A wavelength multiplexer/demultiplexer used to separate two wavelength bands disposed with the end of one wavelength band separated up to a specified distance from the end of another wavelength band. A dielectric multilayer film is provided at the intersection of two mutually-crossing optical waveguides to separate an incident light to the dielectric multilayer film into a transmission light and a reflection light. Note that the distance X from the multilayer film surface on the light incidence side of the dielectric multilayer film up to the center intersection of the above two crossing optical waveguides is set so as to satisfy 0