In a course of updating old map data on a hard disk drive to new map data, a low-priority portion of the old map data that has less importance/necessity is deleted to create a vacant area, and a high-priority portion of the new map data is copied to the vacant area as an update of the map data. Then, the high-priority portion of the old map data is deleted to create another vacant area, and a low-priority portion of the new map data is copied to the vacant area. During the update of the map data, restrictions on the usage of the map data are avoided at all efforts by having the high-priority portion of at least one of the new map data and the old map data maintained on the hard disk drive, while maximizing the utilization of storage resources in a map data management apparatus.