A navigation system comprising a GPS (15) for detecting a current position; a DVD drive (16) for reading a map from a DVD (100) stored with a plurality of partially overlapping maps; a memory card (18) for storing information to identify a portion overlapping with another map, in the map being used for a navigation; and a CPU (11) for judging whether or not the coordinates of the current positionare contained in the overlapping portion, on the basis of the coordinates of the current position and the information stored in the memory card (18), and for controlling, in response to the judgementthat the coordinates of the current position are contained in the overlapping portion, the DVD drive (16) to read the maps sharing the overlapping portion. As a result, the map data to be read can besmoothly switched according to the current position.