An optical scanning apparatus includes a light source device, a rotating polygon mirror, a first optical system, and a second optical system. The light source device has at least one light emitter. The first optical system can cause a light beam emitted from the light source device to be incident on the rotating polygon mirror. The width of the incident light beam is larger than the width of the deflecting surface of the rotating polygon mirror in the main scanning direction. The second optical system leads the light beam deflected by the rotating polygon mirror to a surface to be scanned. The surface to be scanned is scanned with the light beam in the main scanning direction by the rotation of the rotating polygon mirror. The number of the deflecting surfaces of the rotating polygon mirror is ten or less, and each conditional expression is satisfied.