PURPOSE: To make the converting accuracy equal to each other in positive and negative signals, by integrating an input analog signal or that of opposite polarity with a circuit element using in common for both polarities, in an integration circuit.

CONSTITUTION: An input switching circuit 1 consists of changeover switches 2, 3 and switch drivers 4, 5, and the input analog signal Ex is inputted to a terminal t1 and a signal -Ex of opposite polarity as the Ex is inputted to a terminal t2. The integration circuit 6 comprises a resistor R, a capacitor C, an operational amplifier 7, a reset switch 8 and a reset switch driver 9, and they are used in common to both polarities. A comparator circuit 10 consists of two comparators 11, 12 and reference power supplies 13, 14 having a reference voltage Es. Terminals 15, 16 of the comparator circuit 10 are connected to an RS FF19, an OR gate 20 and a polarity display circuit 21 via waveform shaping buffers 17, 18. A gate 20 is connected to a T-FF 23 via a single pulse generating circuit 22.