The input voltage Vin supplied to the DC/DC converter may sometimes vary, especially When its poWer source is a battery. The DC/DC converter must keep the output voltage VOW constant even in such a case. Accordingly, When the input voltage Vin goes doWn, the process for increasing the duty D is performed according to the above described equation That is, the ratio of time period in Which the sWitch S1 is ON is increased.