PURPOSE: To improve the accuracy of measurement without lowering the speed of response by detecting pulse frequency from the number of the pulses to be measured of a sampling period and the time intervals of the samples to be measured and sampling pulses.

CONSTITUTION: The reference clocks P from a pulse generator 1 are converted to sample pulses by a frequency dividing counter 3, and these pulses are counted with a counter 4. On the other hand, the pulses to be measured from a pulse generator 2 are counted by a counter 5, and are applied as reset pulses to the counter 4. The count values of the counters 4, 5 are latched in buffer counters 7, 9, 10, and an arithmetic unit 8 makes operation by using the number N of the pulses to be measured, the time T2 from the pulse to be measured just before until the sampling pulse and the time T'2 from the pulse to be measured just before at the previous sampling until the sampling pulse, and outputs a pulse frequency F. Thereby, the accuracy of measurement is improved.