JP2982041B

PURPOSE: To make it possible to well correct the optical characteristics of a main scanning direction and sub-scanning direction by arrangement of an aspherical reflection mirror and to execute high-density optical scanning having a plane tilt correcting mechanism of a polygonal mirror. CONSTITUTION: The shape within a main scanning plane inclusive of an optical axis is expressed by the equation Z(Y)=AY +BY +CY +DY +EY (A, B, C, D: coefft) when the coordinate in the optical axis direction is defined as Z and the coordinate included within the main scanning plane orthogonal with the optical axis as Y. The coeffts. A and C satisfy the conditions A.C>0. The shape within the sub-scanning plane orthogonal with the main scanning plane parallel with the optical axis is a circular arc nearly satisfying a geometrical optical conjugation relation of a deflection plane and the surface to be scanned in any position in the main scanning direction.