A light scanning apparatus, including: a light source; a deflector having a rotary polygon mirror configured to deflect the light beam emitted from the light source, and a motor configured to rotate the polygon mirror; a plurality of reflecting mirrors configured to reflect the light beam to the photosensitive member; and an optical box on which the light source is mounted, wherein the optical box has an installation wall on which the deflector is installed and a support wall positioned on a side of the photosensitive member with respect to the polygon mirror, the support wall being provided with a support portion configured to support at least one reflecting mirror, a stepped portion having a plurality of steps is formed between the installation wall and the support wall, and a back surface of the stepped portion has a shape following an inside surface of the stepped portion.