This invention relates to map displaying system, in which the reference altitude value determining means obtains a reference altitude value in accordance with the altitude data in the specific area, and the altitude data changing means performs the processing that each altitude both in the plane area and the solid area have the continuity from one area to another. And the 2-dimensional road data storage means stores the 2-dimensional ground road data and the 2-dimensional underground road data separately. Therefore, the 3-dimensional road data preparing means prepares the 3-dimensional ground road data in accordance with the 2-dimensioanl ground road data and the altitude data, while preparing the 3-dimensional underground road data in accordance with the 2-dimensional underground road data and the altitude data.