Abstract:
An optical scanning device and an image forming apparatus of the present invention aim to improve operation in fewer adjustment processes, even when position, attitude, and shape errors are generated in optical element components, thereby deviating an optical path from a design value and degrading performance. An optical scanning device of the present invention includes a light source, a pre-deflection optical device for applying a predetermined characteristic to a light beam irradiated from the light source, a light deflection device for deflecting the light beam having a predetermined characteristic supplied from the pre-deflection optical device in a main scanning direction by reflection, and a post-deflection optical device having an image forming lens for focusing the deflected light beam irradiated from the light deflection device on an image forming surface. Further, the optical scanning device includes a light deflection device adjusting mechanism configured to enable adjustment of an inclination quantity of the light deflection device, a pre-deflection optical device adjusting mechanism, having the light source and the pre-deflection optical device attached thereon, configured to simultaneously enable adjustment of an overall position and attitude of the light source and the pre-deflection optical device, and a post-deflection optical device adjusting mechanism configured to change a position and/or angle in a sub scanning direction of the light beam emitted from the image forming lens. The image forming apparatus of the present invention includes an optical scanning device of the present invention.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an optical scanning device and an image forming apparatus. For example, the invention can be applied to plural-drum type printers and copy machines and the like.  
         [0003]     2. Description of the Related Art  
         [0004]     In an optical scanning device applied to an image forming apparatus such as plural-drum type color copy machines and color printers, as described in Japanese Patent Laid-Open, No. 11-194285, a plurality of beams for each of color components emitted from each of light sources have their optical paths combined by a pre-deflection optical system to enter a polygon mirror. The beam deflected by the polygon mirror passes through an fθ lens which constitutes a post-deflection optical system and then is separated into the beam for each of color components to be irradiated to a drum for each of color components.  
         [0005]     For example, it is assumed that a surface inclination is generated in a polygon mirror by a manufacturing error or an attaching error, causing optical performance degradation. Conventionally, performance degradation is improved by rotationally adjusting a cylindrical lens for each laser beam around the beam moving direction. However, this approach requires adjustment of the cylindrical lens by the number of laser beams (the number of color components) and therefore increases adjustment processes.  
         [0006]     Although the above case is explained by taking a polygon mirror as an example, when performance degradation of other optical element components as common in a plurality of laser beams is improved, optical element components need to be adjusted by the number of laser beams (the number of color components), resulting in increased adjustment processes.  
       SUMMARY OF THE INVENTION  
       [0007]     It is an object of the present invention to, provide an optical scanning device and an image forming apparatus which are capable of improving operation in fewer adjustment processes even when position, attitude, and shape errors are generated in optical element components and thereby deviate an optical path from a design value and degrade performance.  
         [0008]     An optical scanning device of the present invention includes a light source, a pre-deflection optical device for applying a predetermined characteristic to a light beam irradiated from the light source, a light deflection device for deflecting the light beam having a predetermined characteristic supplied from the pre-deflection optical device in a main scanning direction by reflection, a post-deflection optical device having an image forming lens for focusing the deflected light beam irradiated from the light deflection device on an image forming surface, and a light deflection device adjusting mechanism configured to enable adjustment of an inclination quantity of the light deflection device.  
         [0009]     An optical scanning device of the present invention includes a light source, a pre-deflection optical device for applying a predetermined characteristic to a light beam irradiated from the light source, a light deflection device for deflecting the light beam having a predetermined characteristic supplied from the pre-deflection optical device in a main scanning direction by reflection, a post-deflection optical device having an image forming lens for focusing the deflected light beam irradiated from the light deflection device on an image forming surface, and a pre-deflection optical device adjusting mechanism, having the light source and the pre-deflection optical device attached thereon, configured to simultaneously enable adjustment of an overall position and/or attitude of the light source and pre-deflection optical device, and adjust an incident position and/or angle in the sub scanning direction of the laser beam to the light deflection device.  
