PURPOSE: To incorporate a restored transmitter into an operating loop even if a loop monitor is faulty by providing a flag discrimination section discriminating the operating state of the loop monitor.

CONSTITUTION: While the loop monitor 20 is operated normally, an area of a flag FLG is checked by a flag revision section 130 in the reception of a token packet. When the monitor 20 is not operated normally, the received token packet is sent as it is. If the monitor 20 in this state and a transmitter 113 disconnected for the maintenance is brought into the ready state, a restoration signal is sent to transmitters 112, 114 and the restoration of the transmitter 113 is informed to the transmitters 112, 114. The transmitter 112 recognizing the restoration checks a token packet for a prescribed period and uses a flag discrimination section 140 as to whether or not the monitor 20 is operated normally. When not normally operated, loopback release is applied and the transmitter is incorporated into the operating loop.