An azimuth change quantity .theta. of a road during traveling of a vehicle for a time .delta.t is calculated based on road data provided by a navigation system and a vehicle speed provided by a vehicle speed sensor (at step S3 in FIG. 2). On the other hand, an azimuth change quantity .THETA. of the vehicle is calculated by integrating a yaw rate .gamma. obtained from a yaw rate sensor over the time .delta.t (at step S5). A deviation D between the azimuth change quantity .theta. of the road and the azimuth change quantity .THETA. of the vehicle is calculated (at step S6). When the deviation D becomes equal to or larger than a reference value .beta., it is determined that there is a possibility that the vehicle will depart from the road (at step S9), and a predetermined steering torque is applied to a steering device, so that the deviation is converged into zero (at steps S10 and S11).