Light scanning apparatus and image forming apparatus

An image forming apparatus comprising:

• an image forming unit configured to form an image on a sheet;
• a light scanning apparatus including:
• a light source configured to emit a light beam;
• a rotary polygon mirror configured to deflect the light beam emitted from the light source;
• an optical component of an incident optical system configured to introduce the light beam, which has been emitted from the light source, to the rotary polygon mirror;
• an optical box, to which the light source is mounted, configured to contain the rotary polygon mirror and the optical component in an inside of the optical box;
• and
• a supporting base, which is installed on an optical path of the light beam between the light source and the rotary polygon mirror, and is configured to support the optical component,
• wherein the supporting base includes:
• a first dust-proof member configured to seal a boundary portion between the optical box and the supporting base to dustproof the inside of the optical box;
• and
• a protrusion configured to enter an opening, which is formed in a side wall of the optical box from the inside of the optical box in a state in which the supporting base is mounted to the optical box, the light source being fixed to the opening.