To allow a maintenance device to execute check and maintenance for each slave station in a system on-line state.

In the system where one master station 11 and a plurality of slave stations 13 are connected to a transmission line 12 and the master station 11 conducts information exchange processing with each slave station 13 by its own period or its timing while obtaining a token of the transmission line 12, the master station 11 is provided with a connection port 15 whose one terminal is led into the master station that is connected to the transmission line and with a maintenance device 17 connected selectively to the other terminal of the connection port 15 and conducting check and maintenance of each slave station 13 via the transmission line while acquiring a token of the transmission line 12. Then the master station 11 passes the token of the transmission line 12 to the maintenance device 17 in its spare time of information exchange processing with respect to each slave station 13.