A control apparatus of an electric power steering system has first to fourth calculators and an abnormality detector. The first calculator calculates an assist current command value, which corresponds to assist force for reducing steering force of the steering wheel, based on the steering torque of the steering wheel. The second to fourth calculators calculate a compensation current command value, which corresponds to compensation force for compensating for movement of the steering wheel, based on the angular velocity of the steering wheel that has been estimated based on the terminal-to-terminal voltage of the motor. The abnormality detector detects an abnormality of the terminal voltages of the motor. When the abnormality detector detects an abnormality, the second to fourth calculators substitute zero for the steering wheel angular velocity and then calculates the compensation current command value.