A voltage drop of an output voltage of a DC-DC converter, caused by a wiring resistance, can properly be compensated at low cost. The DC-DC converter is connected to a low-voltage battery through an ignition switch and started up by the ignition switch. A transformation unit transforms a voltage inputted from a high-voltage battery and supplies the voltage to the low-voltage battery. A control circuit calculates a wiring resistance between the transformation unit and the low-voltage battery based on the voltage inputted from the low-voltage battery through the ignition switch in connecting the ignition switch and an output voltage and an output current of the transformation unit. The control circuit corrects a command value of the output voltage of the transformation unit based on the calculated wiring resistance to control the output voltage of the transformation unit. The invention can be applied to a DC-DC converter for electric-powered vehicle.