An input waveguide (23) and an output waveguide (24) are provided on a two-dimensional photonic crystal comprising cyclically-disposed vacancies (22), and two spot-like defects (25, 26) deficlient in vacancies (22) are provided between the both waveguides, thereby preventing a decrease in wavelength-error-caused efficiency and cross talk with other wavelengths in a two-dimensional photonic crystal slab multiplexer/demultiplexer.