PURPOSE: To automatically correct a measurement error due to the fluctuation of an internal oscillator caused by the change in temperature, by sequentially compensating the measured value of a main counter by an error reference signal pikced out from an identical signal source.

CONSTITUTION: Two kinds of frequency signals are generated by a crystal resonator 12 and supplied to an oscillator 14. The oscillator produces a comparatively stable signal f1 and a signal f' resulting from the combination of both the kinds of frequency signals. The signal f1 is applied as a time base to a main counter 16. The signal f' is applied to a reference counter 18. The frequency of an input to a terminal 30 is measured by the counter 16. The measured value is applied to a CPU 20. An error reference signal measured by the counter 18 is also applied to the CPU 20. The CPU uses the error reference signal to read a compensation coefficient which is previously stored in a memory 24 in a correction mode. The CPU corrects an error in the measured value due to the fluctuation in the signal f1 and causes a unit 26 to indicate the result of the measurement.