         [0010]     An optical scanning device of the present invention includes a light source, a pre-deflection optical device for applying a predetermined characteristic to a light beam irradiated from the light source, a light deflection device for deflecting the light beam having a predetermined characteristic supplied from the pre-deflection optical device in a main scanning direction by reflection, a post-deflection optical device having an image forming lens for focusing the deflected light beam irradiated from the light deflection device on an image forming surface, and a post-deflection optical device adjusting mechanism, having at lease the image forming lens attached thereon, configured to enable adjustment of a position and/or attitude of the image forming lens and change a position and/or angle in a sub scanning direction of the light beam emitted from the image forming lens.  
         [0011]     The image forming apparatus of the present invention includes an optical scanning device of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a schematic cross sectional view illustrating a color image forming apparatus according to an embodiment of the present invention;  
         [0013]      FIG. 2  is a schematic cross sectional view illustrating an element of a post-deflection optical system of an optical scanning device according to the embodiment;  
         [0014]      FIG. 3  is a schematic plan view illustrating an element of the optical scanning device according to the embodiment;  
         [0015]      FIG. 4  is a schematic cross sectional view illustrating an element position of a pre-deflection optical system of the optical scanning device according to the embodiment by regarding an optical path as a linear optical path;  
         [0016]      FIG. 5  is a plan view for explaining a method for attachment of a light deflection device to the optical scanning device according to the embodiment;  
         [0017]      FIG. 6  is a cross section taken on line VI-VI of  FIG. 5 ;  
         [0018]      FIG. 7  is a plan view for explaining a method for attachment of the pre-deflection optical system to the optical scanning device according to the embodiment;  
         [0019]      FIG. 8  is a cross section taken on line VIII-VIII of  FIG. 7 ; and  
         [0020]      FIG. 9  is a plan view for explaining a method for attachment of an image forming lens (fθ lens) to the optical scanning device according to the embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     (A) Preferred Embodiment  
       [0021]     A preferred embodiment of an optical scanning device and an image forming apparatus of the present invention will be described below with reference to drawings.  
         [0022]      FIG. 1  illustrates a color image forming apparatus incorporating the optical scanning device according to the preferred embodiment of the present invention. In this kind of color image forming apparatus, since there is generally employed four kinds of image data separated into each of colors including yellow (Y), magenta (M), cyan (C) and black (B) and four sets of various apparatuses for forming the image at each of the color components corresponding to each of Y, M, C and B, structure is made such as to identify the image data at each of the color components and the corresponding mechanism by adding Y, M, C and B to each of reference numerals.  
         [0023]     As shown in  FIG. 1 , an image forming apparatus  100  includes first to fourth image forming units  50 Y,  50 M,  50 C, and  50 B for forming the image at each of the separated color components.  
         [0024]     The respective image forming units  50  (Y, M, C, and B) are arranged in the order of  50 Y,  50 M,  50 C and  50 B below an optical scanning device  1  corresponding to each position at which laser beams L (Y, M, C, and B) are emitted for optical-scanning image information of each of the color components by a first folding mirror  33 B and third folding mirrors  37 Y,  37 M, and  37 C of a multi-beam optical scanning device  1  as described in detail with reference to  FIGS. 2 and 3 .  
         [0025]     A transfer belt  52  for transferring transfer material on which an image formed through the respective image forming units  50  (Y, M, C, and B) are transferred is placed below the respective image forming units  50  (Y, M, C, and B).  
         [0026]     The transfer belt  52  is passed over a belt drive roller  56  and a tension roller  54  rotated by a motor (not shown) in the direction of the arrow and is rotated at a predetermined speed in a direction of rotation of the belt drive roller  56 .  
         [0027]     The respective image forming units  50  (Y, M, C, and B), cylindrically formed so as to be rotatable in the direction of the arrow, have photosensitive drums  58 Y,  58 M,  58 C, and  58 B on which an electrostatic latent image is formed correspond to an image exposed by the optical scanning device  1 .  
