An optical scanning system and an image forming apparatus with such optical scanning system, include a light source, an optical deflector having a deflecting surface, for scanningly deflecting a light beam emitted from the light source means, and an imaging optical system for imaging the light beam, deflected by the deflecting surface of the optical deflector, upon a scan surface to be scanned, wherein, when .gamma. (.gamma..noteq.0) refers to an angle which is defined, in a sub-scan sectional plane, between a principal ray of the light beam from the light source means and a normal to the deflecting surface of the optical deflector as the light beam is going to be incident on the deflecting surface, .beta. refers to an imaging magnification of the imaging optical system with respect to a sub-scan direction, W [mm] refers to a light-beam width, in a main-scan direction, of a light beam that passes a light exit surface of an imaging optical element, of the imaging optical system, which element is closest to the scan surface, and .epsilon. (.epsilon..noteq..gamma.) refers to an angle defined, in the sub-scan sectional plane, between an optical axis of the imaging optical system and a straight line that connects a point of deflection of the light beam incident on the deflecting surface of the optical deflector and an imaging point of the light beam being imaged on the scan surface, a relation.beta..beta..times..function..gamma..times..ltoreq..times. ##EQU00001## is satisfied.