Disclosed herein are actuator, planetary-gear apparatus, and structural-unit embodiments configured to reduce noise and vibration caused by operation of planetary gears. An embodiment may include a ring gear having an outer peripheral surface that extends in the axial direction. A first raised portion may be formed on said outer peripheral surface; and a housing having an inner peripheral surface that is provided facing, and with a gap from, the outer peripheral surface of the ring gear. A second raised portion may be formed on said inner peripheral surface. Movement of the ring gear in the circumferential direction is limited by linear contact or point contact between the first raised portion and the second raised portion, for example. The outer peripheral surface and/or the inner peripheral surface may be a surface of a crowned shape that is bowed in the outer radial direction, according to some further embodiments.