         [0028]     In the space surrounding each of the photosensitive drums  58  (Y, M, C, and B), along the rotational direction of respective photosensitive drums  58  (Y, M, C, and B), there are arranged changing devices  60  (Y, M, C, and B), developing devices  62  (Y, M, C, and B), transfer devices  64  (Y, M, C, and B), cleaners  66  (Y, M, C, and B), and discharging devices  68  (Y, M, C, and B). The changing devices  60  supply a predetermined potential on the surface of the respective photosensitive drums  58  (Y, M, C, and B). The developing devices  62  perform development by supplying toner having a color corresponding to an electrostatic latent image formed on the surface of the respective photosensitive drums  58  (Y, M, C, and B). The transfer devices  64 , provided at a position opposed to the respective photosensitive drums  58  (Y, M, C, and B) from the back of the transfer belt  52  with the transfer belt  52  interposed between the respective photosensitive drums  58  (Y, M, C, and B), transfer toner image of the respective photosensitive drums  58  (Y, M, C, and B) to a recording medium transferred by the transfer belt  52 , that is, a paper sheet P. The cleaner  66  removes residual toner remaining on the respective photosensitive drums  58  (Y, M, C, and B) when the respective transfer devices  64  (Y, M, C, and B) transfer toner image onto the sheet P. The discharging devices  68  discharge residual potential remaining on the respective photosensitive drums  58  (Y, M, C, and B) after transfer of toner image by the respective transfer devices  64  (Y, M, C, and B).  
         [0029]     Below the transfer belt  52 , there is disposed a sheet cassette  70  for receiving the sheet P on which the image formed by the respective image forming units  50  (Y, M, C, and B) is transferred.  
         [0030]     A generally semilunar shaped conveying roller  72  for taking out the sheet P received in the sheet cassette  70  one by one from the top is disposed at one end of the sheet cassette  70  and on the side close to the tension roller  54 .  
         [0031]     A registration roller  74  for aligning one end of a paper sheet P taken from the cassette  70  with one end of a toner image formed to the photosensitive drum  58 B of the image forming unit  50 B (black) is deposed between the conveying roller  72  and the tension roller  54 .  
         [0032]     An adsorption roller  76  for providing a predetermined electrostatic adsorption force to a paper sheet P transferred at a predetermined timing by the registration roller  74  is arranged to a position close to the tension roller  54  between the registration roller  74  and the first image forming unit  50 Y and a position opposed to the circumference of the transfer belt  52  corresponding to the position at which the tension roller  54  substantially comes in contact with the transfer belt  52 .  
         [0033]     At one end of the transfer belt  52  and close to the belt drive roller  56 , and on the circumference of the transfer belt  52  substantially coming into contact with the belt drive roller  56 , there are arranged registration sensors  78  and  80  for detecting the position of an image formed on the transfer belt  52  or transferred on the sheet P in the axial direction of the belt drive roller  56  at regular intervals. ( FIG. 1  is a front cross-sectional view of the color image forming apparatus so that the registration sensors  78  located in front of a sheet surface are not shown in  FIG. 1 .)  
         [0034]     A transfer cleaner  82  for removing toner or the sheet P residues deposited on the transfer belt  52  is disposed at a position on the circumference of the transfer belt  52  coming into contact with the belt drive roller  56  and at a position not coming in contact with the sheet P transferred by the transfer belt  52 .  
         [0035]     A fixing device  84  for fixing toner image transferred on the sheet P on the sheet P is disposed in the direction that the sheet P transferred through the transfer belt  52  is separated away from the belt drive roller  56  and further transferred.  
         [0036]      FIGS. 2 and 3  illustrate the multi-beam optical scanning device  1  incorporated in the image forming apparatus shown in  FIG. 1 .  
         [0037]     The multi-beam optical scanning device  1  has light sources  3 Y,  3 M,  3 C, and  3 B for emitting a light beam toward the first to fourth of the respective image forming units  50 Y,  50 M,  50 C and  50 B shown in  FIG. 1  and only one light deflection device  7  as deflecting means for deflecting (scanning) a light beam (laser beam) emitted by the respective light sources  3  (Y, M, C, and B) at a predetermined linear speed toward an image surface disposed in a predetermined position, that is, a circumference surface of the photosensitive drums  58 Y,  58 M,  58 C, and  58 B of the first to fourth of the respective image forming units  50 Y,  50 M,  50 C and  50 B shown in  FIG. 1 . Pre-deflection optical systems  5  (Y, M, C, and B) are disposed between the light deflection device  7  and the respective light sources  3  (Y, M, C, and B) and a post-deflection optical system  9  is disposed between the light deflection device  7  and the image surface.  
