A database compiler sequentially selects element points constituting an outline of a polygon corresponding to polygon data obtained from a map database. When it is determined that one of the selected polygon element points is positioned on a map mesh boundary, a coordinate of the polygon element point is shifted toward an inside portion of the polygon by 1 point. When the polygon is to be displayed in an emphatic manner, the polygon data which has been coordinate-shifted is obtained. When it is determined that a portion of the outline of the polygon corresponding to the obtained polygon data is positioned on one of the map mesh boundaries, the portion of the outline is displayed in a non-emphatic manner and the remaining portions of the outline are displayed in an emphatic manner.