Disclosed is an arm stopper mechanism mounting apparatus including: an output shaft; a housing that is provided with a hole for inspecting the sealability of the interior thereof; a sealing member that seals the hole, and can be detached from the hole when the sealability is inspected; and an arm stopper mechanism that has a steering arm and a stopper. The steering arm includes an output shaft hole into which the output shaft is fitted; two tie rod holes to which the tie rods are respectively attached; and two striking surfaces which strike against the stopper. The stopper includes two contact surfaces. An angle made by the two contact surfaces of the stopper is greater than an angle made by the two striking surfaces of the steering arm, and is 90° or greater.