An optical multiplexer/demultiplexer is provided with a first member, a second member and an optical filter. The first member is a planar waveguide and is formed with an optical waveguide and another optical waveguide. The second member is a planar waveguide and is formed with an optical waveguide and another optical waveguide. The optical filter is a dielectric multi-layered filter and is sandwiched between the end face of the first member and the end face of the second member.