1. A navigation device comprising: a map database storing a plurality of map data that are divided into predetermined update units and respectively include identification information indicating an identification symbol and version information indicating a present update version; receiving means for receiving, via a communication network, update data that are generated in update units corresponding to the update units of the map data and include difference information indicating an update content of update subject map data as a difference with pre-update map data, identification information indicating the identification symbol of the update subject map 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 the update data; and updating means for performing a differential update on the map data using the update data received by the receiving means or read by the reading means, wherein, when performing an update on the map data using the update data stored on the recording medium, the updating means extract the update data having identification information that matches the identification information of the map data and a later update version than the update version of the map data from the plurality of update data stored on the recording medium, and perform a differential update on the map data using the extracted update data.