In an optical scanning apparatus, an aperture is provided between a semiconductor laser in a light source unit and an oscillating mirror, and between a cylindrical lens and the oscillating mirror. When a light beam from the semiconductor laser comes into an reflection surface of the oscillating mirror, the optical scanning apparatus is configured to limit a beam width of the light beam to a width appropriate to the reflection surface, and then to ensure an irradiation position of the light beam in a main-scanning direction to come into the reflection surface of the oscillating mirror, by causing the light beam to pass through an opening of the aperture.