Deterioration of optical signal characteristics is suppressed even when the noise level of an optical signal to be delivered to an opposite station is high. An optical multiplexer/demultiplexer (20) according to an example embodiment includes: an OCM (1) configured to measure a strength of each of optical signals in a plurality of wavelength bands input to a WSS (2) and to determine an optical signal wavelength band and a noise wavelength band based on the measured strengths; the OCM (1) configured to pass the optical signal in the optical signal wavelength band determined by the OCM (1) as a primary signal; a dummy light generation unit (3) configured to generate dummy light in which the optical signal wavelength band has been extinguished; and an optical coupler (4) configured to multiplex the primary signal output from the WSS (2) with the dummy signal into a wavelength division multiplexing optical signal and to output the wavelength division multiplexing optical signal to an optical transmission path.
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