIG. 12  shows Embodiment 3 of the invention.  FIG. 12 ( a ) is a perspective view of exposure unit  30  that is viewed from the lower portion obliquely, and  FIG. 12 ( b ) is a sectional view of the supporting structure for exposure unit  30 .  
         [0189]     Frame object  401  of exposure unit  30  is supported on an image forming apparatus by connecting member  500  having on its tip portion a spherical body supported by an un-illustrated spherical body bearing, and is supported by supporting bar  502  having a form along a circular arc whose center is in the position of third supporting point T 30 . Namely, the supporting bar  502  is inserted through guide member  501  made of circular-arc-shaped tube that is fixed on frame object  401  by supporting member  503 . Owing to an un-illustrated driving means, frame object  401  is guided by guide member  50  representing a second supporting means to rotate on third supporting point T 30  representing a supporting point as shown with arrow V, and thereby, variation of magnification of a scanning optical system of exposure unit  30  in the primary scanning direction is corrected. In this way, the driving means, supporting bar  502  and guide member  501  constitute a magnification adjustment means. The frame object  401  is supported rotatably in the vertical and horizontal directions by connecting member  500  serving as the first supporting means that forms third supporting point T 30  and by spherical body bearing  505 .  
         [heading-0190]     Embodiment 4  
         [0191]      FIG. 13  is a perspective view of an exposure unit related to Embodiment 4 of the invention.  
         [0192]     Exposure unit  30  has a unit construction assembled on supporting member  600 . On lifted portions  601  and  602  both fixed on ends of the supporting member  600 , there is supported supporting shaft  603 . Tubular connecting members  605  and  606  are engaged rotatably with the supporting shaft  603 . The connecting members  605  and  606  are fixed on box-shaped frame object  604 . In the frame object  604 , there are provided polygon mirror unit  31 , fθ lens  32  and second cylindrical lens  33  which constitute a scanning optical system. On the side of the frame object  604  opposite to the side where the connecting members  605  and  606  are provided, there are fixed engagement flakes  607  and  608 . The engagement flakes  607  and  608  engage with driving screw  609  that is provided on an image forming apparatus. On the driving screw  609 , there is fixed gear  610 .  
         [0193]     When the driving screw  609  is driven to rotate by a motor representing an un-illustrated driving source, the engagement flakes  607  and  608  act on the frame object  604  to rotate around shaft  603 , and thereby, an inclination of exposure unit  30  in the primary scanning direction is adjusted. In this way, the motor, the driving screw  609  and the engagement flakes  607  and  608  constitute an inclination adjustment means.  
         [0194]     Though there have been omitted explanations respectively for adjustment of inclination in the primary scanning direction in Embodiment 3 and for correction of variation in magnification in the primary scanning direction in Embodiment 4, it is possible to use adjustment functions in Embodiment 1 and Embodiment 2 or other optional and known adjustment means.  
         [0195]     Even by adjusting the whole of the unit in Embodiment 4 with an adjustment mechanism of Embodiment 3, it is also possible to adjust an inclination in the primary scanning direction and to adjust variation of magnification in the primary scanning direction more surely, although more parts are needed.  
         [0196]     Though exposure unit  30  having the adjustment mechanism explained above is used in the image forming apparatus shown in  FIG. 1 , it is further used, without being limited to the image forming apparatus shown in  FIG. 1 , for an image forming apparatus which will be explained next as Embodiment 5, namely, an image forming apparatus wherein toner images each having a different color are formed on a photoreceptor, and a plurality of color toner images thus formed are transferred onto a recording sheet through an intermediate transfer body, and it is used for an image forming apparatus wherein toner images each having a different color are formed respectively on a plurality of photoreceptors to be superposed successively on a recording sheet to be transferred, or to be superposed successively on an intermediate transfer belt to be transferred collectively on a recording sheet, thus, a full color image is formed.  
         [heading-0197]     Embodiment 5  
         [0198]     Embodiment 5 will be explained as follows, referring to  FIGS. 14-20 .  FIG. 14  is a diagram showing the whole of an image forming apparatus in Embodiment 5 of the invention, each of  FIGS. 15-19  is a diagram showing the connecting structure between exposure units in the image forming apparatus shown in  FIG. 14 , and  FIG. 20  is a diagram showing a detecting means that detects positional relationship between exposure units.  
         [0199]     Belt-shaped intermediate transfer object  50  wherein a semiconducting layer is formed on a conductive base is trained about driving roller  51  and driven-roller  52  to be tensed appropriately by tension roller  53  so that the intermediate transfer object may be rotated as shown with an arrow. Image forming section YU for a yellow image, image forming section MU for a magenta image, image forming section CU for a cyan image and image forming section KU for a black image are provided to face the intermediate transfer object  50 . Each image forming section is provided with cylindrical photoreceptor U 1 , charging unit U 2 , exposure unit U 3 , developing unit U 4  and cleaning unit U 5 . Incidentally, with regard to these structural elements in the aforesaid image forming section, symbols are given only to those in the image forming section for a yellow image. Transfer unit  54  is provided to face each of the image forming sections YU, MU, CU and KU with an inbetween of the intermediate transfer object  50 .  
