PURPOSE: To prevent the entire system from being into the state of system-down by separating a faulted remote device from a digital line if a fault occurs in the remote device in a loop communication system to limit the range of the affect due to the fault.

CONSTITUTION: Information is transmitted from a preceding RE to a succeeding RE in the path of L3i-reception side digital line interface RIF200-dropper/inserter D/I203-transmission side digital line interface SIF206-relay contact rl2-L3j in each remote device RE normally. As being well known, each channel of a digital line has a signal bit, by which each corresponding digital line is controlled and information required for state supervision is transmitted. Since a relay RL1 is operated and a normally ON relay RL2 is restored by the relay contact rl1 at fault, a relay contact rl2 is selected in solid lines in the figure, and a circuit of L3i-relay contact rl2-L3j is formed. That is, REj 2j is separated from the digital line and the signal from the L3i is transmitted to the L3j through the REj 2j.