Abstract: A clamp on type of an ultrasonic flow rate-measuring device attached to an outer wall and having an automatic pipe thickness measuring function is disclosed. The ultrasonic flow rate measuring-device includes an automatic pipe thickness measuring function. The ultrasonic flow rate-measuring device is fixed to the outer diameter of a pipe of which the material is known but the thickness is unknown. The ultrasonic flow rate-measuring device adopts a resonant frequency, which generates a maximum voltage level during frequency sweeping through the entire path.