         [0200]     Sheet feeding cassettes  60 A,  60 B and  60 C each being loaded with recording sheets are provided at the lower portion of the apparatus, while, manual sheet feeding tray  60 D is provided on the side of the apparatus. Sheet feeding sections  61 A,  61 B,  61 C and  61 D each feeding out recording sheet P from each of the sheet feeding cassettes and manual sheet feeding tray are provided. Each of the sheet feeding sections  61 A,  61 B,  61 C and  61 D is composed of a feed-out roller that feeds out stacked recording sheets in succession from the uppermost sheet, a separation conveyance roller that separates one recording sheet to convey it and of a fanning roller. The numeral  56  is a conveyance belt that conveys recording sheet P to fixing unit  57 , the numeral  57  is a fixing unit having therein a heat roller and a pressure roller, the numeral  58  is a cleaning unit that removes remaining toner on the intermediate transfer object  50 , the numeral  59  is a sheet ejection tray and the numeral  64  is a reading unit that reads a document and generates image data.  
         [0201]     In each image forming section, a toner image is formed on photoreceptor U 1  through charging by charging unit U 2 , exposure by exposure unit U 3  and development by developing unit U 4 . With regard to formation of these toner images, transfer is made by transfer unit  54  on intermediate transfer body  50 , and each toner image is controlled to be formed so that toner images are superposed on the intermediate transfer body  50  and a full color image may be formed.  
         [0202]     A full color image formed on the intermediate transfer body  50  is transferred onto recording sheet P by transfer unit  55 , and is fixed by fixing unit  57 . Recording sheet P on which a full color image is formed is ejected to sheet ejection tray  59 .  
         [0203]     Each of image forming sections YU, MU, CU and KU is required to form a toner image of each color on intermediate transfer body  50  at the accuracy of a unit of micron. When this condition is not satisfied, a full color image formed on the intermediate transfer body SQ cannot avoid doubling. Therefore, positions of the image forming sections YU, MU, CU and KU, in particular, the relational position between exposure units U 3  of the units is required to be established highly accurately.  
         [0204]     In the present embodiment, exposure units U 3  of the image forming sections YU, MU, CU and KU, namely, four exposure units are connected vertically, as shown in  FIG. 15 . As shown in FIG.  15 , exposure unit U 3  for yellow images is fixed, at its both ends, on supporting plates  7 A and  7 B representing a supporting means, exposure unit U 3  for magenta images is fixed on supporting plates  8 A and  8 B representing a supporting means, exposure unit U 3  for cyan images is fixed on supporting plates  9 A and  9 B representing a supporting means, and exposure unit U 3  for black images is fixed on supporting plates  10 A and  10 B representing a supporting means, while, supporting plates  7 A and  8 A, supporting plates  8 A and  9 A, supporting plates  9 A and  10 A, supporting plates  7 B and  8 B, supporting plates  8 B and  9 B and supporting plates  9 B and  10 B are mutually connected and fixed.  
         [0205]     As shown in  FIG. 16 , exposure unit U 3  has therein Semiconductor laser  35  representing a light source and an optical system composed of collimator lens  36 , first cylindrical lens  37 , polygon mirror unit  31 , fθ lens  32  and second cylindrical lens  33 . The numeral  39  represents an index sensor, and  38  represents a mirror that guides a light beam to the index sensor  39 .  
         [0206]     The supporting plates  7 A,  7 B,  8 A,  8 B,  9 A,  9 B,  10 A and  10 B each having many protrusions are combined with each other through a fixing means in which each supporting plate is fixed to the other with these protrusions interlocked each other. Each of  FIGS. 17 and 18  shows an example of a combination of supporting plates  8 A and  9 A.  FIG. 17  shows how these supporting plates are positioned before they are combined, and  FIG. 18  shows how the supporting plates are positioned after they are combined. Image forming sections YU, MU, CU and KU are in the state wherein the image forming sections YU, MU and CU are adjusted to their desired positions by an unillustrated image forming section positioning means, with the image forming section KU serving as a standard. Under the state where positions of the image forming sections have been adjusted, each image forming section is made by an unillustrated adjusting means to be floating in the air, and each of mutual clearances of the aforesaid many protrusions has sufficient space including a margin for adjustment in the course of an adjusting step. With many protrusions stated above which are fixed with each other by a connecting means described later, the image forming sections YU, MU, CU and KU are constituted as a large image forming unit.  
