A seal member is attached to a cover which occludes a gear case as a housing unit. The seal member is formed of elastomer and integrally formed with: a plate-shaped portion formed into a disc shape which covers the inner surface of the cover; an inner seal portion which is positioned on the inner peripheral edge of a through-hole, and comes into sliding contact with the outer peripheral surface of an output member; an outer seal portion which is positioned on the periphery of the cover, and comes into contact with the opening end of the housing unit; and anchor portions which latch onto the front surface of the cover via mounting holes provided to the cover. The boundary between the seal member and the cover extends only to the outside of the gear case, and does not extend to the inside of the gear case.