PROBLEM TO BE SOLVED: To provide a small-sized Mach-Zehnder interferometer. SOLUTION: In an optical interferometer 19 having a pair of optical waveguides 21 and 22 extended between a pair of optical coupler areas, the optical waveguides 21 and 22 have optical paths different by length between optical coupler areas which give a maximum transmission volume and a minimum transmission volume, and each of the pair of optical waveguides has a reflector part C which optically couples first and second segment parts A and B.