PURPOSE: To make it possible to attain accurate detection with a short measurement time by detecting with high precision how much a difference in phase between an ultrasonic transmitted signal emitted from a transmitting element and a received signal received by a receiving element varies according to the velocity of a fluid.

CONSTITUTION: A signal output of frequency f1, generated by oscillation circuit 10, is sent out by signal circuit 12, but stepdown circuit 14 converts the output signal into a signal of frequency f2 and mixer circuit 16 generates output signals of frequencies f1+f2 and f1-f2. Tuner/filter circuit 18 extracts and shapes 20 signal f1-f2. Further, the output of receiving element 8 is amplified 22 and mixed by mixer circuit 24 into a signal from circuit 14 to obtain signals of f1+f2 and f1-f2, and tuner/filter circuit 26 extracts and shapes 28 the signal of f1-f2. Output signals from circuits 20 and 28 are supplied to time-difference comparator circuit 30 to measure a difference in time, and pulses from clock frequency f3 from oscillation circuit 32 are passed through gate circuit 34 opening and shutting corresponding to the output of circuit 32 to calculate the quantity of fluid 4 on the basis of the output of circuit 30, and also to make a decision on an error measurement, etc., by processing circuit 36, thereby displaying the quantity of flow, etc., by display unit 38.