PURPOSE: To reduce the cost of an inclination correcting optical system by linearly condensing a laser beam on the mirror surface of a deflector by a distributed index lens which has refractive index distribution only in one direction.

CONSTITUTION: The distributed index lens which has refractive index distribution only in one direction is used instead of a conventional cylindrical lens. Parallel rays of the luminous flux from a laser unit 1 as the light source pass a distributed index lens 7 having refractive index distribution only in one direction, and the image of the section in the subscanning direction is formed no a rotating polygonal mirror but that in the main scanning direction is not formed and rays are parallel as they are. Thus, the distributed index lens 7 is used in an inclination correcting scanning optical system to reduce the cost of the inclination correction optical system.