Disclosed is an optical scanning system and an image forming apparatus using the same, which is designed to reduce the overall size of the system and to make uniform the light spot diameter in a sub-scan direction, inside an effective scan region, as well as to ensure high-speed production of high-quality images, wherein the optical scanning system includes a light source, a condensing optical system, a deflector and an imaging optical system having one or more optical elements, wherein, in at least one optical element of the optical elements, the curvature of the opposite surfaces thereof changes continuously, inside an effective region, from the optical axis toward the peripheral portion thereof so that the sign of the curvature is reversed at the middle, and wherein, when the curvature radius of the surface of that optical element facing the deflector side, at the optical axis and in the sub-scan sectional plane, is denoted by rl.sub.0 while the least quantity of an absolute value of the curvature radius of the surface facing the scan surface side in an effective region of the portion where the sign of the curvature radius is reversed is denoted by |r2|.sub.min, the optical element satisfies a specific relation for them.