When an update instruction to update “A” navigation map information and “B” navigation map information is inputted, a CPU initializes the storage area having stored therein no map information currently in use out of the two storage areas. The CPU then reads, from a CD-ROM, the latest version of the “A” navigation map information or the “B” navigation map information that corresponds to the map information currently in use, and stores it in the initialized storage area. The CPU then updates management information, starts use of the newly stored map information, initializes the storage area having stored therein the map information whose use is stopped out of the two storage areas, reads, from the CD-ROM, the latest version of the “A” navigation map information or the “B” navigation map information not currently in use, and stores it in the initialized storage area.