A control apparatus for an electric power steering system assures stability of the feedback control and control generation of noise from the control circuit when the steering wheel automatically returns to the neutral position by means of the self-aligning torque in order to improve the feeling of the steering operation. In the motor driving circuit connecting four semiconductor elements in the shape of H bridge, the semiconductors of the first arm are driven by the duty ratio D1 while the semiconductors of the second arm are driven by the duty radio D2 defined by the function of the duty ratio D1 (D2 is longer in time than D1).