PURPOSE: To allow the system to cope with a path fault of an optical signal even in the case of high speed optical fiber transmission by sending a signal through a standby line in an opposite direction to a path of the optical signal after the standby line is selected on the occurrence of a fault to the signal path.

CONSTITUTION: When traffic is not sent through all active and standby optical fibers 30, 31, 30, 41 between optical fiber network devices NE2, NE3 due to a lien broken fault 5, a standby optical fiber sending an optical signal in the opposite direction is looped back. That is, the traffic from an clockwise optical receiver/optical transmitter 10 of the NE 2 is looped back and the signal is given to the NE 3 via NEs 1, 4 through a counter clockwise standby fiber 41. The fiber 41 is branched at the input of the counter clockwise optical receiver/optical transmitter 20 in the NE 3 and connected to the input of the clockwise optical receiver/optical transmitter 10. Thus, the traffic is sent clockwise from the NE 2 to the NE 3. Furthermore, the traffic of the transmitter- receiver 20 of the NE 3 enters the NE 1.