A power relay is provided in a power line, which supplies power from a power source to a motor through an inverter circuit, to shut off power supply from the power source to the inverter circuit when an abnormality arises. A microcomputer diagnoses a short-circuit failure of the power relay when a steering wheel is not steered during a normal control operation of assisting the steering operation of the steering wheel. An EPS shuts off power supply by the power relay when the steering wheel is not steered, so that the short-circuit failure of the power relay is diagnosed without affecting the assist operation for the steering wheel.