In an optical multiplexing/demultiplexing method of the present disclosure: a polished surface of an optical fiber core wire having side surfaces that have been polished up to a core or the vicinity of the core, and polished surfaces of a plurality of optical waveguides that are lined up in parallel in a longitudinal plane, have different propagation constants, and have side surfaces that have been polished up to the core or the vicinity of the core are surface-matched; and the polished surface of the optical fiber core wire and the polished surfaces of the plurality of optical waveguides move relative to each other, and are located at a position, in which a desired branching ratio can be obtained, from one end of the optical fiber core wire to the other end of any one optical waveguide of the plurality of optical waveguides.
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