An optical multiplexer/demultiplexer having a high isolation characteristic, a small polarization dependency and capable of being reduced in size is disclosed. An end portion of a core (15) and an end portion of a core (16) are arranged in opposed relation to each other with a thin-film filter (19) interposed therebetween. The end portion of the core (15) far from the filter and the end portion of the core (16) far from the filter are arranged in substantially parallel to each other. The core (17) is arranged in such a manner as to branch from the filter-side end portion of the core (15) and to be optically coupled with the core (15). The end portion of the core (15) near to the filter and the end portion of the core (16) near to the filter are curved. The end portion of the core (15) near to the filter is curved towards the core (17). The end portion of each of the core (15) and the core (16) far from the filter reaches an end of an optical waveguide substrate, and the end portion of the core (17) far from the filter reaches a side of the optical waveguide substrate.