To improve the rigid feeling of steering, improve the efficiency by a reduction in torque loss, and reduce the cost.

A deceleration mechanism for supplying an auxiliary torque 8, a motor 9, and a torque sensor 2 for detecting a steering torque are held by a housing 7. The deceleration mechanism 8 has a small gear 81 and a large gear 82, and it is formed of a special theoretical tooth profile gear mechanism capable of setting the minimum tooth number of the small gear to 4. The torque sensor 2 comprises a torque transmission shaft 21 and a solenoid 22 arranged around the torque transmission shaft 21 to generate a magnetic field passing the torque transmission shaft 21, and the torsional distortion of the torque transmission shaft 21 by the steering torque is detected on the basis of the change of magnetic field by the solenoid 22, whereby the steering torque is detected with no contact.