Optical add/drop nodes are used in a network having a pair of optical transmission paths for transmitting optical signals in opposite directions to each other. Each add/drop node comprises a variable split ratio optical coupler for splitting an optical signal output from a transmitter. The split ratio of the variable split ratio optical coupler is set such that the optical power levels of the signals added through the respective optical add/drop nodes are equal to one another respectively on the pair of optical transmission paths.