Patent ID: 8397859

Claim:
An electric power steering system, comprising: a rack shaft that is able to reciprocate in an axial direction; a motor shaft through which the rack shaft passes, and which is rotated by a motor; and a ball screw device that converts rotation of the motor shaft into axial movement of the rack shaft, wherein the ball screw device is formed by securing a ball screw nut to an axial end portion of the motor shaft and arranging a plurality of balls within a helical ball-rolling path that is formed by aligning a thread groove formed in an inner periphery of the ball screw nut and a thread groove formed in an outer periphery of the rack shaft with each other, wherein a hollow threaded shaft is formed in an axial end portion of the ball screw nut of the ball screw device, and the ball screw nut is screwed to the axial end portion of the motor shaft by screwing the threaded shaft to a threaded portion formed in the motor shaft, and wherein a restriction portion that restricts relative rotation between the ball screw nut and the motor shaft is provided.