To control a motor properly by avoiding unnecessary put-on and put-off of a motor originating from turbulence such as steering on the steering wheel, kick-back, etc.

The control is executed on the basis of the steering condition parameter(s) (for example, steering torque) and its change rate. For the steering condition parameter, two regions are set, i.e., the ON region as exceeding the specified value and the OFF region otherwise. For the change rate, two regions are set. i.e., the ON region ranging between the first and the second specified value and the OFF region otherwise. In case both the steering condition parameter and change rate show values within the respective ON regions, a motor is put on (time ta). In case the steering condition parameter shows a value in the OFF region over a specified period of time and the change rate shows a value in the OFF region over a specified period of time, the motor is put off (time tb).