The present invention addresses the problem of implementing a monolithic integrated type of optical gate switch which can be reduced in size by integration and can avoid the problem of optical coupling loss of an interferometer optical circuit unit and a phase modulation unit. The optical gate switch of the present invention comprises: an optical waveguide wafer the core layer of which comprises quantum wells showing a phase modulation effect produced by transitions between sub-bands; a Michelson interferometer formed on the optical waveguide wafer; and a variable optical intensity attenuation unit which adjusts the optical balance of the interferometer in one arm on the reflection side of the Michelson interferometer.