A mirror surface angle detector which can be used in common with a mirror surface angle adjusting device of a mirror. The mirror surface angle of the mirror is adjusted by oscillation of the mirror by the mirror surface angle adjusting device. The mirror surface angle detector for detecting the mirror surface angle of the mirror is provided separately from the mirror surface angle adjusting device, and is incorporated into the mirror surface angle adjusting device such that engagement claws provided at the mirror surface angle detector engage with engagement holes provided at the mirror surface angle adjusting device. When the mirror surface angle detector is removed from the mirror surface angle adjusting device, the mirror surface angle adjusting device can also be used in a type of vehicle which does not have a mirror surface angle detector in the mirror surface angle adjusting device. Therefore, vehicles of different types can make common use of the mirror surface angle adjusting device.