The paper describes the principle of measuring digitally the flow velocity of local voids, and discusses the accuracy of this method. It is shown that the spectrum of local void velocities can be determined reliably with digital values. The measurement errors of void velocity ug (m/sec) are within about ±9% in cases when the probes have detector head lengths of 0.8~1.0mm, and head diameters of 0.1 mmφ, when the voids have a flow velocity in the range of 0.2 m/sec≤ug≤1.2 m/sec and a radius of curvature of void of 2mm≤R≤16mm.