PURPOSE: To secure a sufficiently high S/N by reducing circulate acoustic waves, especially, surface waves.

CONSTITUTION: The title flowmeter measures the flow velocity or flow rate of a fluid from the difference between the propagation time of acoustic waves from an ultrasonic vibrator 1 on the upstream side to another ultrasonic vibrator 2 on the downstream side and the propagation time of acoustic waves in the opposite direction. In order to scatter surface waves and obtain an S/N higher than a fixed level, a sound absorption material 10 composed of a gel is put on the outer periphery of a pipeline 4 so as to allow circulate acoustic waves to leak.