PURPOSE: To ensure a voice talking between stations and to improve the network operating and maintenance efficiency by selecting an order wire line when a fault takes place in an optical transmission line between stations.

CONSTITUTION: When a fault takes place in a transmission line in a route other than a transmission line set to a non-use state, a switch section 170 provided to a transmitter-receiver connecting to both ends of the transmission line having a fault is used to set the system such that the transmission line having the fault is not in use. Simultaneously, the switch section 170 provided to transmitters-receivers 100-1, 100-n connecting to both ends of the transmission line set to the inoperating state sets the transmission line set in the non- operating state. As a result, the voice talking between stations is ensured by selecting the transmission line when a fault takes place in the transmission line between stations.