Between photonic cross connects (PXCs) 3 and 4, an in-use system path passing through transmission apparatus (WDMs) 5 and 7 are set, and a backup system passing through transmission apparatus (WDMs) 6 and 8 is set. The WDMs 3 and 4 link to the PXCs 5 and 7 and release masking therebetween when the in-use system path is set, and set masking therebetween when the in-use system path is deleted. The WDMs 5 and 7 included in the in-use system path transmit an alarm to a monitoring system when an input light is turned off, and the WDMs 6 and 8 included in the backup system path do not transmit an unintended alarm to the monitoring system even when the input light is turned off.