PURPOSE: To make it possible that a slave station gathers monitoring information of all stations, by giving a function, by which monitoring information received from slave stations in the preceding communication are stored, to a master station and by adding these stored monitoring information to an information transmission command to the slave station.

CONSTITUTION: A master station transmits an information transmission command with a slave station name added, slave station monitoring information which are stored and are received in the preceding communication, and a slave station name for these information collectively to all slave stations A∼C as an ENQ signal. In respect to slave stations A∼C, all stations receive monitoring information of a station which receives the preceding polling; and if the information transmission command is sent for stations themselves, they transmit their own monitoring information to the master station.