1. A map data update method for updating nationwide updated version map data using route-specific update data, comprising the steps of: storing nationwide updated version map data which is comprised of a plurality of map files, in a map storage portion of a navigation apparatus: creating every time a new route is opened in an external route-specific update data creation device, a route-specific update data comprising update data for all routes which have been opened from the time of release of the nationwide updated version map data until the opened date of said new route; storing a plurality of the route-specific update data sets with different creation dates; identifying a first route-specific update data set from among said plurality of the route-specific update data sets and receiving the first route-specific update data set from the external route-specific update data creation device; storing map files of the nationwide updated version map data of the time of release which are to be updated using the first route-specific update data set, as pre-update map files; updating the map files of the nationwide updated version map data of the time of release stored in the map storage portion, using the first route-specific update data set; storing the map data updated using the first route-specific update data set in the map storage portion instead of the nationwide updated version map data; when performing an update using a second route-specific update data set after performing an update using the first route-specific update data set, identifying a second route-specific update data set from among said plurality of the route-specific update data sets and receiving the second route-specific update data set from the external route-specific update data creation device; restoring the nationwide updated version map data of the time of release from the map data updated by the first route-specific update data set using the stored pre-update map files, in which the restoration is performed by replacing map files of the map data updated by the first route-specific update data which have the same names as the stored pre-update map files, by the stored pre-update map files; using the second route-specific update data set, to update the restored nationwide updated version map data: and storing the map data updated using the second route-specific update data set in the map storage portion instead of the map data updated by the first route-specific update data set and storing pre-update map files necessary to restore the nationwide updated version map data of the time of release from the map data updated by the second route-specific update data set.