A first ESD protection circuit is provided between a first high-potential side power supply and a first low-potential side power supply of a first power supply system and a second ESD protection circuit is provided between a second high-potential side power supply and a second low-potential side power supply of a second power supply system. A coupling circuit includes a bidirectional diode and couples the first and second low-potential side power supplies. A first transistor is composed of an n-channel MOS transistor, has a drain coupled to the first high-potential side power supply of the first power supply system, and has a back gate coupled to the second low-potential side power supply of the second power supply system. A resistor element is inserted in series between the drain of the first transistor and the first high-potential side power supply.