         [0207]      FIG. 18 ( a ) is a side view of the combined portion, and  FIG. 18 ( b ) is a front view of the combined portion. As shown in  FIG. 17 , protrusion  82  of supporting plate  8 A is interlocked with protrusion  91  of supporting plate  9 A alternately. The protrusions  82  and  91  respectively have bent portions  84  and  93  at their tip portions, and after the supporting plate  8 A and the supporting plate  9 A are combined, resins RE are filled in a gap between the bent portions  84  and  93  by injecting melted resins RE as shown in  FIG. 18 . The combined portion where resins RE are filled is in the state shown in  FIG. 18  wherein the supporting plates  8 A and  9 A are combined firmly by resisting power of resin RE itself acting on force in the direction F to draw the supporting plates apart, because resins RE are filled in a gap between the bent portions  93  and  84  each having surface RS that crosses direction F to draw the supporting plates  8 A and  9 A apart.  
         [0208]     Supporting plate  7 A is combined with supporting plate  8 A, supporting plate  9 A is combined with supporting plate  10 A, supporting plate  7 B is combined with supporting plate  8 B, supporting plate  8 B is combined with supporting plate  9 B, and supporting plate  9 B is combined with supporting plate  10 B, all in the same way as in the foregoing to fill resins RE by injecting them.  
         [0209]     In the aforesaid way, four exposure units U 3  are combined firmly with each other.  
         [0210]      FIG. 19  shows another-example of combination of exposure units. In this example, bent portions are not provided on many protrusions provided on each of supporting plates  7 A,  8 A,  9 A and  10 A, which is different from the occasion in  FIGS. 17 and 18 .  
         [0211]     In the example shown in  FIG. 19 , after adjoining supporting plates of exposure units such as supporting plates  7 A and  8 A and supporting plates  8 A and  9 A are combined, resins RE are injected in the combined portion so that both of them are combined and fixed. In this case, resin materials which are of the same type as that of resins forming the supporting plates  7 A and  8 A are used as resin RE. The same resin materials used for the supporting plate and for resin RE to be filled make the coefficients of linear expansion for them to be the same, resulting in prevention of a strain of the image forming section caused by temperature rise in an image forming apparatus. When filling melted resins in a gap, a portion on each of supporting plates  7 A,  7 B,  8 A,  8 B,  9 A,  9 B,  10 A and  10 B where resins are filled is affected by heat and its surface is roughened, which makes the supporting plates to be combined more firmly, compared with a combining method which does not apply heat.  
         [0212]     Adjoining supporting plates other than the supporting plates  7 A,  8 A and  9 A are also combined in the same way.  
         [0213]     Incidentally, the combining structure-between exposure units in the present embodiment can naturally be applied also to combination for exposure units  30 ,  30 M,  30 C and  30 K in  FIG. 1 .  
         [0214]     To improve image quality for a color image forming apparatus, it is necessary to assemble after adjusting positions accurately for exposure units U 3  respectively of image forming sections YU, MU, CU and KU. In the present embodiment, the assembled body shown in  FIG. 15  is assembled through the steps explained below.  
         [0215]     First, supporting plates  7 A,  7 B,  8 A,  8 B,  9 A,  9 B,  10 A and  10 B are fixed on exposure units U 3 . The exposure unit U 3  on which the lowermost supporting plates  10 A and  10 B are attached is fixed to a jig with an unillustrated attaching standard. Other exposure units U 3  are stacked in succession from the bottom, and the exposure units other than the lowermost one are held by an arm of an unillustrated jig to be kept in the state of floating. Next, for the lowermost exposure unit, other exposure units are adjusted in terms of position so that they may be within a range of design values. The adjustment is made by measuring the distance between exposure units U 3  accurately.  
         [0216]     Melted resins RE are poured into a gap formed between the aforesaid supporting plates whose positions have been adjusted, and the supporting plates are fixed.  
         [0217]     As shown in  FIG. 20 , on intermediate transfer body  50  trained about driving roller  51 , driven rollers  52  and  53 , there are formed, through exposure of fixed pattern by exposure unit U 3 , many marks ML, MC and MR each being in a shape of “inverted L” for position adjustment. Marks ML are formed on the left end portion, marks MC are formed on the central portion and marks MR are formed on the central portion, all of photoreceptor  50 . Each mark has a shape of “inverted L” composed of a segment being in parallel with primary scanning line SL and a segment crossing the primary scanning line SL at an angle of 45°.  
         [0218]     Information of position detection for the segment being in parallel is used for adjusting a position of each exposure unit U 3  for each of image forming sections YU, MU, CU and KU, and information of position detection for the segment crossing the primary scanning line is used for adjusting a position of each exposure unit U 3  in the primary scanning direction and for adjusting magnification in the primary scanning direction. Positions of marks SSL, SSCL and SSR are detected by sensor SS. The sensor SS is composed of light-projecting element SS 1  of the LED construction which projects a light beam on photoreceptor  50 , light-receiving element SS 2  that receives reflected light from photoreceptor  50  and lens SS 3 .  
