PURPOSE: To ensure a highly accurate drift compensation and to prevent the generation of drift error and a scale-out of data, by preforming a drift compansation with which the drift component is subtracted from an analog signal by an A/D converter and another drift compensation which uses an operation of an A/D-converted data.

CONSTITUTION: One of plural analong signal is selected by an analog data multiplexer M1, and and an optional analog signal is amplified by an amplifier A1. This amplifier signal is fed to a drift compensating differential amplifier A2, and the drift component set to a D/A converter DA1 is subtracted. Then the drift-compensated analog signal is converted into a digital signal through an A/D converter AD1 and then applied to a control part C2. The drift is compensated for the digital signal with an operation carried out at the part C2. At the same time, the entire device such as the multiplexer M1, the converter DA1, etc. is controlled by the part C2. Thus the drift is compensated with high accuracy, and both drift error and scale-out of data can be effectively prevented.