hereinafter , embodiments of the present disclosure will be described in detail with reference to the accompanying drawings . fig1 is a schematic diagram showing an overall construction of an image forming apparatus 100 provided with an exposure device 19 as an optical scanning device according to the present disclosure , the right side of the diagram corresponding to the front side of the image forming apparatus 100 . as shown in fig1 , the image forming apparatus 100 ( here , a monochrome printer ) is provided with a paper feed cassette 2 containing a stack of sheets of paper in a lower part of the apparatus main body 1 . over the paper feed cassette 2 , a paper transport passage 4 is formed which extends substantially horizontally from the front side to the rear side of the apparatus main body 1 and then upward to reach a paper discharge section 3 formed in a top face of the apparatus main body 1 . along the paper transport passage 4 , there are arranged , from an upstream side , a pickup roller 5 , a feed roller 6 , an intermediary transport roller 7 , a registration roller pair 8 , an image forming section 9 , a fusing device 10 , and a discharge roller pair 11 . inside the image forming apparatus 100 , a control section ( cpu ) 30 is arranged for controlling the operation of the respective rollers mentioned above , the image forming section 9 , the fusing device 10 , the exposure device , etc . the paper feed cassette 2 is provided with a paper placement plate 12 , which is supported on a pivot 12 a provided in a rear end part in a paper transport direction so as to be swingable with respect to the paper feed cassette 2 . the stack of sheets of paper ( recording medium ) on the paper placement plate 12 is pressed against by the pickup roller 5 . in a front part of the paper feed cassette 2 , a return roller 13 is arranged in pressed contact with the feed roller 6 . when a plurality of sheets are simultaneously fed out by the pickup roller 5 , they are separated by the feed roller 6 and the return roller 13 so that only the topmost one sheet is transported further . the sheet singled out by the feed roller 6 and the return roller 13 then has its transport direction changed toward the rear side of the apparatus , and is transported to the registration roller pair 8 . the registration roller pair 8 then feeds , with adjusted timing , the sheet to the image forming section 9 . the image forming section 9 forms a predetermined toner image on paper by an electrophotographic process . the image forming section 9 is composed of a photosensitive drum 14 , which is an image carrying body pivoted to be rotatable in the clockwise direction in fig1 ; a charging device 15 , a developing device 16 , and a cleaning device 17 , which are arranged around the photosensitive drum 14 ; a transfer roller 18 , which is arranged to face the photosensitive drum 14 across the paper transport passage 4 ; and an exposure device ( lsu ) 19 , which is arranged over the photosensitive drum 14 . over the developing device 16 , a toner container 20 is arranged for supplying toner to the developing device 16 . in this embodiment , the photosensitive drum 14 is an amorphous silicon ( a - si ) photosensitive body , in which on an electrically conductive substrate ( cylindrical body ), a photoconductive layer based on a - si is formed as a photosensitive layer , and on the top face of it , a surface protection layer is stacked that is formed of an inorganic insulator or an inorganic semiconductor such as sic , sin , sio , sion , or sicn . when image data is fed to the image forming apparatus from a host device such as a personal computer , first , the charging device 15 electrically charges the surface of the photosensitive drum 14 uniformly . next , the exposure device ( lsu ) 19 emits a laser beam to form , on the photosensitive drum 14 , an electrostatic latent image based on the image data fed in . then , the developing device 16 deposits toner on the electrostatic latent image to form a toner image on the surface of the photosensitive drum 14 . the toner image formed on the surface of the photosensitive drum 14 is then transferred to paper fed into a nip portion ( transfer position ) between the photosensitive drum 14 and the transfer roller 18 . the paper having the toner image transferred to it in the image forming section 9 is then separated from the photosensitive drum 14 , and is transported toward the fusing device 10 . the fusing device 10 is arranged on the downstream side of the image forming section 9 with respect to the paper transport direction . thus , the paper having the toner image transferred to it is heated and pressed by a heating roller 22 , which is provided in the fusing device 10 , and a pressing roller 23 , which is in pressed contact with the heating roller 22 , so that the toner image transferred to the paper is fused . the paper thus having undergone image