1. A navigation device comprising: a map database storing map data divided into predetermined update units, each update unit corresponding to a region obtained by dividing a map into a plurality of regions and each update unit respectively including identification information identifying the corresponding region and version information indicating a present update version; receiving means for receiving, via a communication network, update data generated in update units corresponding to the update units of the map data, each generated unit of update data including difference information indicating an update content of the generated update data as a difference with the map data of the corresponding update unit stored in the map database, identification information identifying the corresponding region for the unit of update data, and version information indicating a post-update update version; reading means for reading the update data from a recording medium storing a plurality of units of the update data; and updating means for performing a differential update of the map data stored in the map database using the update data received by the receiving means or read by the reading means, wherein, when performing an update of the map data using the update data stored on the recording medium, the updating means extracts the update data units having identification information that matches the identification information of the update units of map data stored in the map database and having version information indicating a later update version than the update version of the update units of map data stored in the map database from the plurality of units of the update data stored on the recording medium, and performs a differential update of the update units of map data using the extracted update data units.