An output transistor is provided between an input terminal and an output terminal. An error amplifier adjusts the gate voltage of the output transistor such that the voltage that corresponds to the output voltage approaches a predetermined reference voltage. A fluctuation detection capacitor is provided on a path from the input terminal to a grounded terminal, which sets one terminal thereof to a fixed voltage. A current feedback circuit supplies, to the gate of the output transistor, the current that corresponds to the current that flows through the fluctuation detection capacitor. A clamp circuit clamps the gate voltage of the output transistor. The clamp circuit 30 clamps the gate voltage of the output transistor such that the voltage difference between the gate of the output transistor and the input terminal exhibits a predetermined clamp voltage or more.