To improve a communication quality by suppressing the effect of cross talk caused by end face reflected light particular for single-fiber bidirectional optical wavelength multiplex transmission by applying tone signal light in wavelength multiplex transmission.

The system comprises a tone signal generating part for generating tone signal light of which a frequency band is different from that of each of wavelength multiplex signal light and of which a wavelength is different from tone signal light of a party, a coupler for coupling the generated tone signal light with present wavelength multiplex signal light as signal light that a present side transmits, and a tone signal detecting part for separating a part of the opposite signal light which is extracted from a directional coupler, and detecting only the wavelength of the opposite tone signal light.