In an optical beam scanning apparatus in which an optical beam from a light source impinges upon a deflecting system and a deflected optical beam from said deflecting system scans a surface-to-be-scanned through a scanning lens in a main scanning direction, with a curvature of image surface correction method of the present invention, curvature of image surface is reduced without improving the surface accuracy of the scanning lens. A lens holder (14) is freely movable on an optical base (3) along a guide (15). When an adjustment screw (16) is manipulated, the lens holder (14) is moved so that a collimator lens (12) is moved along an optical axis (OA) and positioned. As the collimator lens (12) is moved, the degree of parallel of an optical beam which is emitted toward a galvanometer (30) is changed and the shape of an image surface is adjusted, so that the image surface is positioned within the depth of focus of the optical beam scanning apparatus over an entire scanning range. Hence, it is possible to perform drawing with an excellent image quality.