PURPOSE: To improve the precision of a light reflecting surface with small-sized constitution by fitting a mirror surface body on a shaft fixed to the bottom part of housing by bearings fitted in both bearing fitting surfaces.

CONSTITUTION: The mirror surface boy 7 is fitted on a rotator support shaft 5 by the bearings 9a and 9b which are fitted in the bearing fitting surfaces formed at both end parts of a shaft hole 7a penetrating the mirror surface body 7 in the center and pre-loaded by a pre-load spring 8. Consequently, rotary oscillations of the mirror surface body 7 are reduced and a light reflector 7e is formed at the outer periphery between the bearings 9a and 9b, so surface inclination is reduce. Further, covers 10 and 15 which cover the outward opening end of the shaft hole 8 of the mirror surface body 7 and the opening end of the housing are provided, so the light reflecting surface 7e is never contaminated with bearing lubricating oil. The mirror surface body 7 is nonmagnetic, so magnetic powder is prevented from being attracted to the light reflecting surface 7e and making a light beam disordered. Thus, the precision of the light reflecting surface is improved with the small-sized constitution.