         [0219]     Adjustment of positions between exposure units employing a position detecting means shown in  FIG. 20  is conducted through image forming tests operating an image forming apparatus in which the assembled body shown in  FIG. 15  is incorporated.  
         [0220]     Owing to the assembling steps for an image forming apparatus stated above, there is formed an image forming apparatus capable of forming a color image with high image quality which has neither position shifting in the primary scanning direction nor position shifting in the sub-scanning direction for image forming sections YU, MU, CU and KU, and is free from inclination of the primary scanning line and from variation o magnification in the primary scanning direction.  
         [0221]     Structure 1 makes it possible to adjust an inclination in the primary scanning direction with a high precision under the simple supporting structure and through easy adjustment operation, and thereby to realize an image forming apparatus offering high image quality.  
         [0222]     Structure 2 makes it possible to adjust variation of magnification in the primary scanning direction with a high precision under the simple supporting structure and through easy adjustment operation, and thereby to realize an image forming apparatus offering high image quality.  
         [0223]     Structure 3 makes it possible to adjust an inclination in the primary scanning direction and variation of magnification in the primary scanning direction with a high precision under the simple supporting structure and through easy adjustment operation, and thereby to realize an image forming apparatus offering high image quality.  
         [0224]     Either one of Structures 4-8 makes it possible to adjust a scanning optical system easily and accurately in less adjustment frequency.  
         [0225]     Either one of Structures 9-12 makes it possible to adjust more precisely with less amount of adjustment, because the adjustment is conducted, centering almost around an optical axis of a scanning optical system.  
         [0226]     Either one of Structures 13-16 makes it possible to realize a supporting structure wherein positional shifting caused by rotation is extremely limited, as a spherical body bearing that supports an exposure unit rotatably in all directions, and thereby, an adjustment with a high precision is possible, and an adjustment value can be maintained for a long time.  
         [0227]     Either one of Structures 17-29 makes it possible to realize an adjustment means at a high precision in the case of assembling various members such as a driving screw, an engagement flake and an engagement block for constituting the adjustment means, without requirement of a high assembling precision.  
         [0228]     Either one of Structures 30-32 makes it possible to realize a color image forming apparatus offering high image quality.  
         [0229]     Either one of Structures 27-34 makes it possible to adjust extremely accurately.  
         [0230]     Structure 35 makes it possible to adjust, with a high precision, a variation of magnification in the primary scanning direction with a simple adjusting means.  
         [0231]     Structure 36 makes it possible to adjust, with a high precision, an inclination in the primary scanning direction with a simple adjusting means.  
         [0232]     Structures 37-44 make it possible to realize an image forming apparatus wherein no position shifting is caused between plural exposure units and to realize an image forming apparatus wherein images of high image quality can be formed for a long time.  
         [0233]     Hereinunder, are detailed descriptions of the other embodiments.  
         [0234]     As a color image forming apparatus employing the scanning optical device of the invention, there is used a color image forming apparatus of a tandem type wherein toner images which are formed respectively on a plurality of image carriers to be composed respectively of yellow (Y) toner, magenta (M) toner, cyan (C) toner and black (K) toner are formed, and the toner images thus formed on the plural image carriers are superposed on a transfer material through an intermediate transfer body, or directly.  
         [0235]     The color image forming apparatus shown in  FIG. 21  is called a color image forming apparatus of a tandem type wherein the scanning optical device of the invention is used as an exposure means, and toner images formed on image carriers are transferred onto an intermediate transfer body to be superposed, and then, the superposed toner images are transferred collectively, and the color image forming apparatus is composed of plural sets of image forming sections  810 M,  810 M,  810 C and  810 K, intermediate transfer unit  807 , a sheet conveyance means and fixing means  824 . On the top portion of image forming apparatus main body (hereinafter referred to as an apparatus main body) A, there is arranged document image reading device SC.  
         [0236]     Image forming section  810 Y that forms a yellow image has therein charging means  802 Y, exposure means  803 Y, developing means  804 Y, primary transfer means  805 Y and cleaning means  806 Y, all arranged around image carrier (photoreceptor)  801 Y. Image forming section  810 M that forms a magenta image has therein image carrier (photoreceptor)  801 M, charging means  802 M, exposure means  803 M, developing means  804 M, primary transfer means  805 M and cleaning means  806 M. Image forming section  810 C that forms a cyan image has therein image carrier (photoreceptor)  801 C, charging means  802 C, exposure means  803 C, developing means  804 C, primary transfer means  805 C and cleaning means  806 C. Image forming section  810 K that forms a black image has therein image carrier (photoreceptor)  801 K, charging means  802 K, exposure means  803 K, developing means  804 K, primary transfer means  805 K and cleaning means  806 K. On the image forming section  810 , there are conducted charging, exposure and development, so that images each being of a different color are formed on image carrier  801 .  
         [0237]     Intermediate transfer unit  807  has therein semiconducting endless-belt-shaped intermediate transfer body  870  that is trained about a plurality of rollers and is supported rotatably.  
