To prevent the numbers of necessary optical path and wavelength from being increased without increasing the load of switching processing in an electric path switch.

The traffic characteristics of virtual passes 140 to 142 for passing are monitored, and when the characteristics exceed a previously set threshold, a network node 120 informs a network operation system 170 of the state, the system 170 sets up an optical path 220 between prescribed network nodes 100, 130 and the virtual passes 141, 142 to be multiplexed with poetical paths 160 to 162 previously set in each transmission line are transferred through the path 220. When the traffic characteristics of the passes 141, 142 are less that the threshold, the passes 141, 142 transferred through the path 220 are switched to the paths 160 to 162.