PURPOSE: To simplify the titled device in constitution and to reduce the circuit in size by counting up the number of bits corresponding to the value of a polarity bit in an A/D converted digital data, and switching the gain of a gain switching amplifier in accordance with the counted result.

CONSTITUTION: The A/D converter 11 having a fixed input range in a main part has a certain fixed full scale range, and if a convertion command is applied, outputs converted digital data consisting of n bits (n>2) arranged in parallel or series after a fixed period. The gain of the gain switching amplifier GA1 is set up to 1, 2, 4, 8, and 16 times and the gain of the amplifier GA1 is switched in accordance with the counted value of a counter, i.e. the contents of a counter UDC3. In the range switching process, there is an input range selecting period A/D converting an input data with a prescribed gain, "1" e.g., and selecting the input range in accordance with the converted value and the gain of the amplifier GA1 is set up in accordance with the determined input range and A/D converted. The gain of the amplifier GA1 is corrected in accordance with the converted value, then A/D converted again.