A DC conversion apparatus includes a switch converting a DC voltage of a DC power source to a high-frequency voltage by being turned ON/OFF via a primary winding of a transformer, a series circuit of a switch and a clamp capacitor being connected to both ends of the primary winding, a synchronous rectifying circuit performing synchronous rectification of a high-frequency voltage generated at a secondary winding, a smoothing circuit smoothing a rectified output of the synchronous rectifying circuit by a smoothing reactor and a smoothing capacitor, a capacitor storing energy stored in the smoothing reactor due to a current backflow of the synchronous rectifying circuit caused under light load condition, a switch being turned ON in synchronization with the switch and returning the energy stored in the capacitor to the clamp capacitor on the primary side via the transformer, and a control circuit turning the switch and the switch ON/OFF states alternately.