Assist force that assists an operation of a steering system of a vehicle is applied to a rack shaft by transmitting driving force of a motor to the rack shaft via a motor shaft and a ball screw nut. A step is formed on the outer periphery of the ball screw nut. An end portion of a third housing is arranged to face a bearing such that the distance that the ball screw nut is able to move relative to the motor shaft in axial direction and the relative movable length is longer than a predetermined length.