Supersonic water-drop-removing mirror assembly and water removing method has a periphery of the double-walled vibrating unit sandwiched by the mirror holder from fore and backside at predetermined points on nodes of generated standing wave. The mirror holder has projections in point contact with the backside of double-walled vibrating unit at predetermined points corresponding to nodes of the standing wave in central portion of double-walled vibrating unit. The projections are formed of an elastic material which absorbs vibration. The mirror plate is first vibrated by the supersonic vibrator with its vibration frequency set at a first level to remove water drops on a front surface of the mirror plate. Next, the vibration frequency of the supersonic vibrator is changed from the first level to a second level so that positions of nodes of standing wave to be generated by said supersonic vibrator are shifted. Subsequently, the mirror plate is vibrated again with the vibration frequency of the supersonic vibrator set at second level to remove the water drops which remain on the front surface of the mirror plate at positions of the nodes of the standing wave which were generated when the vibration frequency was set at the first level.