<p>A ball screw nut through which a ball screw shaft is inserted is connected to a first circular tubular member, which is placed around the ball screw shaft, such that the circular tubular member and the ball screw shaft do not rotate relative to each other. The ball screw nut and the first circular tubular member are rotatably supported by a second circular tubular member via a ball bearing having two rows of balls. One end of the ball screw nut is fitted by splines to one end of the first circular tubular member. Two rows of raceways of the ball bearing are formed on the outer peripheral surface of the ball screw nut.</p>
