An optical scanning apparatus includes a deflector for deflecting a beam emitted from a light source, and scanning optical system having at least one scanning lens for causing the beam deflected by the deflector means to be imaged on a surface to be scanned. At least one scanning lens constituting the scanning optical system has a plurality of positioning reference surfaces for effecting the positioning of the scanning lens in the direction of the optical axis thereof, and is disposed so that a straight line linking the points on two positioning reference surfaces together or a plane formed by the points on three or more positioning reference surfaces may be non-orthogonal to the optical axis. In a main scanning cross-section, the shape of the effective portion of the scanning lens is asymmetrical with respect to the optical axis.