PURPOSE: To achieve a higher response characteristic with a widened dynamic range, by a method wherein the cycle of an input signal is counted by a clock and a voltage inversely proportional to the cycle is generated to reduce ripples at a low frequency area.

CONSTITUTION: An input signal 7 is inputted into a monostable multi-vibrator 1 to produce an edge pulse 8. The cycle of the edge pulse 8 is counted with a counter 2 and a reference clock 9 to be stored into a register 3. A value inversely proportional to an output of the register 3 is outputted with a computing circuit 4 to be converted into analog from digital with a D/A conversion circuit 5. This reduces variations due to dispersion in characteristic of a circuit element with less ripples at a low frequency area. This also widens a dynamic range and provides a better response characteristic with respect to variations in the frequency.