A control apparatus of an electric power steering system has calculating means, controlling means, determining means suppressing means, canceling means. Calculating means calculates an assist current value that corresponds to the assist force based on steering torque of a steering wheel. Controlling means executes assist control of the motor based on the assist current value. Determining means determines an abnormality in the circuit. Suppressing means suppresses the assist control when an abnormality in the circuit is determined. If the determination of an abnormality has continued longer than a predetermined time since the suppression of the assist control, the suppressing means opens relay means that permits electric power to be supplied to the motor. When it is determined that the circuit returns to normal with the assist control suppressed, the canceling means cancels the suppression of the assist control.