A fluid detection sensor , comprising: an optical sensor comprising a substrate , a light emitting element array comprising a plurality of light emitting elements which are arranged in a row along a longitudinal direction of the substrate on an upper surface of the substrate and a light receiving element array comprising a plurality of light receiving elements which are arranged in a row along the longitudinal direction of the substrate corresponding to each of the light emitting elements , on the upper surface of the substrate , the light emitting element array and the light receiving element array being arranged on the substrate along the longitudinal direction of the substrate ; a flow path member comprising a tubular body which is transparent and is arranged along the longitudinal direction facing the optical sensor , an inside of the tubular body constituting a flow path configured for a plurality of fluids which are substantially immiscible to flow therethrough as a slug flow ; and a reflecting member placed on a side opposite to the optical sensor with respect to the flow path member and away from the flow path member , wherein the flow path member is arranged so that a center position of the flow path is offset from a center of arrangement of the light emitting element array and the light receiving element array on a light receiving element array side, or the flow path member is arranged so that a center position of the flow path is offset from a center of arrangement of the light emitting element array and the light receiving element array on a light emitting element array side, and wherein the fluid detection sensor is configured for detecting changes in light which occur in accordance with a movement of the plurality of fluids by the light receiving element array receiving light which is emitted by the light emitting element array toward the slug flow in the flow path and is reflected by the reflecting member . an optical sensor comprising a substrate , a light emitting element array comprising a plurality of light emitting elements which are arranged in a row along a longitudinal direction of the substrate on an upper surface of the substrate and a light receiving element array comprising a plurality of light receiving elements which are arranged in a row along the longitudinal direction of the substrate corresponding to each of the light emitting elements , on the upper surface of the substrate , the light emitting element array and the light receiving element array being arranged on the substrate along the longitudinal direction of the substrate ; a flow path member comprising a tubular body which is transparent and is arranged along the longitudinal direction facing the optical sensor , an inside of the tubular body constituting a flow path configured for a plurality of fluids which are substantially immiscible to flow therethrough as a slug flow ; and a reflecting member placed on a side opposite to the optical sensor with respect to the flow path member and away from the flow path member , wherein the flow path member is arranged so that a center position of the flow path is offset from a center of arrangement of the light emitting element array and the light receiving element array on a light receiving element array side, or the flow path member is arranged so that a center position of the flow path is offset from a center of arrangement of the light emitting element array and the light receiving element array on a light emitting element array side, and wherein the fluid detection sensor is configured for detecting changes in light which occur in accordance with a movement of the plurality of fluids by the light receiving element array receiving light which is emitted by the light emitting element array toward the slug flow in the flow path and is reflected by the reflecting member . <EOS>