Light beams from a laser light source are made to produce a line image, via an optical part which forms a line image, on a reflecting surface of a rotating polygon mirror, and the beams of the reflected light from the reflecting surface are focused by a scanning-lens system so that they scan the surface to be scanned. The scanning-lens system has a cylindrical lens which has a positive refractive power and is located in that position which is next to the surface to be scanned, as well as an aspheric lens where at least one surface has such a profile that its curvature radius increases from the central section to each end section in a direction normal to the scanning direction.
