An axially extending extension is formed on a bearing retainer provided adjacent to a valve block of a joint and press-fitted into the interior of a housing. The extension has at its outer peripheral end a bearing by way of which the bearing retainer is rotatably supported by a differential gear case located outside thereof. This allows moment arising from rotational secondary torque received by the housing to be transmitted from the bearing retainer extension via the bearing to the outside differential gear case, thereby preventing the moment from acting on a main shaft which is a centrally located output shaft.