PURPOSE: To make the reflected light from the parallel flat plate not reach a scanning surface and also to obtain a satisfactory optical performance by making a laser luminous flux enter from the direction inclined to a subscanning direction with respect to the direction orthogonal to reflection surfaces of a polygon mirror.

CONSTITUTION: Polygon mirror 11 as a light reflector has plural reflection surfaces 11a on its surrounding and is rotationally driven around a rotational axis 12. The polygon mirror 11 is sealed in a hermetically sealed housing 18 and the part of the housing 18 on which the luminous flux is made incident is constituted of a transparent parallel flat plate 19. The reflection surfaces 11a of the polygon mirror 11 are made parallel with the rotational axis 12 and the parallel flat plate 19 is made parallel with the rotational axis 12 and the reflection surfaces 11a. That is, when the normals of the reflection surfaces 11a of the rotating polygon mirror 11 cross the parallel flat plate 19 at right angle, the reflection surfaces 11a become parallel with the parallel flat plate 19. The laser beam is made incident on the polygon mirror 11 by being made to have an angle β to the subscanning direction from the scanning center (line) of a main scanning direction.