Provided is an ultrasonic flow switch which can be stably mounted on pipes. A casing part which integrally or independently holds the ultrasonic wave elements is mounted on an outer surface of the pipe by a clamp part. An acoustic couplant which is formed of a soft elastic body is pressed to an outer surface of the pipe such that at least one of the first and second ultrasonic wave elements is acoustically coupled to the pipe. A collapsing amount of the acoustic couplant at a portion where the collapsing amount of the acoustic couplant caused by the pipe is the largest is restricted by a collapsing amount restricting part.