PURPOSE: To make the precision of measurement constant automtically even when the pulse width of a gate signal changes, by computing the pulse width of the gate signal based on a counted value of the number of gate signals and a counted value of clock signals, and thereby controlling a time of measurement automatically.

CONSTITUTION: Gate 12 and 21 are controlled by a gate signal from a gate signal generating circuit 16 and the number of input signals and the number of clock pulses are counted by counting circuits 13 and 22, respectively. Counted values thus obtained are divided by an arithmetic unit 28 and thereby an average frequency is decided. To this arithmetic unit 28 is also impressed a counted value of the number of generation of gate signals, and, through the division thereof with the counted value from the circuit 22, the width of a gate pulse is calculated. Then, a cycloe of measurement corresponding to the precision of measurement set in the arithmetic unit 28 is computed on the basis of the pulse width, a gate 15 is opened and closed through the intermediary of a control circuit 26, and thereby a time of measurement is controlled automatically in accordance with the precision of measurement. As the result, the precision of measurement is made to be constant according to the set precision even when the width of the gate pulse changes, and thus the measurement of the average frequency is performed.