An ultrasonic flowmeter has oscillators 1, 2, a transmitter 31 for transmitting burst waves BURST, amplifiers 34, 35 for amplifying a received signal, a first comparator 36 for checking whether the crest of the amplified signal falls within a predetermined range, a second comparator 37 for checking whether the amplified signal exceeds a predetermined threshold, a third comparator 38 for detecting zero-cross points in the amplified signal, and a controller 40 including a time counter for measuring the propagation time from when the burst waves are transmitted until when the amplified signal reaches a zero-cross point. The time counter has a low-speed clock, a mid-speed clock whose frequency is calibrated with the low-speed clock and whose propagation time is shorter than that of the low-speed clock, and a high-speed clock whose frequency is calibrated with the mid-speed clock and whose propagation time is shorter than that of the mid-speed clock.