In each of the divided regions of the map, a redundancy of the region is estimated from the characteristics quantity such as fractal dimension of line segments of road. Then, regions with the same redundancies are combined to form a larger area so that the map is divided according to the redundancy. The areas thus formed are summarized by the summarization methods conforming to their redundancies and are combined into one map. In the map thus generated, only necessary information can be presented, with redundant information eliminated.