A motor-driven power steering system equipped with a control unit (9) which controls the turn of a DC motor (13) as an auxiliary steering force on the basis of a steering torque signal from a steering torque sensor (2) that detects the rotational force of a steering wheel (1) and on the basis of a vehicle-speed signal from a vehicle-speed sensor (10), and which turns an electromagnetic clutch (16) on or off in order to effectively or ineffectively control the rotational force of the DC motor (13) as the auxiliary steering force. Further provided is a fail-safe relay (91) having a normally open contact (91b) on a line for supplying power to the electromagnetic clutch (16). Under an abnormal condition such as breakage in the power source wiring, the electromagnetic clutch (16) is shut off because of no energization of the fail-safe relay (91), and no auxiliary steering force is applied by the DC motor (13).