A support portion (23) has three screw insertion portions (23d1, 23d2, and 23d3) which is fixed to a caliper, and arranged so as to surround an output shaft (39). Therefore, it is possible to firmly fix a case (21) to the caliper at these fixing locations. Additionally, since the fixing locations are determined so as to collectively define a “plane”, it is possible to effectively inhibit a sway of the case (21) with respect to the caliper. Furthermore, it is possible to inhibit a twist of the case (21), and it is possible to sufficiently support a vehicle requiring large torque such as a truck.