To provide an actuator for operating a movable member capable of being efficiently manufactured with the least trouble in finishing a ball screw groove of a ball screw mechanism, and reducing the cost.

In this actuator device wherein a nut 17 constantly receives the axial load from the movable member relatively to a screw shaft 16, and the movable member is displaced by relatively movement in the axial forward and backward directions of the nut 17 to the screw shaft 16, while constantly receiving the load, the ball screw groove of the nut 17 has a load face 20a closely abutted on a ball 18 on the basis of the load on its face at a forward direction-side of the nut 17 with respect to a groove bottom part as the boundary, and has a load face 19a closely abutted on the ball 18 on the basis of the load, on its face opposite to the forward direction-side of the nut 17 with respect to the groove bottom part as the boundary, and only the load faces 19a and 20a are finished.