System and method of communication, map data transmitting device, and terminal unit

A communication navigation system that performs navigation, using map data stored in a map database site and current position data about navigation terminals (T), by transmitting the map data to the navigation terminals (T) via the Internet (IN), wherein the map data includes data units respectively corresponding to a plurality of display elements for display on a map, and the data units are given respective priority levels. The map database site is provided with a server (SV) for transmitting the data units via the Internet (IN) according to the respective priority levels. Each navigation terminal (T) is provided with a transmitter/receiver (15) for sequentially receiving the data units transmitted from the server (SV), and a display subsystem (13) for sequentially displaying the display elements respectively corresponding to the data units received by the transmitter/receiver (15), on the map.