<p>A navigation system has an updated map data acquisition means (6) for acquiring updated map data, and map data is updated by writing the updated map data from the updated map data acquisition means (6) into a free space of a map data storage means (2). The map data updating is made even if a navigation function is being used through the use of not-yet-updated map data, stored in the map data storage means (2), by a control means (5) for controlling functions of the entire navigation system.</p>
