on the other hand the spare transponder 108 connected to the network device 102 through the optical coupler 113 in the light diverging switch 104 is also transferred to the initial state by receiving the signal output from the network device 102 in the initial state of auto negotiation next the active transponder 107 carries out the auto negotiation by communicating with the network device 102 step 3 b and establishes the link in the process the spare transponder 108 receives the optical signal output from the network device 102 during the auto negotiation and by returning a signal as a response to the optical signal behaves in the same manner as if the auto negotiation is carried out with the network device 102 in harmony with the auto negotiation between the active transponder 107 and the network device 102 step 3 b as a result the spare transponder 108 establishes the link with the network device 102 in an auxiliary manner the active transponder 107 after establishing the link with the network device 102 transfers the link establishment information to the active transponder 105 of the station a using the second maintenance signal step 4 b the data pattern of the second maintenance signal can be discriminated from that of the first maintenance signal by using a data pattern different from that of the first maintenance signal for transfer of the link disconnection information described above the active transponder 105 that has received the link establishment information stops the transfer of the first maintenance signal and outputs the optical signal from the network device 101.