PURPOSE: To enable a communication between a primary station and the other secondary station even when a fault occurs at one part of the secondary station, or a power source is not supplied.

CONSTITUTION: A transmitting and receiving part 12 having a receiving terminal Rx and a transmitting terminal Tx, CPU 13 which outputs a clear command to a WDT 16 at every constant time interval, and WDT 16 which starts a clock when the power source is supplied, drives a relay 17 until a clock value is beyond a prescribed value, and stops the driving of the relay 17 when the clock value is beyond the prescribed value, are connected through an inside bus 15 with one another. As for the relay 17, the first pole make contact a1 is connected with the receiving terminal Rx of the transmitting and receiving part 12, the first pole brake contact b1 is connected with the second pole brake contact b2, the first pole common c1 is connected with an upstream side loop circuit 18, the second pole make contact a2 is connected with the transmitting terminal Tx of the transmitting and receiving part 12, and the second pole common c2 is connected with a downstream side loop circuit 18.