A road surface reaction torque detector has a low-pass filter element whose time constant is determined in accordance with a vehicle speed that is detected by a vehicle steed detector and a motor rotation speed that is detected by a motor rotation speed detector. A road surface reaction torque estimation value is obtained in such a manner that the road surface reaction torque detector performs low-pass filter processing on steering shaft reaction torque signal that is outputted by a steering shaft reaction torque signal outputting means. As a result, an accuracy of estimation of road surface reaction torque is improved.