PURPOSE: To improve the sense of steering during high speed running by generating auxiliary steering power when the steering power exceeds the first set point and car speed is below a specified one and generating the auxiliary steering power independently of the car speed when the steering power exceeds the second higher set point.

CONSTITUTION: When steering power exceeds the first set point and is below the second set point during steering, a steering power sensor 22 issues an output signal to the contact 21 of a relay 19. At this time, if the car speed is below a specified value, a car speed sensor 18 issues a low speed signal. Thus, a solenoid 20 is excited and the contact 21 is closed. As a result, since the solenoid 24 of a relay 23 is excited and a contact 26 is closed, a motor 10 is started. If the car speed exceeds the specified value, since the sensor 18 will not issue the signal, the contact 21 is opened and the motor will not be started. If the steering power exceeds the second set point, the sensor 22 directly issues the output signal to the solenoid 24 without interposing the contact 21 and the contact 26 is closed. Consequently, the motor 10 is started in this case independently of the car speed.