PURPOSE: To surely judge whether a vehicle is deviated from the route or not or whether it is traveling along the route or not.

CONSTITUTION: A perpendicular line is drawn downward from the position of the vehicle with reference to a route link within a prescribed search area, and the vertical distance to the link is computed. When the perpendicular line cannot be drawn downard, the distance to any end point of the link is computed. The shortest distance out of the computed distances is selected when its value is a deviation reference distance or higher, it is judged that the vehicle is deviating from the route. Since the perpendicular line is drawn downward and the distance is found irrespective of the length of the link, the computation accuracy of the distance is enhanced. Consequently, the link can be made long, and the capacity of the memory can be reduced.