PURPOSE: To report fault information by looping back fault information of a subordinate ring line at a tributary station, allowing a fault occurrence station to cut off an order wire line and allowing a master station to release the cut of order wire line.

CONSTITUTION: An overhead byte inputted from a west side line WL is separated in a separate section 1W, in which a signal and monitor information of an order wire line are extracted. An overhead byte inputted from an east side line EL is separated in a separate section 1E, in which a signal and monitor information of an order wire line are extracted. The digital signal of the order line is D/A-converted by a D/A converter section 2W and sent to a host ring line L1 through an adder section A. On the other hand, the monitor information is looped back by a loopback section 3W and multiplexed on a digital signal of the order wire line at a multiplexer section 5W and sent to the west line WL. Thus, even when line fault takes place, the monitor information is reported to a master station of the lower rank ring line L2.