         [0238]     Images each being of a different color formed by image forming sections  810 Y,  810 M,  810 C and  810 K are transferred, in synchronization, by primary transfer means  805 Y,  805 M,  805 C and  805 K onto rotating intermediate transfer body  870  successively to be superposed, thus, a composite color image is formed. Recording medium (hereinafter referred to as sheet) P loaded in sheet feeding cassette  820  is fed by sheet feeding means  821  to be conveyed, through a plurality of intermediate rollers  822 A,  822 B,  822 C and  822 D and registration roller  823 , to secondary transfer means  805 A where superposed color images are transferred collectively on sheet P. The sheet P on which the color images have been transferred is fixed by fixing means  824 , and is interposed by sheet ejection rollers  825  to be placed on sheet ejection tray  826  that is located outside the apparatus.  
         [0239]     On the other hand, after color images are transferred on sheet P by secondary transfer means  805 K, the sheet P is curvature-separated from intermediate transfer body  870 ; and toner remaining on the intermediate transfer body  870  is removed by cleaning means  806 A.  
         [0240]     In the course of image forming processing, the primary transfer means  805 K is in pressure contact with photoreceptor  801 K constantly. Other primary transfer means  805 Y,  805 M and  805 C are respectively in pressure contact with their corresponding photoreceptors  801 Y,  801 M and  801 C only when color images are formed.  
         [0241]     Secondary transfer means  805 A is in pressure contact with intermediate transfer body  870  only when the sheet P passes through the secondary transfer means so that secondary transfer is conducted.  
         [0242]     An exposure means used for the color image forming apparatus in  FIG. 21  will be explained next.  FIG. 22  shows a scanning optical device representing an exposure means, and scanning optical device  803 Y will be explained as an example, because scanning optical devices  803 Y,  803 M,  803 C and  803 K each representing an exposure means are the same in terms of mechanism. On scanning optical device main body  831  formed on the scanning optical device  803 Y, there are fixed optical parts and others. With regard to the scanning optical device, divergent light emitted from semiconductor laser light source device  839  is collimated by collimator lens  833 . A section of the collimated light is formed by aperture-stop  834 , and it is converged, only in the sub-scanning direction, by first cylindrical lens  835  having power only in the sub-scanning direction, then, it passes through second cylindrical lens  838  that corrects inclination of deflector  836  to be converged on the surface to be scanned as a light spot which scans on the surface to be scanned in the W direction at a constant speed through deflector  836  and fθ lens  837 .  
         [0243]     S 0  represents an optical synchronization sensor, and it detects (optical synchronization detection) a light beam that is reflected by mirror M 0  and enters optical synchronization sensor S 0  to be deflected and to scan, and then, it starts writing after a certain period of time. Thus, there is conducted optical synchronization wherein each toner image of a color image in the primary scanning direction agrees in terms of position with other toner images.  
         [heading-0244]     (Embodiment 6)  
         [0245]     Embodiment 6 will be explained as follows, referring to a top view, an illustration and a timing chart shown in  FIG. 23 .  
         [0246]     The scanning optical device shown in the present example is arranged to be movable around a point in the vicinity of an optical axis of the second cylindrical lens  838 , that point serving as a rotation center, and when one end of the second cylindrical lens  838  is moved upward or downward, an inclination of a deflected scanning line emitted from deflector  836  and fθ lens  837  is changed.  
         [0247]     In the present example, detection sensor S 1  representing a detection means detects that a light beam passing through the second cylindrical lens  838  has arrived at an upper limit position or a lower limit position to be allowed, and its arrangement is shown in  FIG. 23 ( a ) Namely, upper limit detection mirror M 1  and lower limit detection mirror M 2  for detecting respectively the upper limit position and the lower limit position to be allowed are arranged to be shifted each other in the optical scanning direction, between an incidence position to optical synchronization sensor S 0  for a light beam passing through the second cylindrical lens  838  and a position to start writing.  
         [0248]     The upper limit detection mirror M 1  arranged reflects a light beam when the upper limit position is exceeded by the light beam, and the reflected light enters detection sensor S 1 . The lower limit detection mirror M 2  arranged reflects a light beam when the lower limit position is exceeded by the light beam, and the reflected light enters detection sensor S 1 . When a light beam exceeds the allowable upper limit or lower limit, the detection sensor S 1  detects that the light beam has arrived at an allowable limit.  
         [0249]     With regard to judgment about whether the limit value of the light beam detected by the detection sensor S 1  is an upper limit or a lower limit, if the detection sensor S 1  detects that a light beam has arrived at the limit value, after t 1  sec, for example, from optical synchronization detection of the light beam conducted by optical synchronization sensor S 0 , as shown in FIGS.  23 ( b ) and  23 ( c ), there is made judgment that arrival at the upper limit has been detected. On the other hand, if the detection sensor S 1  detects that a light beam has arrived at the limit value, after t 2  sec, for example, from optical synchronization detection, there is made judgment that arrival at the lower limit has been detected.  
