than that of the cylindrical portion 1c the cylindrical portion 1a contains a brushless motor 3 therein a hollow rotational shaft 5 is disposed to pass through the inside of the brushless motor 3 the hollow rotational shaft 5 is rotatably supported by the inner periphery of the cylindrical portion 1c with a bearing 7 as represented on the left hand side of fig1 meanwhile the hollow rotational shaft 5 is rotatably supported by the inner periphery of the cylindrical portion 1a with a bearing 9 as represented on the right hand side of fig1 the brushless motor 3 is connected to a power source not shown of a motor vehicle through a harness 11 the hollow rotational shaft 5 is connected to a planetary gear unit 13 disposed in series at two stages an output side of the second stage of the planetary gear 13 is connected to a nut member 15 the nut member 15 is attached to the housing 1 such that the nut member is not permitted to move linearly with respect to the housing however the nut member 15 is rotatably supported by an inner periphery 1ab of the cylindrical portion 1a with a bearing 17 and is thus rotatable with respect to the housing 1 in.