A control unit alternately repeats a first state in which a first switch and a third switch are on and a second state in which a second switch and a fourth switch are on. A voltage detection unit compares the output voltage that occurs at one terminal (OUT terminal) of an output capacitor with a negative threshold voltage, and generates an abnormality detection signal which is asserted when the output voltage is higher than the threshold voltage Vt. When the abnormality detection signal is asserted, the control unit switches the on resistance of the first switch to a value that is higher than that in a normal state.