In an electric power steering system, an electric motor generates output power for power-assisting steering operation of a steering wheel, a torque sensor generates a plurality of electric output signals corresponding to a torque applied to the steering wheel, and a control unit controls the steering operation of the electric motor in accordance with at least one of the electric output signals of the torque sensor. The control unit detects a difference between rotation speeds of left and right steered wheels, checks whether any one of the plurality of output generating parts is in failure, and specify a failing-device based on the rotation speed difference and the plurality of output signals.