After a current value flowing through a coil L is detected, it is converted into a voltage value, is then inputted to an inverting input terminal of a comparator 4, and is then compared with a voltage indicating the difference between the value of a fed back output voltage and a reference voltage. A voltage source 20 is provided at the inverting input terminal of the comparator 4 so that an offset voltage is added to the voltage value to be inputted to the inverting input terminal of the comparator 4.