An optical scanning device includes a vertical-cavity surface-emitting laser light source that emits laser beams perpendicular to a top surface thereof; a first optical system that couples the beams from the light source; a deflecting unit that deflects the beams; a second optical system that guides the beams from the first optical system to the deflecting unit; a third optical system that focuses the beams deflected by the deflecting unit into an optical spot on a scanned surface; and a light-quantity adjusting element disposed between the light source and the deflecting unit and having a substrate formed of a first and second surfaces. The first surface of the light-quantity adjusting element is coated with neutral density coating and the second surface is coated with antireflection coating so that reflectance of the second surface is made smaller than reflectance of the first surface.