An optical scanning device includes a light source, a light-beam splitting unit, a deflector, and a scanning optical system. The light-beam splitting unit splits a light beam from the light source into a plurality of light beams, so that the light beams are each incident to any one of reflecting surfaces of the deflector while having a phase difference of approximately π/2. The scanning optical system receives the light beams from the deflector and projects each of the light beams onto a corresponding target surface.