An optical scanning device corrects a scanning line curvature and prevents image quality degradation, while spatially separating a plurality of light beams at oblique angles of incidence despite its compact magnifying optical scanning device construction. The optical scanning device directs, to the deflection surface of an optical deflector via a cylindrical lens, the plurality of light beams emitted by a light source having a plurality of light emission sections, guides, via an f lens system to a surface being scanned, the plurality of light beams deflected by the optical deflector so that the plurality of light beams scan the surface being scanned. The f lens system has a plurality of rotationally asymmetrical lenses, the optical axes of which are substantially parallel with the light beams incident on the respective lenses, and each of the generatrices that connect the vertices of a radius curvature on a lens surface in the sub scanning direction of the rotationally asymmetrical lenses is curved in a sub scan direction.