An optical scanning device includes: a common optical deflector that deflects light beams from light source devices; and a scanning optical system that focuses the deflected light beams on different scanning surfaces. All the light beams from the light source devices are incident on the optical deflector in directions oblique to a normal line of the scanning surfaces in a main-scanning cross section and to a normal line of a deflecting reflection plane of the optical deflector in a sub-scanning cross section. The scanning optical system includes individual lenses which are individually arranged for the respective light beams and have a plane shape symmetric in the main-scanning direction and satisfies a predetermined condition, and an optical axis of the individual lens is arranged obliquely to the normal line of the scanning surface in the main-scanning cross section so as to satisfy a predetermined condition.