an optical scanning device has a light deflector deflecting a light beam from a light source , a synchronization detection sensor determining timing of starting scanning in main scanning direction based on timing of detecting the light beam scanned in main scanning direction by the light deflector , and a pre - sensor imaging optical system imaging the light beam reflected from the light deflector on the synchronization detection sensor . the pre - sensor imaging optical system moves the imaging position of the light beam on the synchronization detection sensor in a direction making the timing of detecting the light beam earlier or later according to whether variation in temperature causes the magnification of the scanning optical system in main scanning direction to increase or decrease respectively .