When the steered wheels are driven, there is a problem of the torque steering that the steering is disturbed by the difference in the driving forces between a pair of left and right steered wheels. Before such a problem, there is a problem that, since the vehicle is apt to run straight ahead more strongly as the driving force increases even when a uniform driving torque is given to a pair of steered wheels, a larger force is required to steer the vehicle leftward or rightward by overcoming such a tendency of running straight ahead. In view of the above, the steering assist force is increased according to an increase of the driving torque of the steered wheels in a manner of harmonizing the steering assist force to the magnitude of the steering resistance due to the driving torque of the steered wheels, so as to improve the steering feeling.