PURPOSE: To attain high accurate measurement of an analog quantity and high speed waveform analysis by counting an output of a V/F converter sequentially for a prescribed time at plural counters and outputting the result of a count of a counter whose count is finished as a digital value while other counter makes counting.

CONSTITUTION: When an output signal from a V/F converter 10 is counted for a prescribed time by a counter 13, other counter 14 starts count for a prescribed time. The result of count is switchingly outputted as a digital value corresponding to an analog signal in the order from the counter whose count is finished. Thus, desired accuracy of digital conversion is implemented even at a short sampling interval without use of a high speed counter especially and high accurate measurement of an analog quantity and high speed waveform analysis are attained.