formation in the image forming section 9 and fusing device 10 is discharged to the paper discharge section 3 by the discharge roller pair 11 . after transfer , the toner remaining on the surface of the photosensitive drum 14 is removed by the cleaning device 17 , and the electric charge remaining on the surface of the photosensitive drum 14 is destaticized by a destaticizer ( not shown ). the photosensitive drum 14 is then electrically charged by the photosensitive drum 14 again so that image formation proceeds likewise . fig2 is a main scanning sectional view schematically showing an internal construction of the exposure device 19 according to a first embodiment of the present disclosure . as shown in fig2 , the exposure device 19 is provided with an ld unit 40 , a collimator lens 41 , a cylindrical lens 43 , a polygon mirror 45 , scanning lenses 47 a and 47 b , a planar mirror 50 , an sos ( start - of - scan ) lens 51 , and a bd sensor 53 . the ld unit 40 is provided with a laser diode ( ld ) as a light source , and emits a light beam ( laser beam ) resulting from optical modulation based on an image signal . the collimator lens 41 forms the laser beam emitted from the ld unit 40 into a substantially parallel beam . the cylindrical lens 43 has a predetermined refractive power only in a sub scanning direction of the laser beam . the parallel beam having passed through the collimator lens 41 and entered the cylindrical lens 43 exits from it in the form of a convergent beam in the sub scanning direction while remaining a parallel beam in a main scanning section , and is imaged as a linear image on a deflecting surface ( reflective surface ) of the polygon mirror 45 . the polygon mirror 45 is a rotary multiple - face mirror in the shape of a regular polygon ( here , a regular hexagon ) having a plurality of deflecting surfaces ( reflective surfaces ) as side faces , and is rotated at a predetermined speed in the counter - clockwise direction in fig2 by a driving means ( not shown ) such as a motor . the scanning lenses 47 a and 47 b are lenses having fθ characteristics . the laser beam reflection - deflected by the polygon mirror 45 is , through the scanning lenses 47 a and 47 b , imaged as a spot with a predetermined size on the photosensitive drum 14 , and this achieves scanning in the main scanning direction ( from top to bottom in fig2 ). outside the effective exposure region , at the scanning - start side is arranged the planar mirror 50 , and at the scanning - end side are arranged the sos lens 51 and the bd sensor 53 . the laser beam having been deflected by the polygon mirror 45 , then passed through an end part of the scanning lens 47 a , and then been reflected by the planar mirror 50 then passes through the sos lens 51 and enters the bd sensor 53 . in phase with the timing with which the bd sensor 53 detects the laser beam , the bd sensor 53 outputs a signal to a time counting portion ( not shown ). the bd sensor 53 can be one of various optical sensors , such as a photodiode , a phototransistor , or a photo ic . although in fig2 the bd sensor 53 is arranged at the scanning - end side outside the effective exposure region of the photosensitive drum 14 ( scanned surface ), the bd sensor 53 can instead be arranged at the scanning - start side or anywhere else so long as it does not affect the scanning of the scanned surface . fig3 and 4 are side views showing how the laser beam reflected from the planar mirror 50 passes through the sos lens 51 and enters the bd sensor 53 . as the sos lens 51 , an aspherical lens having a positive power is used . it is possible to use as the sos lens 51 a spherical lens having a positive power , but it then produces spherical aberration in the laser beam . it is thus preferable to use an aspherical lens that corrects spherical aberration . also , using an aspherical lens helps reduce the size of the sos lens 51 , and thus , also from the viewpoint of compactness of the exposure device 19 , it is preferable to use an aspherical lens . suppose that the laser beam l that emanates from the planar mirror 50 , then enters the sos lens 51 through a surface r 1 , then exits from it through a surface r 2 , and is imaged on the bd sensor 53 enters the sos lens 51 at a point of entrance p . then , in this embodiment , the optical axis o of the sos lens 51 is offset , with respect to the point of entrance p , in the direction opposite from the advancing direction ( the direction indicated by arrow a in the diagrams ) in the main scanning direction . the distance ( optical axis offset ) from the optical axis o of the sos lens 51 to the point of entrance p of the laser beam l is , for example , 20 mm . the light entrance surface ( surface r 1 ) of the sos lens 51 is , for example , an aspherical surface with a radius of curvature of 21 . 075 mm and a conic constant of − 0 . 