Abstract: Traveling conditions of a vehicle are detected. A present position of the vehicle is calculated on the basis of the detected vehicle traveling conditions. A memory holds data representing preset reference points located at intersections and at unforked straight roads, data representing whether the respective preset reference points are located at the intersections or at the unforked straight roads, and other data. A target reference point toward which a vehicle is traveling is selected from the preset reference points. When the vehicle passes the target reference point, the calculated vehicle position is forcedly set to the target reference point. A decision is made as to whether or not the target reference point is located at an intersection or at an unforked straight road.