A control apparatus for an electric power steering system is provided, which controls a motor for applying assist steering torque to a steering line of a vehicle at least according to steering torque signal of the steering line, vehicle velocity signal and motor current signal. The control apparatus includes a target current controller for setting target current signal for driving the motor, a deviation calculator for computing a deviation between the target current and motor current signals, a deviation adjuster for adjusting the deviation with a variable gain according to the vehicle velocity signal, a proportional element for executing proportional control for the motor, an integral element for executing integral control for the motor and an gain adjuster for adjusting the gain according to a differential of the steering torque signal.