         [0250]     In the present example, the upper limit detection mirror M 1  and the lower limit detection mirror M 2  are arranged to be shifted each other in the optical scanning direction. Therefore, the allowable upper limit and the lower limit of a light beam can easily be detected by a single piece of detection sensor S 1 , and a device to detect the upper limit and lower limit can be simplified.  
         [heading-0251]     (Embodiment 7)  
         [0252]     Embodiment 7 will be explained as follows, referring to a top view, an illustration and a timing chart shown in  FIG. 24 .  
         [0253]     The scanning optical device shown in the present example is arranged to be movable around a point in the vicinity of an optical axis center of the second cylindrical lens  838  serving as a center of oscillation, and when one end of the second cylindrical lens  838  is moved upward or downward, an inclination of a deflected scanning line emitted from deflector  836  and fθ lens  837  is changed.  
         [0254]     In the present example, optical synchronization sensor. S 0  conducts optical synchronization detection for scanning light passing through the second cylindrical lens  838 , and it detects that the light beam passing through the second cylindrical lens  838  has arrived at the allowable upper limit position or lower limit position and its arrangement is shown in  FIG. 24 ( a ). Namely, upper limit detection mirror M 1  and lower limit detection mirror M 2  for detecting respectively the upper limit position and the lower limit position to be allowed are arranged to be shifted each other in the optical scanning direction, between an incidence position to optical synchronization sensor S 0  for a light beam passing through the second cylindrical lens  838  and a position for writing.  
         [0255]     The upper limit detection mirror M 1  arranged reflects a light beam when the upper limit position is exceeded by the light beam, and the reflected light enters optical synchronization sensor S 0 . The lower limit detection mirror M 2  arranged reflects a light beam when the lower limit position is exceeded by the light beam, and the reflected light enters optical synchronization sensor S 0 . The optical synchronization sensor S 0  detects optical synchronization and detects that the light beam has arrived at an allowable limit of the upper limit or the lower limit.  
         [0256]     With regard to judgment about whether the limit value of the light beam detected by the optical synchronization sensor S 0  is an upper limit or a lower limit, if the arrival at the limit value is detected after t 1  sec, for example, after optical synchronization detection of a light beam is conducted by optical synchronization sensor S 0  as shown in FIGS.  24 ( b ) and  24 ( c ), there is made judgment that arrival at the upper limit has been detected. On the other hand, if the limit value is arrived, after t 2  sec, for example, from optical synchronization detection, there is made judgment that arrival at the lower limit has been detected.  
         [0257]     In the present example, the upper limit detection mirror M 1  and the lower limit detection mirror M 2  are arranged to be shifted each other in the optical scanning direction, and an arrangement is made so that reflected lights from these mirrors may enter optical synchronization sensor S 0  that conducts optical synchronization. Therefore, sensors for detecting the upper limit and the lower limit are not needed, and detection for the upper limit and the lower limit is simplified and is made to be easy.  
         [heading-0258]     (Embodiment 8)  
         [0259]     Embodiment 8 will be explained as follows, referring to  FIGS. 25-27 .  
         [0260]     On the scanning optical device shown in the present example, there is provided second cylindrical lens  838  representing a movable optical element. The second cylindrical lens  838  is arranged to be movable around a point in the vicinity of its optical axis center serving as a center of oscillation, and when one end of the second cylindrical lens  838  is moved upward or downward, an inclination of a deflected scanning line which has been deflected by the deflector  836  and passed through fθ lens  837  is changed.  
         [0261]     The present example is arranged to be of the structure to move one end of the second cylindrical lens  838  upward or downward by rotating a movable shaft provided in that structure, wherein a wire that is stretched by a spring is wound around the rotatable shaft, and a wire position detecting means that detects a position of the wire is provided along the stretched wire, so that the upper limit or the lower limit of the movement of the second cylindrical lens  838  may be detected by detecting the position of the wire.  FIG. 25 ( a ) is a perspective view of the scanning optical device of the present example, and  FIG. 26  shows primary portions of the present example. Incidentally,  FIG. 26 ( a ) is a side view of the primary portions of the present example,  FIG. 26 ( b ) is a top view of the primary portions and  FIG. 26 ( c ) is a side view of the primary portions.  
         [0262]     On the second cylindrical lens  838 , there is provided, in the vicinity of the optical axis center, center shaft  838   a  serving as the center of oscillation, and the center shaft  838   a  is placed on V-shaped supporting shaft section  831   b  provided on base board  831   a  of the scanning optical device main body  831  to be pressed downward by elasticity of blade spring member  831   c , thus, the second cylindrical lens  838  is capable of oscillating around the center shaft  838   a  that serves as the center of oscillation. An inclination of the deflected scanning line is changed by oscillation of the second cylindrical lens  838 .  
