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.