         [0038]     In this case, the direction where the respective laser beams are deflected (scanned) by the light deflection device  7  is referred to as a main scanning direction. The direction perpendicular to the main scanning direction and the direction perpendicular to each axis line as the reference of a deflection movement provided to the laser beam by the light deflection device so that the laser beam scanned (deflected) by the light deflection device is in the main scanning direction, are referred to as a sub scanning direction.  
         [0039]     As shown in  FIGS. 3 and 4  ( FIG. 4  represents a laser beam L as an example), the pre-deflection optical systems  5  include the respective light sources  3  (Y, M, C, and B) for each of color components constituted by a semiconductor laser element, finite focus lenses  13  (Y, M, C, and B) for providing a predetermined convergence to a laser beam emitted from the respective light sources  3  (Y, M, C, and B), stops  14  (Y, M, C, and B) for providing a cross-sectional shape to the laser beam L passed through the finite focus lenses  13  (Y, M, C, and B), and cylinder lenses  17  (Y, M, C, and B) for further providing a predetermined convergence to the laser beam passed through the stops  14  (Y, M, C, and B) in the sub scanning direction. The pre-deflection optical systems  5  adjust the cross-sectional shape of the laser beam emitted from the respective light sources  3  (Y, M, C, and B) into a predetermined shape and guide the beam to a reflection surface of the light deflection device  7 .  
         [0040]     The cyan laser beam LC emitted from the cylinder lens  17 C has its optical path folded by a folding mirror  15 C and then passes through an optical parts  19  for synthesizing an optical path to be guided to the reflection surface of the light deflection device  7 . The black laser beam LB emitted from the cylinder lens  17 B has its optical path folded by a folding mirror  15 B and then is reflected by the optical parts  19  for synthesizing an optical path to be guided to the reflection surface of the light deflection device  7 . After the yellow laser beam LY emitted from the cylinder lens  17 Y passes through above the folding mirror  15 C, the yellow laser beam LY transmits through the optical parts  19  for synthesizing an optical path and is guided to the reflection surface of the light deflection device  7 . The magenta laser beam LM emitted from the cylinder lens  17 M has its optical path folded by a folding mirror  15 M and passes through above the folding mirror  15 B. Thereafter, the magenta laser beam LM is reflected by the optical parts  19  for synthesizing an optical path and is guided to the reflection surface of the light deflection device  7 . In this example, the folding mirrors  15 M,  15 B, and  15 C, and the optical parts  19  for synthesizing an optical path are not shown in  FIG. 4 .  
         [0041]     The post-deflection optical system  9  includes an image forming lens (fθ lens) group  21  for focusing a laser beam deflected by a polygon mirror  7   a  on the respective photosensitive drums  58  (Y, M, C, and B) and first folding mirrors  33  (Y, M, C, and B), second folding mirrors  35  (Y, M, C, and B), and third folding mirrors  37  (Y, M, C, and B) for separating a synthetic laser beam passed through the image forming lens group  21  into a laser beam of each of color components and guiding on the corresponding photosensitive drums  58  (Y, M, C, and B). In the first embodiment, the image forming lens (fθ lens) group  21  has two lenses  21   a  and  21   b.    
         [0042]     The above-mentioned FIGS.  1  to  4  are drawings used for explaining a positional relationship of each component. The first embodiment has the feature in attachment of a predetermined component so that an attitude and the like are adjustable. All the relations of the features are not accurately shown in the above-mentioned FIGS.  1  to  4 . A method of attachment of a predetermined component (optical element components) feature will be described in reference with FIGS.  5  to  9 .  
         [0043]     The light deflection device  7 , made up of the polygon mirrors  7   a  and a polygon motor  7   b,  is attached as shown in  FIGS. 5 and 6 . In this case,  FIG. 5  is a plan view. (In the case of  FIGS. 1 and 2 , they are considered to be a bottom view.)  FIG. 6  is a cross section taken on line VI-VI of  FIG. 5 .  
