PURPOSE: To enable the normal detection of the velocity and rate of flow by attenuating a noise component before said component arrives at an ultrasonic flowmeter.

CONSTITUTION: An ultrasonic wave is generated on account of fluid oscillation, fluid friction, etc. and propagates in a conduit 16 when the fluid to be measured in the conduit 16 is pressure reduced in a reducing valve 17 and is reduced down to a prescribed pressure. The ultrasonic wave passes the opening 4 of a noise attenuator 24 and is prevented from advancing straightforward as the wave is attenuated and shut off by an acoustical absorbent 8 pasted in a cover 3. The remaining ultrasonic energy passes a hole 7 and advances downstream while the energy is absorbed and attenuated by the acoustical absorbent 8 pasted on the inside surface of a cylindrical body 9; at the same time, the rate of attenuation is increased and the influence on a flowmeter 15 is thoroughly eliminated. Materials, such as nonwoven cloth, foamed resins or the like, which absorb and attenuate easily ultrasonic waves are used as the sound absorbing material.