PURPOSE: To obtain an A/D conversion output which is not affected by temperature drift, etc., by sampling offset to the A/D converter periodically to deduct it from a normal input and correcting the offset at all times.

CONSTITUTION: In the input part of an A/D converter 1, a selecting signal output means 10, switches 4 and 6 which switch input voltages to the A/D converter 1 to a voltage V to be converted and a voltage 0 by a selecting signal SEL outputted from this means 10, and an inverter 5 are provided. Also, a correcting arithmetic means 11 composed of a microcomputer, etc., is provided in the output part of the converter 1. When the selecting signal SEL is 'L', the switches 4 and 6 are turned on and off respectively, and when the selecting signal is 'H', they are turned off and on inversely. When a conversion output value Vout after correction is calculated by conversion outputs Vout 1 and Vout 2 of the converter 1 respectively, the offset of the A/D converter 1 can be removed.