PURPOSE: To prevent the mirror surfaces of a polygon mirror from varying in a reflection factor in a main scanning direction by preventing powdery dust from sticking partially on the mirror surfaces.

CONSTITUTION: Fins 40 which extend almost from the longitudinal center of the respective mirror surfaces to the corner parts 30b of the polygon mirror 30 are fitted to the peripheral edge parts of both plate surfaces 30c of the polygon mirror 30. The fins 40 has their surfaces slanted gradually leaving the plate surfaces 30c of the polygon mirror 30 in the rotating direction. Then when the polygon mirror 30 is rotated, the flow of air is guided by the fins 40 from one rotation-directional end to the opposite end on the mirror surfaces 30a as shown by an arrow to be made uniform.