In a flash A/D converter including a plurality of differential amplifier circuits and a plurality of voltage comparator circuits, a regulator circuit is provided. The regulator circuit automatically regulates a bias voltage of each of the plurality of differential amplifier circuits in a differential amplifier circuit array to make an output dynamic range for the differential amplifier circuits match an input dynamic range for the plurality of voltage comparator circuits. Therefore, even if the input dynamic range for the voltage comparator circuits is narrowed with reduction in a power supply voltage, the output dynamic range for the differential amplifier circuits and the input dynamic range for the voltage comparator circuits match, thus resulting in a high A/D conversion accuracy.