         [0044]     The light deflection device  7  is fixed by fixing the polygon motor  7   b  to a flat plate-shaped motor board  150 . The motor board  150  is fixed to a fixed member  151  such a housing by a plurality of fixing screws  152  and  153  (2 screws are shown in the figures). A predetermined amount of clearance is formed between the motor board  150  and the fixed member  151  by a linear protruding portion  154  provided to top surface of the fixed member  151  and a set screw  155  threadedly engaged with a female screw provided to the fixed member  151 . As shown in  FIG. 5 , a straight line connecting the fixing screws  152  and  153  is perpendicular to the direction where the protruding portion  154  is extended as viewed from a projection plane. The set screw  155  is positioned on this straight line. The direction of the protruding portion  154  is set to be parallel when the polygon mirror  7   a  is at a predetermined angle. The predetermined angle is the angle at which optical performance deterioration is largest in an image surface.  
         [0045]     At this point, the fixing screws  152  and  153  are loosed to move the set screw  155  back and forth so that the motor board  150  is rotated around the protruding portion  154  as a fulcrum. Thereafter, the fixing screws  152  and  153  are tightened to adjust the reflection surface inclination of the polygon mirror  7   a.  After the adjustment, as shown in  FIG. 6 , the reflection surface of the polygon mirror  7   a  and the protruding portion  154  are positioned on the straight line at a predetermined angle.  
         [0046]     The pre-deflection optical system for each of color components is disposed as shown in  FIGS. 7 and 8 .  FIG. 7  is a plain view.  FIG. 8  is a cross section taken on line VIII-VIII of  FIG. 7 . Regarding the top portion of the pre-deflection optical system board mentioned blow, only components on the illustrated arrow line will be explained here. Other components are omitted.  
         [0047]     The light sources  3  (Y, M, C, and B), the finite focus lenses  13  (Y, M, C, and B), the stops  14  (Y, M, C, and B), the cylinder lenses  17  (Y, M, C, and B), the folding mirrors  15  (Y, M, C, and B), and the optical parts  19  for synthesizing a optical path are fixed on the same flat plate-shaped pre-deflection optical system board  200 . The pre-deflection optical system board  200  is fixed to a fixed member  201  such as a housing by a plurality of (for example, 2 screws) fixing screws (not shown). A predetermined amount of clearance is formed between the pre-deflection optical system board  200  and the fixed member  201  by a linear protruding portion  204  provided on top surface of fixed member  201  and a set screw  205  threadedly engaged with a female screw provided to the fixed member  201 . As viewed from a projection plane, the straight line of the protruding portion  204  is perpendicular to the direction where an optical path of all laser beams from the respective light sources  3  (Y, M, C, and B) is combined toward the polygon mirror  7   a.  The set screw  205  is sufficiently spaced in the direction perpendicular to the protruding portion  204 .  
         [0048]     At this point, all fixing screws (not shown) are loosed to move the set screw  205  back and forth so that pre-deflection optical system board  200  is rotated around the protruding portion  204  as a fulcrum. Thereafter, all the fixing screws are tightened to adjust an incident position and angle in the sub scanning direction of a laser beam to the reflection surface of the polygon mirror  7   a.    
         [0049]     Two fθ lenses  21   a  and  21   b  which are the main components of the post-deflection optical system  9  are fixed as shown in the plain view of  FIG. 9 . Although a cross section is omitted, components below the fθ lens board mentioned below are disposed the same as  FIG. 4 .  
         [0050]     The two fθ lenses  21   a  and  21   b  are fixed to the same flat plate-shaped fθ lens board  250 . The fθ lens board  250  is fixed to a fixed member (not shown) such as a housing with a plurality of fixing screws  252  and  253  (2 screws are shown in the figure). A predetermined amount of clearance is formed between the fθ lens board  250  and the fixed member by a linear protruding portion  254  provided to top surface of the fixed member and a set screw  255  threadedly engaged with a female screw provided to the fixed member. As shown in  FIG. 9 , a straight line connecting the fixing screws  252  and  253  is perpendicular to the direction where the protruding portion  254  is extended as viewed from a projection plane. The set screw  255  is positioned on this straight line. As shown in the plain view of  FIG. 9 , the protruding portion  254  is set so as to be perpendicular to an optical axis in which a light beam focused to a performance deterioration point passes through an fθ lens group.  
