An electric power steering system (20) comprises a control device (32) which controls a direct-current motor (49) for operatively providing auxiliary torque to a steering shaft (26) in dependence on steering torque (Ts) acting on the steering shaft. The control device (32) includes a control signal generating circuit (70) which generates a PWM signal (t3, T4, T5) for controlling the motor in dependence on the steering torque, and a motor drive circuit (80) which receives the PWM signal and impresses an on-off voltage to the motor, the drive circuit (80) having a close circuit (49-86-80a, 49-83-80d) in which an electric current (Ia) due to an inductance of the motor flows while the on-off voltage is off. The steering system (20) further comprises a mechanism (76) for detecting a steering speed (Ns) of a steering wheel. The control signal generating circuit (70) generates the PWM signal in dependence, in a positive steering state in which the rotating direction of the steering wheel is coincident with the acting direction of the steering torque, on a first function (125, 126) and, in a steering wheel returning state in which the rotating direction of the steering wheel is not coincident with the acting direction of the steering torque, on a second function (127, 128), having the steering torque and the steering speed as parameters thereof.