Abstract: An apparatus for ultrasonically determining fluid flow passing through a measurement section includes a tube which has two ends and an average predetermined inner cross-sectional area; the tube constitutes the measurement section. A sound-emitting transducer is disposed near one end of the tube, which has a first sound-transfer surface, and a sound-receiving transducer disposed near the other end of the tube, which has a second sound transfer surface. Each sound transfer surface is disposed at a sufficiently large predetermined distance from a corresponding end of the tube, so as to permit the fluid flowing through the tube. A fluid-receiving chamber communicates with the one end of the tube, and has a first average cross-sectional area, and a fluid-discharge chamber communicates with the other end of the tube, and has a second average cross-sectional area.