PURPOSE: To subject a flow rate to ultrasonic measurement excellently by low-voltage excitation without being affected by the diameter of a pipeline and an extraneous substance mixed in, by measuring an oscillation frequency of PLL in the state wherein an ultrasonic propagation line in connected thereto, and thereby determining a flow speed.

CONSTITUTION: An ultrasonic propagation line 1 is connected to PLL 2, and oscillation frequencies f1a and f2a of PLL 2 in the first and second states wherein the states of transmission reception of transmitter receivers 6a and 6b are changed by a selector 7 at the time of the oscillating operation of PLL 2 so that the phases of an ultrasonic wave 17 passing through the propagation line 1 and an oscillation signal of a voltage-controlled oscillator 16 are in accord with each other are measured by a frequency measuring means 3. Moreover, the first and second oscillation frequencies f1b and f2b of PLL 2 on the occasion when the number of ultrasonic waves is increased by a known quantity, e.g. by one cycle, by a frequency shifting means 4 are measured. Then a flow speed V is determined on the basis of the formula, and a flow rate is measured. By this method wherein a time required for propagation of ultrasinic waves is not utilized, the flow rate is subjected excellently to the ultrasonic measurement by low-voltage excitation without being affected by the diameter of a pipeline and an extraneous substance mixed in. In the formula, D denotes the diameter of the pipeline, and θ the opposed angle of the transmitter-receivers.