A protection circuit includes: a high-side switch connected to a power terminal to which a predetermined power supply voltage VBB is supplied from an onboard battery; and an NMOS transistor MT1 connected to the high-side switch and configured to prevent an electrical conduction to the high-side switch when the onboard battery is reverse-connected to the power terminal, wherein a semiconductor integrated circuit is protected from a breakdown due to the reverse connection of the external power supply. A semiconductor integrated circuit apparatus includes the above-mentioned protection circuit configured to protect a semiconductor integrated circuit connected between the power terminal and the ground terminal, from an electro-static discharge breakdown. The protection circuit is connected to the clamp circuit unit inserted between the power terminal and the ground terminal, and is configured to protect the clamp circuit unit from a breakdown when the external power supply is reverse-connected.