PURPOSE: To prevent vehicle behavior from becoming unstable by reason that an auxiliary steering angle corresponding to a differential value of a main steering angle becomes excessive in the case of steering at a high speed.

CONSTITUTION: The upper limit of a differential value [)d/dt)θ]L of a steering angle as shown in (b) is restricted in such a way that as a steering angular velocity (d/dt) θ becomes fast, a rising rate is set to become small, and that as a steering angle θ is lowered, the rising rate is set to become small. By using this upper limit as a differential term in an operation expression for an auxiliary steering angle, vehicle behavior can be prevented from becoming unstable by reason that the auxiliary steering angle becomes excessive particularly in the vicinity of a neutral postion in the case of steering at a high speed.