PURPOSE: To enable to measure accurately pulse density over all speed range by extracting a pulse having interval near predetermined interval from pulse train to be measured and using pulse repetition number of the pulse to be measured within the interval and that of the interval.

CONSTITUTION: The frequency of first clock pulse PC1 is divided to form second clock pulse PC2 to extract the pulses PV1, PV3 of the inrerval near the pulse PC2 of the pulse train PV to be measured and immediately before that. The intervals of these two pulses are counted by the pulse PC1 to obtain the counted value NCO1 and the pulse repetition number NV11 of the pulse to be measured within the pulses PV1, PV3. Thus, the pulse density NV=NV1/NC0 is obtained. Thus, the accuracy becomes =±1/NC0. When the frequency division ratio of the pulse PC2 is set to 1/m, it becomes S1/m. Accordingly, it can measure accurately the pulse density over the all speed range.