The pad interface circuit includes a first stack MOS transistor having a first terminal connected to a pad and a bulk connected to a first supply voltage; a second stack MOS transistor having a first terminal connected to a second terminal of the first stack MOS transistor and a second terminal, a gate terminal, and a bulk that are connected to the first supply voltage; and a voltage level sensing circuit generating a feedback voltage by using a pad voltage applied from the pad. In addition, the feedback voltage is applied to a gate terminal of the first stack MOS transistor.