An optical scanning apparatus including a deflection unit configured to deflect a light beam emitted from a light source so that the light beam scans on a photosensitive member, a casing which is configured to house the light source and the deflection unit inside the casing and which is provided with an aperture that transmits the light beam deflected by the deflection unit out from the casing, a first light transmission member provided on the aperture for dust-proofing an interior of the casing, which is configured to transmit the light beam incident on the aperture, and a support member configured to support a second light transmission member for transmitting a light beam that has been transmitted through the first light transmission member and dust-proofing the first light transmission member, wherein the second light transmission member is detachable from the support member.