Optical scanning apparatus

An optical scanning apparatus comprising:

• a light source;
• a deflection unit configured to deflect a light beam emitted from the light source so that the light beam scans on a photosensitive member;
• a casing to which the light source is attached and in which the deflection unit is housed, the casing being provided with an aperture through which the light beam deflected by the deflection unit passes;
• a first light transmission member, fixed to the casing, configured to seal the aperture to be dust-proof for a space inside the casing, and transmit the light beam;
• a second light transmission member configured to transmit a light beam that has been transmitted through the first light transmission member, the second light transmission member being detachable from the optical scanning apparatus;
• and
• a guide member provided to the casing, configured to guide the second light transmission member in a longitudinal direction of the second light transmission member and hold the second light transmission member,
• wherein a space sealed from the space inside the casing and a space outside the optical scanning apparatus is formed by the guide member, the second light transmission member held by the guide member, and the first light transmission member so that the first light transmission member is protected from dust.