A changing-speed calculator monitors a current value of an output current flowing through a load circuit to calculate a change speed of the current value. A correction-amount obtaining unit reads from a correction-amount table a correction amount of a duty ratio corresponding to the change speed of the output current. A reference-voltage comparator compares a voltage value of an output voltage to be applied to the load circuit and a predetermined reference voltage with each other, and then notifies a duty-ratio determining unit of the comparison result. The duty-ratio determining unit performs feedback control according to the comparison result of the voltage values to correct the duty ratio. After correcting the duty ratio, the duty-ratio determining unit further corrects the duty ratio by the correction amount obtained by the correction-amount obtaining unit.