a brushless motor that generates assistant steering force to be supplied to a steering mechanism of an automotive vehicle;

a controller that acquires a level of three-phase voltage used for driving the brushless motor;

a motor driver that drives the brushless motor based on the level of the three-phase voltage acquired by the controller; and

a current detector that detects a level of three-phase current flowing in the brushless motor,

wherein the controller includes:

a current value converter that converts the level of the three-phase current into a d-axis current detection value and a q-axis current detection value,

a target current calculator that calculates a target value of a current supplied to the brushless motor on the basis of an input value including at least a steering torque,

a current value restrictor that restricts the target value calculated by the target current calculator so as not to be greater not only than a first restrict value based on a square sum of the d-axis current detection value and the q-axis current detection value but also than a second restrict value based on the squares of the level of the three-phase current, and

a voltage value determining unit that determines a level of three-phase voltages on the basis of target value restricted by the current value restrictor, the d-axis current detection value, and the q-axis current detection value.