         [0263]     On the base board  831   a , there is attached rotary shaft  841  that is rotatable. The rotary shaft  841  has male screw portion  841   a  and has, on its tip portion, cylindrical wire winding section  841   b  on which wire  850  is wound and worm gear  842  is mounted solidly.  
         [0264]     On the male screw portion  841   a  of the rotary shaft  841 , there is mounted, on a screw-engagement basis, nut member  843  which is composed of two disc-shaped members located at the upper and lower portions and a female screw portion located at the central portion and has a horizontal-H-shaped sectional view.  
         [0265]     Side end portion  838   b  of the second cylindrical lens  838  is forked into two portions. Inside the upper disc-shaped member among the two disc-shaped members of the nut member  843 , there is provided elastic object  843   a  which is in a shape of a blade spring and is urged, and the side end portion  838   b  forked into two portions of the second cylindrical lens  838  is interposed tightly without any play between two disc-shaped members of the nut member  843 . Therefore, the nut member  843  moves upward or downward when rotary shaft  841  rotates, and accordingly, the side end portion  838   b  of the second cylindrical lens  838  also moves upward or downward.  
         [0266]     The worm gear  842  mounted solidly on the rotary shaft  841  is engaged with worm  845  that is connected through gear G 1  provided on a motor shaft of motor M that is mounted on scanning optical device main body  831  to be capable of rotating regularly and in the opposite direction and through intermediate gear G 2 , and when the motor M rotates, the rotary shaft  841  rotates clockwise or counterclockwise.  
         [0267]     Depending on the direction of rotation of the wire winding section  841   b  positioned on the upper portion of the rotatable rotary shaft  841 , wire  850  is further wound around the wire winding section  841   b  or unwound, depending on the direction of rotation of the rotary shaft  841 , to be moved toward the right side or the left side, because wire  850  wound around the wire winding section  841   b  is stretched by tension spring  851 .  
         [0268]     In one structure, wire position detecting means  852  that detects movement of wire  850  is provided along the wire  850  to detect the position of movement of the wire  850 , and this detection makes it possible to detect a range of movement for which the second cylindrical lens  838  can move, namely, to detect the tolerance limit of the upper limit or the lower limit of a light beam passing through the second cylindrical lens  838 .  
         [0269]     In another structure, as shown in  FIG. 26 ( c ), there is provided pulley  853  representing a wire direction changing means that changes the direction of wire  850  which is in the state of extension, to lead the wire  850  to the reverse side of base board  831   a , and wire position detecting means  852  is provided on the reverse side of base board  831   a  where no optical element of the scanning optical device is provided, to utilize a space effectively and to downsize an apparatus.  
         [0270]     The wire position detecting means  852  that detects a range of movement allowed for the wire  850  will be explained. As illustrations in  FIG. 27  show, the wire position detecting means  852  includes a detecting method of detection by an optical detector, a detecting method of detection by a mechanical switch and a detecting method of detection by conductive detecting.  
         [0271]      FIG. 27 ( a ) is wire position detecting means  852  employing an optical detector, in which light-emitting element- 855   a  and light-receiving element  855   b  are provided to face each other with wire  850  between as shown in (1), and when light shielding laminar member  855   c  which is fixed on the wire  850  is present newly between the light-emitting element  855   a  and light-receiving element  855   b , light emitted from the light-emitting element  855   a  is prevented from arriving at the light-receiving element  855   b , thus, light detection from ON to OFF is carried out. The numeral  854  represents guide members provided to interpose the wire  850 , and when laminar member  855   c  exists, the guide members  854  guide the laminar member  855   c  to be between the light-emitting element  855   a  and the light-receiving element  855   b . Incidentally, with regard to the light shielding laminar member  855   c , it may be a single laminar member having a length equivalent to that of movement allowed for wire  850  as shown in (2), or, a distance between two laminar members  855   c  may be arranged to be the same as the length of movement allowed for wire  850  as shown in (3).  
         [0272]      FIG. 27 ( b ) is wire position detecting means  852  employing a mechanical switch, in which the state shown in (1) turns out to be the state shown in (2) when actuator  856   a  of microswitch  856  touches laminar member  856   b , and light detection from OFF to ON is carried out. With regard to a method to detect the length of movement allowed for wire  850  in this case again, either a method based on a single sheet of fixed member  856   b  having the length of movement allowed (See  FIG. 27 -( a ) (2)), or a method based on two fixed members  856   b  wherein a distance between them is arranged to be the same as the length of movement allowed (See  FIG. 27 ( a ) (3)) is practicable.  
