A navigation system comprises: a GPS for detecting a current position; a DVD drive for reading a map from a DVD on which a plurality of partially overlapping maps are recorded; a Memory card for storing information to identify a portion overlapping with another map, in a map being used for navigation; and a CPU for judging, based on coordinates of a current position and the information stored in the Memory card, whether the coordinates of the current position are contained in the overlapping portion or not, and for controlling, in response to a judgment that the coordinates of the current position are contained in the overlapping portion, the DVD drive to read from the DVD the map sharing the overlapping portion. Consequently, map data to be loaded can be smoothly changed according to the current position.