A network system includes a plurality of communication nodes. In the network system, data transmitted from a server apparatus (1) is received by a PC (2) and a PDA (3). Each of relay nodes 4 obtains a communication network parameter concerning the communication network connecting the communication nodes with each other. Each of the relay nodes (4) also determines a communication route, based on a format conversion parameter concerning format of data transmitted from the server apparatus (1) and types of format conversion at format converters (5a) and (5b), and the obtained communication network parameter.