A plurality of terminals (321, 322, 323, 324, 231) on a communication network are connectable to a transmission line (10) to communicate mutually. Each terminal includes a single device or a composite device essentially consisting of a plurality of devices. Transmission and reception of messages to and from the plurality of terminals on the communication network is performed using an address system assigned to each terminal. The address system comprises a device address or DA (131) assigned to the individual terminals, a sub­device address or SDA (132) assigned to respective devices constituting each terminal and having a single or a plurality of values SDA, and a function address or FA (133) assigned to respective functions possessed by each device and having a single or a plurality of values of FA, whereby even when a terminal to be controlled is a composite device, only a function desired to be controlled can readily be controlled and extensibility of the system can be improved to facilitate control processing in the system through communication.