DC-DC converter with improved efficiency in a dual voltage network of a vehicle

The invention improves efficiency of a DC-DC converter. When it is determined that a high-voltage battery is not being charged in Step S1, a switching frequency of a switching circuit of the DC-DC converter is set in Step S4 based on an output current of the DC-DC converter. On the other hand, when it is determined that the high-voltage battery is being charged in Step S1, the switching frequency is set to a previously set switching frequency during the charge in Step S6. In Step S7, the switching frequency of the switching circuit is changed to the value set in Step S4 or S6. The invention can be applied to a DC-DC converter for an electric-powered vehicle or the like.