PURPOSE: To obtain the accuracy of integration that is not affected by the characteristics of a switch and the accuracy of a feedback resistance, by using no pulse waveform for feedback but using a capacitor and feeding back the electric quantity charged at the capacitor.

CONSTITUTION: An analog input signal E is integrated by an analog integrating circuit consisting of a resistance 21, a capacitor 23 and an amplifier 22. The output of the integrating circuit is converted into a pulse train of the positive or negative direction through a plus/minus deciding circuit 24. A charge/discharge timing control circuit 25 controls an FET switch 26 to charge a capacitor 27 up to the -V or +V voltage by the positive or negative pulse. When the charging of the capacitor 27 is completed, the electric quantity stored at the capacitor 27 is fed back to the amplifier 22 through a feedback resistance 28. For this purpose, the circuit 25 controls the switch 26.