The digital map data processing system of the invention aims to improve the appearance of a map without increasing the labor in automatic generation of road polygons required for generation of map data. The digital map data processing system of the invention reads network data 40, which expresses multiple roads by nodes and links, and generates polygon data 50 including road polygons by processing the network data 40. The road polygon generation process includes generation of rectangular polygons of specified widths about respective links, generation of edge lines around polygons, and elimination of unnecessary edge lines. The digital map data processing system selectively applies a suitable terminal processing technique to specify terminal geometry of each object polygon, based on connection with each connection polygon and geometries and attributes of the object polygon and the connection polygon. This desirably improves the appearance at a connection of adjacent polygons.