PURPOSE: To precisely detect a neutral point of a steering angle sensor by providing a neutral point determining means determining a steering wheel angle being detected by a steering angle sensor as the neutral point.

CONSTITUTION: A mean steering wheel angle θha is set as a neutral steering wheel angle θhn showing a neutral point of a steering wheel angle θh. When a vehicle runs at a high speed of 60km/h or more, it is judged that the vehicle runs straight on the basis of that a state where a displacement amount in the lateral direction of a preceding vehicle is less than 1m is continued for 30 seconds or more at an inter-vehicle distance Y of 35 to 70m from the preceding vehicle, and since a mean value θha of the steering wheel angle θh at that time is determined as a neutral point (the neutral steering wheel angle θhn) of a steering angle sensor 10, the neutral point is precisely detected.