A DC-DC converter includes first and second capacitors connected in series, a switching part, and a control circuit part which includes a voltage difference calculating part for calculating a difference between voltages of the first and second capacitors, a duty ratio controller for controlling duty ratio of on- and OFF-durations to decrease the voltage difference of the first and second capacitors on the basis of the calculated difference. A power supplying system including the DC-DC converter controls balance between first and second capacitors in voltage on the basis of the powering/regenerating discriminating signal.