An optical scanner forms an electrostatic latent image on a photosensitive member by scanning the photosensitive member with a light beam. The optical scanner includes: an incident optical system which at least comprises: a light beam emission device configured to emit a light beam; and a cylindrical lens configured to condense the light beam emitted from the light beam emission device, and a scanning optical system which at least comprises: a light deflecting device configured to reflect the light beam having passed through the cylindrical lens to deflect the light beam in a main scanning direction for scanning the photosensitive member; and a scanning lens configured to focus the light beam deflected by the light deflecting device on the photosensitive member to form an electrostatic latent image thereon. The incident optical system and the scanning optical system are divided by a light shielding wall.