         [0051]     At this point, the fixing screws  252  and  253  are loosed to move the set screw  255  back and forth so that the fθ lens board  250  is rotated around the protruding portion  254  as a fulcrum. Thereafter, the fixing screws  252  and  253  are tightened to adjust a synthetic laser beam from the polygon mirror  7   a  in a moving direction of the sub scanning direction.  
         [0052]     Next, an adjustment method for setting the absolute value of an inclination quantity of the reflection surface of the polygon mirror  7   a  at zero will be explained.  
         [0053]     (1) As a first step, the motor board  150  fixing the light deflection device  7  (polygon motor  7   b ) is placed on the fixed member  151 . In this case, the pre-deflection optical system board  200  is not placed on the fixed member  201 . In addition, the fθ lens board  250  is not placed on the fixed member. (The whole post-deflection optical system may not be placed.) (2) Next, the polygon mirror  7   a  is adjusted so as to set a surface inclination quantity at zero at a scanning angle. (The fixed member (housing)  151  is the reference.) (3) The pre-deflection optical system board  200  mounting the pre-deflection optical systems  5  (Y, M, C, and B) is fixed to the fixed member  201 . The whole post-deflection optical system  9  is placed on the fixed member  201  including the placement of the fθ lens board  250  to the fixing member. (4) Before a laser beam from the pre-deflection optical systems (Y, M, C, and B) mounted on the pre-deflection optical system board  200  reaches the polygon mirror  7   a,  the pre-deflection optical system board  200  is adjusted so as to make the beam passing position close to a designed value at a predetermined point. This means that the position and attitude of the pre-deflection optical systems  5  (Y, M, C, and B) are adjusted. After adjustment of the pre-deflection optical system board  200 , a discrete element of the respective pre-deflection optical systems  5  (Y, M, C, and B) may be adjusted so as to make the beam passing position close to a designed value at a predetermined point. (5) After the laser beam deflected by the polygon mirror  7   a  at a scanning direction passes through the two fθ lenses  21   a  and  21   b  fixed to the fθ lens board  250 , the fθ lens board  250  is adjusted so as to make the beam passing position close to a designed value at a predetermined point. This means that the position and attitude of the post-deflection optical system, especially the fθ lenses  21   a  and  21   b,  are adjusted.  
         [0054]     According to the first embodiment, the following effects can be produced.  
         [0055]     Performance deterioration based on the reflection surface inclination of the polygon mirror  7   a  is prevented by adjusting the reflection surface itself. Comparing with the conventional method of adjusting the respective cylinder lenses  17  (Y, M, C, and B), only one part of polygon mirror adjustment facilitate the adjustment work.  
         [0056]     In this case, the position and attitude of the respective pre-deflection optical systems  5  (Y, M, C, and B) are adjusted together. Further, the position and attitude of the fθ lenses  21   a  and  21   b  are also adjusted together. Therefore, the performance can be further improved.  
       (B) Another Example of an Adjustment Method  
       [0057]     The adjustment method for the adjustment constitution of the above-mentioned embodiment is not limited to the above-mentioned example. The adjustment method shown as (B-1) to (B-7) mentioned below can also be applied.  
         [0058]     (B-1) This adjustment method also sets the absolute value of an inclination quantity of the reflection surface of the polygon mirror  7   a  at zero in a similar manner to the adjustment method described in the above-mentioned embodiment.  
         [0059]     In the adjustment method, the process (4), that is, the pre-deflection optical system board  200  is adjusted so as to make the beam passing position close to a designed value at a predetermined point before a laser beam from the pre-deflection optical systems (Y, M, C, and B) mounted on the pre-deflection optical system board  200  reaches the polygon mirror  7   a,  is omitted from the adjustment method explained in the above-mentioned embodiment. In the case where this adjustment method is fixedly applied, adjustment constitution to the pre-deflection optical system such as the pre-deflection optical system board  200  can be omitted.  
         [0060]     (B-2) This adjustment method also sets the absolute value of an inclination quantity of the reflection surface of the polygon mirror  7   a  at zero in a similar manner to the adjustment method described in the above-mentioned embodiment.  
