PURPOSE: To enable communication between a control station and a subordinate station to be performed even when a communication line is disconnected by adding a line for alternation, providing a function to repeat polling from the control station at the subordinate station, and repeating the polling from the control station by another subordinate station.

CONSTITUTION: Polling sequence 5 setting an address addressed to the subordinate station A 2a a PA52 for the subordinate station A 2a is transmitted to the control station 1. The station A 2a returns an information message 6a including data 62 to be transmitted to the station 1. The station 1 returns ACK (affirmative response) 7a to the station A 2a. Furthermore, the station 1 transmits the sequence 5 setting an address (c) addressed to the subordinate station C 2c as the PA (polling address) 52 to the station A 2a. The station A 2a returns an information message 6a including data 62 '0' representing the fact that the station C 2c is not connected to its own station. The station 1 returns ACK 7a to the station A 2a, and also, returns the sequence 5 setting an address (d) addressed to the subordinate station D 2d as the PA52 to the station A 2a. The station A 2a returns the information message 6a including the data 62 representing the fact that the station D 2d is not connected to its own station.