725 , and the light exit surface ( surface r 2 ) of the sos lens 51 is a flat surface . next , a description will be given of how the timing with which the bd sensor 53 detects the laser beam is adjusted by the sos lens 51 . as the environmental temperature rises , the refractive index and radius of curvature of the sos lens 51 vary such that the positive power of the sos lens 51 decreases . here , the radius of curvature varies as the volume of the lens increases with variation in temperature , and the refractive index varies according to the temperature coefficient of refractive index variation . thus , theoretically , both the refractive index and the radius of curvature vary with variation in temperature . usually , the positive power of the sos lens 51 is affected more by the refractive index , though depending on the material of the lens . accordingly , as shown in fig3 , the laser beam l which would be imaged on the bd sensor 53 in a room - temperature environment now becomes a laser beam l 1 having an optical path shifted in the advancing direction ( the direction indicated by arrow a ) in the main scanning direction . thus , a laser beam l 2 that has entered the sos lens 51 with earlier timing than the laser beam l is imaged on the bd sensor 53 . that is , the detection by the bd sensor 53 takes place with earlier timing . on the other hand , as the environmental temperature falls , the refractive index and radius of curvature of the sos lens 51 vary such that the positive power of the sos lens 51 increases . thus , as shown in fig4 , the laser beam l which would be imaged on the bd sensor 53 in a room - temperature environment now becomes a laser beam l 1 having an optical path shifted in the direction opposite to the advancing direction in the main scanning direction . thus , a laser beam l 2 that enters the sos lens 51 with later timing than the laser beam l is imaged on the bd sensor 53 . that is , the detection by the bd sensor 53 takes place with later timing . in this way , when the environmental temperature rises and the magnification of the scanning lenses 47 a and 47 b in the main scanning direction increases , the timing of the detection by the bd sensor 53 can be made earlier so that scanning is started with earlier timing . by contrast , when the environmental temperature falls and the magnification of the scanning lenses 47 a and 47 b in the main scanning direction decreases , the timing of the detection by the bd sensor 53 can be made later so that scanning is started with later timing . that is , variation in the magnification in the main scanning direction ( variation in the scanning width ) resulting from variation in the environmental temperature can be distributed between the scanning - start and scanning - end sides . by distributing variation in the magnification in the main scanning direction between the scanning - start and scanning - end sides in this way , it is possible to suppress image displacement on the recording medium . fig5 is a main scanning sectional view schematically showing an internal construction of the exposure device 19 in a second embodiment of the present disclosure . fig6 and 7 are side views showing how the laser beam reflected from the planar mirror 50 passes through the sos lens 51 and enters the bd sensor 53 . in this embodiment , as the sos lens 51 , an aspherical lens having a negative power is used . the optical axis o of the lens is offset , with respect to the point of entrance p , in the advancing direction ( the direction indicated by arrow a in the diagrams ) in the main scanning direction . the distance ( optical axis offset ) from the optical axis o of the sos lens 51 to the point of entrance p of the laser beam l is , for example , 20 mm . the light entrance surface ( surface r 1 ) of the sos lens 51 is , for example , an aspherical surface with a radius of curvature of 21 . 075 mm and a conic constant of − 0 . 725 , and the light exit surface ( surface r 2 ) of the sos lens 51 is a flat surface . in other respects , the exposure device 19 here is configured similarly as that in the first embodiment . it is possible to use as the sos lens 51 a spherical lens having a negative power , but from the viewpoints of spherical aberration correction and compactness of the exposure device 19 , it is preferable to use an aspherical lens . next , a description will be given of how the timing with which the bd sensor 53 detects the laser beam is adjusted by the sos lens 51 . as the environmental temperature rises , the refractive index and radius of curvature of the sos lens 51 vary such that the negative power of the sos lens 51 decreases . accordingly , as shown in fig6 , the laser beam l which would be imaged on the bd sensor 53 in a room - temperature environment now becomes a light beam l 1 having an optical path shifted in the advancing direction ( the direction indicated by arrow a ) in the main scanning direction . thus , a laser beam l 2 that has