A first transistor is an N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor) with a first terminal of the conduction channel, the gate, and the back gate thereof connected to a terminal to be protected. A second transistor is an N-channel MOSFET with a first channel of the conduction channel, the gate, and the back gate thereof connected to a fixed voltage terminal, and with a second terminal of the conduction channel thereof connected to the second terminal of the conduction channel of the first transistor. The first transistor and the second transistor are floating MOSFETs formed in a common N-type well formed in a P-type semiconductor substrate, and are provided in the form of separate devices. A common connection node which connects the first transistor and the second transistor is connected to an N-type well for device separation.