PURPOSE: To avoid breakdown of the whole system even when a trouble occurrs in one master station, by making an arrangement that two master stations can collect monitoring data of the same slave station simultaneously.

CONSTITUTION: When the 1st master station 1 becomes the talker station, the master station 1 tries the listening and confirms that no poling signal from the 2nd master station 2 exists on a poling circuit 6, and then, starts the data collection from each slave station 3, 4, and 5 by succeessively transimitting the poling signal to each slave station 3, 4, and 5. When it is judged that the 2nd master station sends the poling signal to the poling circuit 6, the 1st master station does not become the talker station and works as the listener station and stores slave station addresses in the poling signal transmitted from the 2nd master station 2. The 1st master station 1 collects monitoring data sent from the slave station of the said address in the next listening period together with the second master station 2.