PURPOSE: To automatically and permanently correct the offset voltage by securing the difference between the digital data obtained by converting the analog signal and the data stored in a storage circuit respectively.

CONSTITUTION: An analog multiplexer uses the analog signal sources of the channels CH1-CHm as the input and selectively outputs one of these channels. Then the reference voltage of a reference voltage generating circuit 5 is inputted to a channel CH1 and held by a sample and hold circuit 2. Then the reference voltage is converted into a 3-bit signal by an A/D converting circuit 3 and stored in a storage circuit 6. A subtractor circuit 7 calculates the difference between the data stored in the circuit 6 and the reference voltage based on the value obtained by applying the A/D conversion to the analog input signals of the channels CH2-CHm. In such a way, the offset voltage can be corrected.