A ball screw nut has a pair of through holes and a first passage that allows the through holes to communicate with each other and that opens radially outward. In each of the through holes, a ball screw mechanism has a circulation member having a second passage in which a ball can move between a ball raceway and the first passage. The ball screw mechanism has a circulation path for communicating the two connection points set on the ball raceway by the first passage and each of the second passages. A driven pulley has a thin portion that defines a radial clearance between an inner periphery of the driven pulley and an outer periphery of the ball screw nut. The ball screw mechanism has a pressing lid that presses down each of the circulation members from the outer periphery of the ball screw nut and that covers the first passage.