Abstract: In an image forming reflecting mirror for a constant speed optical scan, a light beam is convergent in a cross scan-corresponding direction with a point near a starting point of deflection as a starting point of the light beam, and the light beam is deflected at an equal angular velocity and is converged to form a light spot on a scanned face so as to optically scan the scanned face at a constant speed. The image forming reflecting mirror has a shape on a deflecting-reflecting face represented by the following general formula, Y.sup.2 =2RmX-(K+1)X.sup.2, where X designates a coordinate in the direction of an optical axis and Y designates a coordinate in a direction perpendicular to the optical axis with a position on the optical axis as an origin, and Rm designates a radius of curvature on the optical axis and K designates a conical constant.