A mirror (18) having a specular surface changeable in orientation using an actuator (16) is provided with an angle detection capability at reduced cost. A plate pivot (10) can change its orientation together with the specular surface so as to orient in a direction in which the specular surface is orienting. A slide block (20) is stressed toward the plate pivot (10) so that rotation of the plate pivot (10) causes the slide block (20) to move in a direction substantially perpendicular to the specular surface by an amount proportional to an amount of the rotation of the plate pivot (10). A magnet (26) moves in the direction of movement of the slide block (20) by an amount proportional to an amount of the movement of the slide block (20), and a magnetism sensing element (202) detects magnetism derived from the magnet (26) and outputs a signal corresponding to the amount of movement of the magnet (26).