PURPOSE: To determine a zero position rapidly so as to enable the realization of steering angle control of high accuracy in an early stage by judging whether a vehicle is in the rectilinear state on a relatively lenient basis in a stage where a zero position is not yet obtained after the start of operation.

CONSTITUTION: A steering angle sensor 17 is provided to obtain relative steering angles every mement to the steering angle at the reset time, and wheel rotating speed sensors 21-24 are provided to obtain wheel rotating speed wheel by wheel. In a control unit C, the rotating speed difference between both wheels in a wheel group formed of lateral wheels is obtained first. In the case of the difference being the specified value or less, a vehicle is judged to be in the rectilinear state, and the relative steering angle at the time of judging the rectilinear state is made the zero position of the steering angle sensor 17. In a stage where the zero position is not determined, the vehicle is judged to be in the rectilinear state when the vehicle speed is the reference vehicle speed or higher and the state of the rotating speed difference between both wheels being within the rotating speed difference corresponding to the reference steering angle continues for the specified time or longer, and the relative steering angle at this time is set to the zero position.