PURPOSE: To prevent chattering from taking place by comparing an analog voltage with a voltage applying prescribed shift to a step voltage obtained from the voltage division of a reference voltage and outputting a data when both coded data are coincident.

CONSTITUTION: A changeover switch SW17 of a comparison voltage generating circuit 13 generating a step voltage through the voltage division of a reference voltage VR by a voltage division circuit 15 is thrown to the position A, a voltage VY of the circuit 15 is inputted to a comparator 12 and compared with an analog voltage Vi. A sequential approximation register SAR18 outputs a digital coded data DA corresponding to the voltage VY through the result of comparison and stores it to a temporary storage circuit 19 and a data latch circuit 22. Then the SW17 is thrown to the position B, a voltage Vy' shifting (16) the voltage VY by a prescribed amount is inputted to a comparator 12, where the data DB coded by the SAR18 and the data DA from the circuit 19 are compared (21) and when they are coincident, the data DA is outputted from the circuit 22. When they are dissident, the content of the circuit 19 is rewritten into the DB, which is compared (21) with the data obtained by throwing again the SW17 to the position A, and the coincident data is outputted via the circuit 22 and it is repeated.