         [0061]     In this adjustment method, the process (5), that is, the fθ lens board  250  is adjusted so as to make the beam passing position close to a designed value at a predetermined point after the laser beam deflected by the polygon mirror  7   a  at a scanning direction passes through the two fθ lenses  21   a  and  21   b  fixed to the fθ lens board  250 , is omitted from the adjustment method explained in the above-mentioned embodiment. In the case where this adjustment method is fixedly applied, adjustment constitution to the fθ lens  21   a  and  21   b  such as the fθ lens board  250  can be omitted.  
         [0062]     (B-3) This adjustment method sets a relative error of an inclination quantity of the reflection surface of the polygon mirror  7   a  at zero. In this case, the zero relative error means that an inclination quantity is appropriate judging from the relative relation of the pre-deflection optical system, the light deflection device, and the post-deflection optical system without regard to an angle seen from a reference surface such as the housing.  
         [0063]     (1) The motor board  150  fixing the light deflection device  7  (polygon motor  7   b ), the pre-deflection optical system board  200  mounting the pre-deflection optical systems  5  (Y, M, C, and B), and the fθ lens board  250  mounting the fθ lenses  21   a  and  21   b  are placed. (2) Before a laser beam from the pre-deflection optical systems (Y, M, C, and B) mounted on the pre-deflection optical system board  200  reaches the polygon mirror  7   a,  the pre-deflection optical system board  200  is adjusted so as to make the beam passing position close to a designed value at a predetermined point. This means that the position and attitude of the pre-deflection optical systems  5  (Y, M, C, and B) are adjusted. After adjustment of the pre-deflection optical system board  200 , a discrete element of the respective pre-deflection optical systems  5  (Y, M, C, and B) may be adjusted so as to make the beam passing position close to a designed value at a predetermined point. (3) After a laser beam from the pre-deflection optical systems  5  (Y, M, C, and B) group is reflected toward a scanning direction by the polygon mirror  7   a,  a reflection surface angle (inclination quantity) of the polygon mirror is adjusted so as to make the beam passing position close to a designed value. In this case, the measurement position is the position before the laser beam reaches the nearer fθ lens  21   a.  (4) After the laser beam deflected by the polygon mirror  7   a  at a scanning direction passes through the two fθ lenses  21   a  and  21   b  fixed to the fθ lens board  250 , the fθ lens board  250  is adjusted so as to make the beam passing position close to a designed value at a predetermined point. This means that the position and attitude of the post-deflection optical system, especially the fθ lenses  21   a  and  21   b  are adjusted.  
         [0064]     (B-4) This adjustment method also sets a relative error of an inclination quantity of the reflection surface of the polygon mirror  7   a  at zero. In this adjustment method, the motor board  150  is not adjusted. In the case where this adjustment method is fixedly employed in a machine, adjustment constitution of an inclination quantity of the reflection surface of the polygon mirror  7   a  such as the motor board  150  can be omitted.  
         [0065]     (1) The motor board  150  fixing the light deflection device  7  (polygon motor  7   b ), the pre-deflection optical system board  200  mounting the pre-deflection optical systems  5  (Y, M, C, and B), and the fθ lens board  250  mounting the fθ lenses  21   a  and  21   b  are placed. (2) After a laser beam from the pre-deflection optical systems  5  (Y, M, C, and B) group is reflected toward a scanning direction by the polygon mirror  7   a,  the pre-deflection optical systems (Y, M, C, and B) mounted on the pre-deflection optical system board  200  are adjusted so as to make the beam passing position close to a designed value. In this case, the measurement position is the position before the laser beam reaches the nearer fθ lens  21   a.  After adjustment of the pre-deflection optical system board  200 , a discrete element of the respective pre-deflection optical systems  5  (Y, M, C, and B) may be adjusted so as to make the beam passing position close to a designed value at a predetermined point. (3) After the laser beam deflected by the polygon mirror  7   a  at a scanning direction passes through the two fθ lenses  21   a  and  21   b  fixed to the fθ lens board  250 , the fθ lens board  250  is adjusted so as to make the beam passing position close to a designed value at a predetermined point.  
