To provide an ultrasonic flowmeter, capable of measuring a flow rate with high accuracy, as well as, with high efficiency by a transmitting-receiving vibrator, and of being consitutted, simply by shortening the approach section in front of the transmitting-receiving vibrator.

Two board-like partition walls 40 are arranged along a flow direction in a linear inertmediate channel 21a, and an opening width W, which is a long side L of the linear middle channel 21a, is divided in the width direction, to form three-layered divided channels 21a' having each equal opening width W'. One circular communication hole 41 is formed, penetrating in the width direction corresponding to a measuring line of an ultrasonic beam on each partition wall 40, and each thin plate-shaped or sheet-shaped adjustment net 42, constituted of a reticulated material, is overlaid on each communication hole 41.