Since a nut 45 is provided with paths for connecting rolling paths 23b, 45f and circulating paths 45c, that is, return paths 45d at end caps 45b, 45b attached to both ends of a main body 45a, a receiving portion for receiving power transmitted from an electric motor 35, that is, a driven gear portions 45e is provided at a center of an outer peripheral face on outer sides in a radius direction of the rolling paths and therefore, when power is transmitted from the electric motor 35 to the driven gear portion 45e, smooth operation can be carried out by restraining the nut 45 from being twisted.