To provide a method of accurately and rapidly determining a maximum value of a correlation which is an accurate time difference equivalent to a flow velocity by comparatively simple signal processing.

In the ultrasonic flow meter, a discrete value is determined by subjecting an upstream side reception signal and a downstream side reception signal to an analog-to-digital conversion, an interrelationship between the upstream side signal and the downstream side signal is determined, Hilbert transformation is performed on a calculation result determined by a correlation calculating means, a phase relationship is determined from the calculation result determined by the correlation calculating means and a calculation result determined by the Hilbert transformation, and the time difference is calculated via a maximum value detecting means from a calculation result of the phase relationship determined by a phase calculating means.