A DC-DC converter for outputting dc voltage whose peak value is higher than an input dc voltage value and high-frequency voltage corresponding to a command value is superimposed. An insulated switching power supply 27 and a series regulator 26 are connected in parallel to a dc power supply 1. The insulated switching power supply 27 generates output voltage V.sub.2 having a certain level, and the series regulator 26, output voltage V.sub.1 whose voltage varies depending on a command value (command voltage varying at a high frequency). Then, the sum of the output voltages V.sub.1 and V.sub.2 is fed to the load 9.