         [0273]      FIG. 27 ( c ) is wire position detecting means  852  employing a method to detect the state of electric continuity, in which a conductive ball representing fixed member  857   e  is fixed on conductive wire  850  which is grounded. On the apparatus main body side, on the other hand, there is provided laminar detecting member  857   a  on which a circular detecting hole that conducts when the fixed member  857   e  arrives is provided. To the detecting member  857   a , there are connected small power supply  857   b  and continuity detecting means  857   c , and thereby, it is detected that the wire  850  has come to the limit of its movement allowed. The symbol (1) shows an example wherein two fixed members  857   e  are fixed on the wire  850  to be away each other by an amount of the limit for movement allowed for the wire so that the detection may be carried out, while, (2) shows an example wherein two sets of detecting members  857   a  are fixed to be away each other by the amount of movement allowed for the wire  850  so that the detection may be carried out.  
         [0274]     By virtue of the wire position detecting means explained above, it is detected that the wire has come to the limit of its movement allowed. When the control section is constructed to control to stop the drive of motor M based on its detection information, a light beam that conducts optical scanning on a photoreceptor can be adjusted within a range which does not deviate from the allowed range of change.  
         [0275]     Regarding an adjustment of variation in magnification in the direction of scanning line, an example of the invention will be explained using FIGS.  25 ( a ),  25 ( b ) and  25 ( c ).  
         [0276]     With regard to positioning of the scanning optical device on the image forming apparatus, pins  863   a  and  863   b  protruded from frame  861  of the image forming apparatus are inserted in the scanning optical device. A round hole is formed on a casing of the scanning optical device at the position thereon corresponding to the pin  863   a , while an elongated hole is formed with its major axis being in parallel with the primary scanning direction on the same casing of the scanning optical device at the position thereon corresponding to the pin  863   b . Further, the rear end portion of the scanning optical device is supported on adjustment screw  864  protruded from frame  862  of the image forming apparatus. Pressing member  867  presses on the rear end of the scanning optical device in the horizontal and vertical directions. In the horizontal direction, the scanning optical device is positioned by pins  865   a  and  865   b  which are protruded from the scanning optical device and come in contact respectively with frame  861  and wedge-formed plate  866  provided on the frame  861 .  
         [0277]     The wedge-formed plate  866  is a rotatable member whose thickness on a certain section perpendicular to the rotation direction of the member varies gradually as the section moves in the rotation direction of the member, and when the member rotates, a portion of the scanning optical device closer to the pin  865   b  that is in contact with the wedge-formed plate can rotate relatively to the frame  861 , by an amount of a play of the round hole formed on the casing of the scanning optical device, on the center of rotation represented by the position where the pin  865   a  is in contact with the frame  861 . The rotation makes it possible to change a distance between the surface to be scanned in a scanning plane and the scanning optical device, and thereby, variation of parallelism between an image carrier and the scanning optical device caused by size variations of constituent parts of the image forming apparatus can be corrected, resulting in a possibility to correct variance of magnification in the primary scanning direction. After an amount of error is measured automatically or visually by the chart for detecting the magnification error, the wedge-formed plate  866  is rotated up to the desired value manually or automatically so that the correction may be made.  
         [0278]     An amount of protrusion from the scanning optical device for each of the pins  865   a  and  865   b  may be fixed. However, if the amount of protrusion is arranged to be adjusted by a screw or the like, it is possible to previously adjust, in the course of assembly of the scanning optical device, variations of magnification in the primary scanning direction caused by variations of each part of the scanning optical device for the ideal position corresponding to the frame of the image forming apparatus, for example, variations of a focal length of the lens and variations of the casing of the scanning optical device. When the scanning optical device adjusted accurately to the reference point for mounting is mounted on the image forming apparatus, the wedge-formed plate  866  has only to be operated for positional variations of an image carrier for the reference point of mounting for the scanning optical device in the image forming apparatus, thus, an adjustment margin can be made small, and an adjustment means can be made small in size. It is further possible to make an amount of a play between the pin  863   a  and the hole that is formed on the casing of the scanning optical device so that the pin  863   a  may be inserted thereinto to be small, and to provide an image forming apparatus which is highly reliable. Incidentally, correction of the total magnification and that of the writing timing are conducted by a known electrical correction means.  
         [0279]     In Embodiments 6,7 and 8, the following effect can be attained:  
         [0280]     A limited position allowed for a light beam emitted and deflected is detected by a single sensor, which simplifies a necessary apparatus.  
         [0281]     A limited position allowed for a light beam emitted and deflected is detected without providing a sensor for that purpose, which further simplifies a necessary apparatus.  
         [0282]     In the Embodiments, since information about movement of an optical-element conducting correction of a position of a scanning line is converted into a movement of a wire, a wire position detecting means can be arranged at the corner portion of the apparatus which is away from the optical element, which makes an apparatus to be small in size.  
         [0283]     In Embodiment 8, a wire is lead to the rear side of an apparatus, and a position for a wire position detecting means to be arranged can be selected freely, which further makes an apparatus to be small in size.  
         [0284]     Further in Embodiment 8, the adjustment of the variation in the magnification can be made easy by rotation of the wedge member.