A regulator circuit stabilizes the input voltage applied to an input terminal before outputting the output voltage via an output terminal. An output transistor is provided between the input terminal and the output terminal. An error amplifier adjusts the voltage applied to the control terminal of the output transistor such that a 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 the grounded terminal. One terminal of the fluctuation detection capacitor is set to a fixed electric potential. In a case that the input voltage is lower than the voltage at the other terminal of the fluctuation detection capacitor, an undershoot suppressing circuit forcibly reduces the gate voltage of the output transistor.