In an optical scanning apparatus in which a movable plate 653 pivots at a high speed, friction between the movable plate 653 and air generates heat. The amount of generated heat is relatively large in a main scanning direction but relatively small in a direction of a pivot axis. A light beam is converged in a sub scanning direction Y within a sub-scanning surface, and is focused as a line image in the vicinity of a deflection mirror surface 651 in a deflector 65. This reduces the size of the deflection mirror surface 651 in the direction Y of the pivot axis (the sub scanning direction). Meanwhile, a second distance is longer than a first distance in the main scanning direction, and the movable plate 653 and vertical side portions (second adjacent portions) 652b are widely apart from each other in the main scanning direction X. This reduces air resistance against the movable plate 653, and hence, the amount of heat generation.