PURPOSE: To have natural steering sense free from incompatibility by giving an aid steering force to the steering system in accordance with the target steering force set on the basis of the phase difference between the sideways acceleration of a car and the sideways acceleration for the steering angle and direction of the steering system.

CONSTITUTION: An aid steering force is given to a steering system A by a means B. The actual steering force in the steering system A, steering angle, and angular velocity in steering are sensed by sensing means C-E. Further the car speed is sensed by a means F. The sideways acceleration generated in the car is calculated by a means G on the basis of the steering angle, angular velocity in steering, and car speed. The steering direction of the steering system A is sensed by a means H. The phase difference between the sideways accelerations for the steering angle is presumed by a means I. Further the target steering force is set by a means J on the basis of the sideways acceleration, steering direction, and phase difference. The abovementioned means B is controlled by a means K so that the actual steering force becomes identical to the target steering force.