Provided is an ultrasonic flow switch which can be stably mounted on pipe. A clamp part is mounted on an outer surface of the pipe. A casing part which integrally or independently holds the first and second ultrasonic wave elements is fixed to the clamp part by a sensor fixing screw. The clamp part is configured to restrict displacement of the casing part in an axial direction of the pipe and in a circumferential direction of the pipe and to allow displacement of the casing part in a radial direction of the pipe in a state where the casing part is not fixed.