When it is determined that a malfunction has occurred in a rotational angle sensor, an electronic control unit changes the control manner from the normal assist control to the sensorless assist control. In the sensorless assist control, an assist stop command unit receives the information concerning the motor rotational angular speed .omega.m from an angular speed conversion unit. Then, the motor rotational angular speed .omega.m is compared with the threshold value .omega.m1. If the motor rotational angular speed .omega.m is equal to or higher than the threshold value .omega.m1, an assist enabling signal is transmitted to a PWM voltage production unit to continue the power assist. On the other hand, if the motor rotational angular speed .omega.m is lower than the threshold value .omega.m1, a stop command signal is transmitted to the PWM voltage production unit to stop the power assist.