A display screen indicates a map on a first scale. After a vehicle passes a guide point, determinations are made as to whether or not an on-map position of a next guide point is outside a predetermined large-area indication deciding frame in the display screen, and as to whether or not the on-map position of the next guide point is inside a predetermined detailed indication deciding frame in the display screen. A second scale in a range where the current position of the vehicle and the on-map position of the next guide point are in the display screen is computed in response to results of the determinations. The display screen successively indicates the map on the first scale, maps on different intermediate scales between the first and second scales, and a map on the second scale in either a reducing order or a magnifying order at prescribed time intervals.