The present invention is aimed at providing a two-dimensional photonic crystal wavelength multiplexer/demultiplexer capable of multiplexing and demultiplexing both TE and TM-polarized lights. In the wavelength multiplexer/demultiplexer, first and second resonators having the same resonance wavelength .lamda.r are provided between first and second waveguides and which are separately provided in a two-dimensional photonic crystal having a photonic band gap for the TE polarization. A first polarization converter for converting a TM-polarized light to a TE-polarized light is provided on the first waveguide 121 between the first and second resonators and. Similarly, a second polarization converter for converting a TE-polarized light to a TM-polarized light is provided on the second waveguide between the first and second resonators and. Among the light propagating through the first waveguide, the TE-polarized light with wavelength .lamda.r is introduced from the first resonator into the second waveguide, whereas the TM-polarized light is converted to a TE-polarized light by the first polarization converter and then introduced through the second resonator into the second waveguide.