From among high upland area determination data stored in a hard disk drive, only a portion corresponding to a required segmental region is read into an external memory along with traveling of a vehicle. Determining of a high upland area is then executed based on the portion of the high upland determination in the external storage device. When a high-upland division is present in a vicinity of a present position, a portion of the high upland area determination data is read into the external memory. The read portion of the high upland area determination data corresponds to a high upland area, which covers all the high-upland divisions adjacently existing starting from the high-upland division in the vicinity of the present position. When a high-upland division is not present in the vicinity of the present position, a portion of the high upland area determination data corresponding to a segmental region having a predetermined minimum range centering on the present position is read into the external memory.