An input buffer circuit achieving rail-to-rail operation maintains a uniform common mode output voltage even though an input signal having any voltage level is inputted. The input buffer circuit has a differential amplifier structure receiving two differential input signals. A first input part has a first inverter circuit into which a first differential input signal is inputted, and a second input part has a second inverter circuit into which the second differential input signal is inputted. The first inverter circuit has a first output node connected to a diode structure having an operating current twice the operating current of the first inverter circuit, and outputs a first output signal. Rail-to-rail operation is achieved, and a common mode output voltage is provided uniformly, with reduced current consumption.