It is the objects of the invention to increase the degree of freedom of channel selecting and to decrease kinds of elemental hardwares. In a transmitter, the wavelengths of lightwaves of respective channels are different from each other by the aids of wavelength control units, and channel IDs are inserted as the overhead signals of respective channels by channel marker generating units, before the electrical signals are converted into lightwave signals. In the receiver, in order to obtain a desired channel from WDM channels, wavelength tunable filters are swept by filter control circuit, and filters, which receives lightwave signals, are found. Then, the channel ID is extracted from the overhead signal of the transmission frame in the state of an electrical signal. The channel ID thus obtained is compared with an exact and desired ID by a channel ID comparison unit. If the two aforementioned IDs do not coincide, the above mentioned operations, involving sweeps of the wavelength tunable filters, are repeated.