To solve the problem of a conventional ultrasonic flowmeter, wherein reflected waves interfere from piezoelectric body oscillator 2 or 3 through boundary surface between the oscillator and a back material or the end surface of the back material and cause malfunctions, and accurate flow rate of measurement liquid cannot be measured.

A U-shaped flow channel 7 is formed in a measurement tube case 6, piezoelectric ceramics 12 of a laminated structure where internal electrodes 9 and 10 and a backing material 11 are disposed on both sides of the oscillator 8 is mounted on one of the positions facing via a linear channel 7a of the U-shaped flow channel 7, and a piezoelectric ceramics 17 of a laminated structure, where internal electrodes 14 and 15 and a backing material 16 are disposed on both sides of the oscillator 13 is facedly mounted on the piezoelectric ceramics 12.