An optical scanning system, including light source means, deflecting means (6; 166) configured to scanningly deflect a light beam from said light source means in a main scan direction, and an imaging optical system configured to image, upon a surface to be scanned, the light beam deflected by a deflecting surface (6a; 166a) of said deflecting means (6; 166), wherein said deflecting surface (6a; i66a) is configured to perform reciprocating motion by which the surface to be scanned is reciprocatingly scanned in the main scan direction with the light beam deflected by said deflecting surface (6a; 166a) of said deflecting means (6; 166), wherein the deflecting surface (6a; 166a) is distortable due to application of acceleration thereto as a result of its reciprocation, wherein as a result of reflection of the light beam by the distorted deflecting surface (6a; 166a), a first phase difference of wavefront aberration is produced, in a main scan section containing the main scan direction and an optical axis of the imaging optical system, between a marginal ray and a principal ray of the light beam reflected by the deflecting surface (6a; 166a) at an effective deflection angle of the same corresponding to a largest scan position in an effective scan region on the surface to be scanned; and as a result of transmission of the light beam through said imaging optical system, a second phase difference of wavefront aberration is produced in the main scan section between a marginal ray and a principal ray of the light beam reflected by the deflecting surface (6a; 166a) at the effective deflection angle thereof; characterized in that at least one optical surface of said imaging optical system is of aspherical shape in the main scan section, wherein said aspherical shape is adapted such that said first and second phase differences are made opposite to each other, wherein, when δL1 + is used to refer to an optical path difference, in the main scan section, between a marginal ray at a scan end portion side of the light beam and a principal ray of the same reflected by said distorted deflecting surface (6a; 166a) at the effective deflection angle thereof, the difference being produced as a result of reflection of the light beam by said distorted deflecting surface (6a; 166a), δL1 - is used to refer to an optical path difference, in the main scan section, between a marginal ray at a scan central portion side of the light beam and the principal ray of the same reflected by said distorted deflecting surface (6a; 166a) at the effective deflection angle thereof, the difference being produced as a result of reflection of the light beam by said distorted deflecting surface (6a; 166a), δL2 + is used to refer to an optical path difference, in the main scan section, between the marginal ray at the scan end portion side of the light beam and the principal ray of the same reflected by said distorted deflecting surface (6a; 166a) at the effective deflection angle of the same, the difference being produced as a result of transmission of the light beam through said imaging optical system, and δL2 - is used to refer to an optical path difference, in the main scan section, between the marginal ray at the scan central portion side of the light beam and the principal ray of the same reflected by said distorted deflecting surface (6a; 166a) at the effective deflection angle of the same, the difference being produced as a result of transmission of the light beam through said imaging optical system, said imaging optical system satisfies the following relation: 0.8 ≤ δL ⁢ 2 - - δL ⁢ 2 + δL ⁢ 1 + - δL ⁢ 1 - ≤ 1.2