An optical scanning apparatus scans a target surface with a light beam along a main scanning direction. The apparatus includes a light source, an optical deflector which includes a rotating multi-faceted mirror, each surface of the multi-faceted mirror acting as a reflecting surface to deflect the light beam from the light source, and a scanning optical system configured to guide the light beam deflected by the optical deflector to the target surface. An angle which is formed between the light beam toward the reflecting surface of the optical deflector and the corresponding reflecting surface and which includes a reflected light reflected from the corresponding reflecting surface is always obtuse.