To make an electric motor compact to improve a vehicle loadability thereof, by providing a spiral groove formed on an outer periphery of a steering shaft with variable pitches.

The steering force is directly transmitted from a pinion 22a of an input shaft 20 to a rack tooth 30a formation part of a steering shaft 30, and the output of an electric motor 60 is transmitted from an output shaft to the steering shaft 30 through a holder 50, an outer race 42, a ball 41, a rolling ring 40 and a spiral groove 30b under the control by a controller. The steering torque is properly assisted by the output of the electric motor 50. Further the spiral groove 30b is formed with various pitch in such manner that the pitch is large at a central part in an axial direction of the spiral shaft 30b, and is small at the both ends in the axial direction. Whereby the rolling ring 40 is engaged with the central part in the axial direction of the spiral groove 30b in the linear movement of a vehicle, and the rolling ring 40 is engaged with the both ends in the axial direction of the spiral groove 30b in a large steering angle of the vehicle, that is, the optimum assistant characteristics can be set, the electric motor 60 can be made compact, and the vehicle loadability of the motor can be improved.