Provided is a light scanning apparatus, including: a light source; a rotary polygon mirror; an optical member guiding a light beam; an optical box on which the light source is mounted and which contains the rotary polygon mirror and the optical member; a cover covering an opening of the optical box; a fixation unit fixing the cover on the optical box; the cover having a dust-proof member which is sandwiched between the cover and a side wall of the optical box; and the dust-proof member including an abutment portion against which the side wall is brought into abutment, and non-abutment portions provided on both sides of the abutment portion and separated from the side wall, and the dust-proof member including a groove in one of the non-abutment portions which is located on a side opposite to another one located on a side where the fixation unit is provided.