PURPOSE: To execute an automatic measurement continuously extending over a long period with high accuracy and without executing an adjustment, irrespective of a state of a fluid, by setting each received ultrasonic signal to an almost constant crest by making it pass through an automatic crest regulator, and thereafter, supplying it to a pulse height discriminator.

CONSTITUTION: First of all, an output of a transmitter 4 is supplied to ultrasonic transmitter-receivers 2a, 2c by an operation of a switching circuit 3, an ultrasonic wave transmitted from the ultrasonic transmitter-receiver 2a is received by an ultrasonic transmitter-receiver 2b of a downstream side, and in the same way, an ultrasonic wave transmitted from the ultrasonic transmitter-receiver 2c is received by an ultrasonic transmitter-receiver 2d. Those received signals are amplified by a pre-amplifier in the switching circuit 3 and main amplifiers 5a, 5b of the post-stage, respectively, and inputted to automatic crest regulators 6a, 6b. The signal adjusted to an almost constant crest, irrespective of a received crest, in said regulator is inputted to pulse height discriminators 7a, 7b, compared with reference voltage, and when it exceeds the reference voltage, a square wave of a constant crest is outputted. Those square waves are inputted to a signal processing device 8, a shift of the timing of both the square waves is measured, and a velocity of flow and a flow rate are derived.