In an A/D converter, each preamp 102 includes a preamp gain adjusting circuit 109. The preamp gain adjusting circuit 109 suppresses the gain of the preamp 102 and restricts a positive-negative output potential difference of the preamp only when the positive-negative output potential difference of the preamp 102 exceeds a reference potential. Accordingly, in the case where the frequency of an input signal to the A/D converter is high, even when the gain of the preamp is increased due to fabrication process variation, temperature variation or supply voltage variation, output strain of the preamp is minimally caused, and the characteristic degradation of the A/D converter can be suppressed.