An interface circuit is provided between a first circuit block and a second circuit block that operates using a power supply system differing from that of the first circuit block. An electrostatic discharge protection circuit that include a PN diode and a diffused resistor is formed in order to prevent electrostatic discharge destruction of a gate insulating film of a transistor that forms the interface circuit. The electrostatic discharge protection circuit may be formed using the remaining basic cells of a gate array that forms the second circuit block. An electrostatic discharge protection circuit formed of a bidirectional diode may be connected between a first low-potential power supply and a second low-potential power supply.