To provide a low-crosstalk optical multiplexer/demultiplexer which is decreased in crosstalks by eliminating the coupling between waveguides.

This optical multiplexer/demultiplexer is formed with an array waveguide group 5 in which a plurality of the waveguides exist side by side between the sectorial slab waveguide 3 disposed on an input waveguide side and the sectorial slab waveguide 4 disposed on an output waveguide side. Clads 7 which part the respective waveguides of the array waveguide group 5 from each other are composed of materials containing elements to absorb the light of a signal light wavelength band.