Abstract: An object of the present invention is to accomplish the automatic preparation of road data in which roads and intersections are expressed by polygons that show accurate agreement with the complicated road shapes on city maps. In simple polygon preparation processing 3, the respective line segments of road network data 2 in which roads are expressed as line segments are expanded in the direction of the width dimension, thus producing simple road polygon data 4 which has a width that is slightly greater than the width of the roads in the city map data 5. Next, in scissors data preparation processing 6, scissors data 7 which defines the outlines of roads is prepared from the city map data 5 by connecting shape lines in the vicinity of roads. Next, in road polygon preparation processing 9, road polygon data 9 which shows good agreement with the shapes of roads in the city map data is prepared by trimming the simple road polygons along the road outlines defined by the scissors data.