A small variable demultiplexer capable of improving yield and stabilizing characteristics can be obtained. The small variable demultiplexer includes an SiO.sub.2 layer serving as a base medium, a waveguide array, a heater serving as an external refractive index modulating unit and a heat sink serving as the external refractive index modulating unit. The waveguide array is arranged on the SiO.sub.2 layer. The waveguide array is formed of the plurality of Si photonic wire waveguides serving as waveguides. The heater and the heat sink provide a refractive index gradient which is formed in the direction perpendicular to the extension direction of the Si photonic wire waveguide, and is variable depending on the plurality of Si photonic wire waveguides. The Si photonic wire waveguide has a one-dimensional photonic crystal structure in the extension direction of the Si photonic wire waveguide.