Navigation system

A navigation system detects a present position of a movable body and performs a navigation for the movable body on the basis of map information. The navigation system is provided with: a storage device (15) of non-volatile type capable of writing the map data thereinto and reading the map data therefrom; a map data transferring device (11) for obtaining the map data corresponding to a predetermined area, and transferring and storing the map data corresponding to the predetermined area to the storage device; an empty area judging device (11) for judging whether or not an empty storage area having a predetermined capacity is reserved in the storage device when the map data transferring device is to transfer the map data corresponding to the predetermined area to the storage device; and an erasing device (11) for determining unnecessary data among data stored in the storage device on the basis of a predetermined condition, and erasing the unnecessary data from the storage device if the empty area judging device judges that the empty storage area is not reserved.