An optical scanning device includes a light source which emits a light beam, a deflector, an incident optical system and one scanning lens. The deflector reflects and deflects/scans the light beam emitted from the light source. The scanning lens includes a first face facing the deflector and a second face on an opposite side to the first face, and performs imaging of the deflected/scanned light beam on a surface to be scanned. In a main scanning cross section, when an angle relative to an optical axis of an incident light beam which enters the scanning lens from the first face is θin, and an angle relative to an optical axis of an outgoing light beam which is output from the second face toward the surface to be scanned is θout, in an entire scanning region, a condition of 0.9