An optical multiplexer/demultiplexer comprises one light guide main line which guides lights of a plurality of wavelengths, and two or more microring optical resonators directly optically coupled to the one light guide main line. The two or more microring optical resonators are arranged in any two of layers higher than, level with, and lower than a plane where the one light guide main line is disposed, and/or the two or more microring optical resonators are arranged on both sides of the one light guide main line in a light guide direction.