         [0066]     (B-5) This adjustment method also sets a relative error of an inclination quantity of the reflection surface of the polygon mirror  7   a  at zero. In this adjustment method, the pre-deflection optical system board  200  is not adjusted. In the case where this adjustment method is fixedly employed in a machine, adjustment constitution to the pre-deflection optical system such as the pre-deflection optical system board  200  can be omitted.  
         [0067]     (1) The motor board  150  fixing the light deflection device  7  (polygon motor  7   b ), the pre-deflection optical system board  200  mounting the pre-deflection optical systems  5  (Y, M, C, and B), and the fθ lens board  250  mounting the fθ lenses  21   a  and  21   b  are placed. (2) After a laser beam from the pre-deflection optical systems  5  (Y, M, C, and B) group is reflected toward a scanning direction by the polygon mirror  7   a,  a reflection surface angle (inclination quantity) of the polygon mirror is adjusted so as to make the beam passing position close to a designed value. In this case, the measurement position is the position before the laser beam reaches the nearer fθ lens  21   a.  (3) After the laser beam deflected by the polygon mirror  7   a  at a scanning direction passes through the two fθ lenses  21   a  and  21   b  fixed to the fθ lens board  250 , the fθ lens board  250  is adjusted so as to make the beam passing position close to a designed value at a predetermined point.  
         [0068]     (B-6) This adjustment method also sets a relative error of an inclination quantity of the reflection surface of the polygon mirror  7   a  at zero. In this adjustment method, the fθ lens board  250  is not adjusted. In the case where this adjustment method is fixedly employed in a machine, adjustment constitution to the post-deflection optical system such as the fθ lens board  250  can be omitted.  
         [0069]     (1) The motor board  150  fixing the light deflection device  7  (polygon motor  7   b ), the pre-deflection optical system board  200  mounting the pre-deflection optical systems  5  (Y, M, C, and B), and the fθ lens board  250  mounting the fθ lenses  21   a  and  21   b  are placed. (2) Before a laser beam from the pre-deflection optical systems (Y, M, C, and B) mounted on the pre-deflection optical system board  200  reaches the polygon mirror  7   a,  the pre-deflection optical system board  200  is adjusted so as to make the beam passing position close to a designed value at a predetermined point. After adjustment of the pre-deflection optical system board  200 , a discrete element of the respective pre-deflection optical systems  5  (Y, M, C, and B) may be adjusted so as to make the beam passing position close to a designed value at a predetermined point. (3) After a laser beam from the pre-deflection optical systems  5  (Y, M, C, and B) group is reflected toward a scanning direction by the polygon mirror  7   a,  a reflection surface angle (inclination quantity) of the polygon mirror is adjusted so as to make the beam passing position close to a designed value. In this case, the measurement position is the position before the laser beam reaches the nearer fθ lens  21   a.    
         [0070]     (B-7) The adjustment methods of the above-mentioned embodiment and the above respective adjustment methods (B-1) to (B-6) adjust either three groups or two groups from three adjustable components.  
         [0071]     However, only one group of component groups may be adjusted in the present invention.  
       (C) Other Embodiments  
       [0072]     While the invention is described in various modifications of embodiments in the explanation of the above-mentioned embodiments, other modifications of the embodiment can be described hereinbelow.  
         [0073]     The constitution in which the motor board  150 , the pre-deflection optical system board  200 , and the fθ lens board  250  and the like are slightly roratable is not limited to the above-mentioned embodiments. For example, the protruding portion can be replaced by two protrusions spaced along the extension direction of the protruding portion. In this case, such two protrusions are preferably placed at other points of an isosceles triangle in which the set screw position is the apex.  
         [0074]     In the above-mentioned embodiments, although the multi-beam optical scanning device in which four beams are entered on the same surface of the polygon mirror is described, the present invention is applied to a multi-beam optical scanning device in which other beam numbers such as two beams, seven beams, and eight beams are entered on the same surface of the polygon mirror. Moreover, the present invention is applied to either a color type or monochrome type multi-beam optical scanning device including one or more optical parts for synthesizing a beam optical path which synthesizes a plurality of beam optical paths (beam splitter or half mirror) Further, the present invention is applied to an optical scanning device in which the only one beam is entered to one surface of the polygon mirror.