PURPOSE: To eliminate the need for a maintenance transmission line having been required for exclusive maintenance use by connecting plural stations in a ring by a 2-way transmission line.

CONSTITUTION: Each of stations 1-4 is provided with a line changeover control section 5 of a same configuration to all the stations and fault detection sections 20, 21 and the stations 1-4 are connected in a ring by two-way optical fiber transmission lines 6a, 6b. The fault detection sections 20, 21 are active in a fault state. The fault detection sections 20, 21 always detect a signal sent to the incoming side transmission line 6a or 6b and discriminates it that a fault takes place in the incoming side transmission line 6a or 6b when no signal is detected and output an alarm signal notifying the fault to the line changeover control section 5 of its own station and sends the signal to other stations.