An input-output interface circuit of the present invention includes an input-output terminal, an input buffer, a first MOS transistor of a first conductivity type formed in a floating well region, an output buffer for outputting a signal externally through the input-output terminal, an electrostatic protection circuit, and a floating well potential adjusting circuit, wherein the electrostatic protection circuit has a first resistance, and a diode connected between another end of the first resistance and a high level power supply potential, and the floating well potential adjusting circuit has a second resistance having one end connected to the input-output terminal, and a second MOS transistor of the first conductivity type having one end connected to another end of the second resistance, another end connected to the floating well region, and a gate connected to the high level power supply potential.