A steering assist operation is performed, which includes a motor control unit that controls a motor, and the steering assist operation does not require a rotational angle sensor. A current drive unit drives the motor at an axis current value of a rotating coordinate system that rotates in accordance with a control angle, which is calculated by a control angle calculation unit. The control angle calculation unit obtains, at every predetermined calculation cycle, a present value of the control angle by adding an addition angle to an immediately preceding value of the control angle. An addition angle calculation unit calculates the addition angle to be added to the control angle through a proportional-integral control based on a torque deviation from a command torque set by a command torque setting unit.