entered the sos lens 51 with earlier timing than the laser beam l is imaged on the bd sensor 53 . that is , the detection by the bd sensor 53 takes place with earlier timing . on the other hand , as the environmental temperature falls , the refractive index and radius of curvature of the sos lens 51 vary such that the positive power of the sos lens 51 increases . thus , as shown in fig7 , the laser beam l which would be imaged on the bd sensor 53 in a room - temperature environment now becomes a laser beam l 1 having an optical path shifted in the direction opposite to the advancing direction in the main scanning direction . thus , a laser beam l 2 that enters the sos lens 51 with later timing than the laser beam l is imaged on the bd sensor 53 . that is , the detection by the bd sensor 53 takes place with later timing . in this way , as in the first embodiment , when the environmental temperature rises and the magnification of the scanning lenses 47 a and 47 b in the main scanning direction increases , the timing of the detection by the bd sensor 53 can be made earlier so that scanning is started with earlier timing . by contrast , when the environmental temperature falls and the magnification of the scanning lenses 47 a and 47 b in the main scanning direction decreases , the timing of the detection by the bd sensor 53 can be made later so that scanning is started with later timing . that is , variation in the magnification in the main scanning direction ( variation in the scanning width ) resulting from variation in the environmental temperature can be distributed between the scanning - start and scanning - end sides . as the environmental temperature varies , the refractive index and radius of curvature of the scanning lens 47 a also vary . as shown in fig2 and 5 , the laser beam reflected from the planar mirror 50 passes through the scanning lens 47 a in an end part thereof . thus , the optical path of the laser beam that is imaged on the bd sensor 53 shifts with variation in the refractive index and radius of curvature of the scanning lens 47 a . specifically , as the magnification of the scanning lenses 47 a and 47 b in the main scanning direction increases , the optical path shifts so as to make the detection timing of the bd sensor 53 earlier . by contrast , as the magnification of the scanning lenses 47 a and 47 b in the main scanning direction decreases , the optical path shifts so as to make the detection timing of the bd sensor 53 later . however , varying the detection timing of the bd sensor 53 with variation in the refractive index and the radius of curvature of the scanning lens 47 a is not sufficient to distribute variation in the magnification in the main scanning direction between the scanning - start and scanning - end sides . this is the reason that the sos lens 51 needs to be arranged between the scanning lens 47 a and the bd sensor 53 . the embodiments described above are in no way meant to limit the scope of the present disclosure , which thus allows for many modifications and variations without departing from the spirit of the present disclosure . for example , although in the embodiments described above a single sos lens 51 is used to adjust the detection timing of the bd sensor 53 , it is also possible to use as the sos lens 51 a compound lens composed of two or more lenses . although the embodiments described above deal with a single - beam scanning device provided with a single ld , the present disclosure is equally applicable to a multiple - beam scanning device provided with a plurality of lds . needless to say , an optical scanning device according to the present disclosure is applicable not only to a monochrome printer as shown in fig1 but to any other image forming apparatus that employs an exposure device , such as a monochrome copier , color copier , color printer , digital multifunction peripheral , and facsimile machine . for example , by applying the present disclosure to a color image forming apparatus that employs a multiple - beam scanning device , it is possible to distribute variation in the magnification in the main scanning direction ( variation in the scanning width ) of each of the laser beams emitted from a plurality of lds between the scanning - start and scanning - end sides , and thereby to effectively suppress color displacement that results from displacement among images of different colors composing a full - color image . the present disclosure finds application in optical scanning devices that detect scanning start timing by use of a synchronization detection sensor . according to the present disclosure , it is possible to provide an optical scanning device that permits variation in the magnification in the main scanning direction resulting from variation in the environmental temperature to be distributed between the scanning - start and scanning - end sides with a simple construction using an sos lens .