To provide an amplifier circuit which operates stably with respect to a wide-range input common voltage and has low current consumption, and to provided an A/D converter.

The amplifier circuit PA100 operates between a first power source VDD and a second power source GND. The amplifier circuit includes: a couple of transistors N1, N2 in which input signals VinP, VinN are input to the control terminals thereof; a couple of load resistors P1, P2 provided between the transistor N1, N2 and the first power source; a constant current source CS provided between the second power source and the couple of transistors N1, N2; and a first switch N3 connected in series with the constant current source CS between the second power source and the couple of transistors N1, N2 and in which ON/OFF is controlled in accordance with a clock signal.