To provide the small-sized A/D converter that is simply manufactured where the gain is quickly and accurately changed.

Plural switched capacitor amplifier stages 21a,... have a selectable gain respectively and the entire gain an analog input signal is controlled at a pre-stage of the A/D converter 22. The A/D converter circuit 20 has an integrated circuit board 29 on which the A/D converter 22 and plural switched capacitor amplifier stages 21a,... are formed, and the A/D converter 22 is a monolithic integrated circuit. Furthermore, each gain of plural switched capacitor amplifier stages 21a,... is controlled based on a digital gain control word. A clock circuit 26 is connected to the A/D converter 22 and plural switched capacitor amplifier stages 21a,....