PURPOSE: To enable a highly accurate measurement of frequency regardless of the continuation time by repeatedly reading an input signal of a relatively low frequency with a short continuation time in terms of the sampling period.

CONSTITUTION: When an input signal is inputted into a counting control circuit 1, an input gate circuit 2 is opened for a certain time and allows the input signal to be sent to a digital memory 5 and a counter 6 through an A/D converter 3. The counter 6 adds by 1 at each inversion of the digitalized signal from positive to negative and produces an output by multiplying the measured value M times. The memory 5 repeatedly reads digitalized signals until the counts of a counter 7 become equal to an output value of the counter 6. On the other hand, while the counts of the counter 7 is not 0, a gate circuit 10 opens and allows a reading clock signal to be sent to a cycle counter 11 and closes as soon as the output of a gate control circuit 9 becomes the logic value 1. An arithmetic circuit 12 calculates the frequency of a input signal from counts of the counter 7 and the cycle counter 11.