To provide a demodulator capable of suppressing deterioration in an extinction ratio while suppressing a large size of a device.

The demodulator includes: a branching means branching a differential phase modulated optical signal into first and second branched light beams and emitting the first and second branched light beams to first and second optical paths; a first dielectric disposed on the first optical path and having a refractive index higher than an average refractive index of the second optical path; and a multiplexing interference means multiplexing the first branched light beam which has passed via the first optical path and the second branched light beam which has passed via the second optical path and making the light beams interfere with each other. The difference between lengths of the first and second optical paths and the refractive index of the first dielectric are set so that the first branched light beam which has passed via the first optical path may be delayed by 1 bit to the second branched light beam which has passed via the second optical path.

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