An optical scanning apparatus includes a light source for emitting light, a deflecting device including a deflecting element for deflection-scanning a surface to be scanned with the light emitted from the light source and including a motor for driving the deflecting element, and an optical system casing including a supporting surface for disposing thereon the deflecting device and including a wall provided to stand on the supporting surface and to face the deflecting element. At a portion at which the wall stands on the supporting surface, an opening is provided so as to extending along the wall.