PURPOSE: To eliminate the display of unnecessary part of map information and to display a map rapidly and understandably more than ever by providing an area deciding device.

CONSTITUTION: The map information is stored in a storage device 2 and divided into groups such as a road group, a river group and a house group, which are given as vector data shown by, e.g., a coordinate string between a start point and an end point. A control device 5 reads out the map information from the storage device 2 and displays every group, which should be surely displayed, in a display device 1. As for other groups, an area deciding device 4 decides whether or not their coordinate data read out from the storage device 2 is that in a prescribed area 4. If the data is that in the prescribed area, it is displayed in the display device 1, whereas if the data is that out of the prescribed area, it is not displayed. Thus, the unnecessary map information can be eliminated, the time for displaying the map is shortened, and an easily understandable map can be obtained.