PURPOSE: To perform A/D conversion for an average value with an input greater in impulsive input and ripple component, by making a returned integration reference voltage to convert the difference of input integration values into a digital value, proportional to the integration time.

CONSTITUTION: When an input signal is inputted, ripple components are wave- shaped at a ripple detection circuit 3, an input integration timing a multiple of the period of this signal is formed, and an analog switch 10 is closed with this timing. During this switch is closed, the input is integrated and given to a negative input terminal of a comparator 8. When the output of the integration circuit is negative, the output of the comparator 8 goes to high level and a returned timing signal of the integration circuit is outputted from a logical product circuit 13. A clock is inputted to a counter 5 from a logical product circuit 12 during the returned integration period and the returned integration time is counted. Since the input integration value is proportional to the average value of the input and the returned time is proportional to the average value of the input signal, the average value of the input signal is digital-converted with the counter 5.