PURPOSE: To allow multistage change of reflectance by providing three or four plane mirrors having different reflectances or colors and each attached to a surface of a mirror holder having a structure of regular triangle or square in cross section.

CONSTITUTION: A mirror holder 3 is a prism structure of regular triangle in cross section and has surfaces each of which firmly carries one of plane mirrors 4A to 4C having different reflectances or colors. A magnet 7 of a nonmagnetic material is attached through a fitting member 6 to each of the vertexes of the triangle which define the outermost circumference of the mirror holder 3 when rotated. There are provided rotary shafts 5A, 5B projecting from the opposite side surfaces 3d, 3e of the mirror holder 3 at the respective centers of gravity of the regular triangles, and the shafts are rotatably fitted in bearing holes 2d of a mirror housing 2. A gear 8 rotatingly driven by a motor 10 is firmly secured to the shaft 5A so as to rotate the mirror holder 3.