In a DC-DC converter of the type in which an inductance stores energy when a main switching transistor is turned on and releases the stored energy when the main switching transistor is turned off, a semiconductor active element is used as a flywheel element which is turned off when the main switching element is turned on, but is turned on to provide the path for the current established by the release of the stored energy, and the signal which is substantially in synchronism with the switching operation of the main switching transistor is applied to the control terminal of the semiconductor active element so as to cause it to turn on and off.