A light scanning apparatus to be used in an electrophotographic image forming apparatus, the light scanning apparatus including: a housing having an outer wall provided with an opening; a light source configured to emit a light beam; a deflecting device provided in the housing and configured to deflect the light beam emitted from the light source such that the light beam emitted from the light source scans a photosensitive member; an optical element arranged on an optical path of the light beam in the housing; and an elastic member provided on the housing to block up the opening, wherein the elastic member is formed of a material which is deformable to increase and decrease a volumetric capacity of an internal space formed by the housing and the elastic member in accordance with a temperature fluctuation inside the housing.