The multiplexer/demultiplexer has directional couplers (5,6) on both sides of a phase shift range (9) with a difference Delta L in the optical path lengths of two waveguides (2,3). The arrangement is constructed such that the coupling ratio for the light intensity with a wavelength lambda2 is set to at least 50 per cent, wherein light in the wavelength band including a wavelength lambad1 and light in the band including a wavelength lambda2 is multiplexed and demultiplexed. Each directional coupler (5,6) is constructed such that the coupling ratio for light intensity of wavelength lambda2 is at least 50 per cent and that for light intensity with a wavelength lambda3 in a band close to that of wavelength lambda1 is 0 per cent or 100 per cent. Light in the band including wavelengths lambda1 and lambda3 and light in the band containing wavelength lambda2 are multiplexed and demultiplexed.
