An optical scanning device that scans a plurality of surfaces to be scanned in a main scanning direction by using a light beam includes: a plurality of light sources; a light deflector that deflects a plurality of light beams emitted from the light sources; and a scanning optical system that individually guides each one of the light beams deflected by the light deflector to a corresponding one of the surfaces to be scanned. The scanning optical system includes one scanning lens shared by the light beams, and at least one surface of the scanning lens has a plurality of optical surfaces corresponding to the plurality of light beams disposed in a sub-scanning direction with a flat surface provided between the optical surfaces.