To keep a primary current at a specified value, regardless of the ripple of power voltage, without performing complicated control, in a self-excited flyback system of DC-DC converter for an electronic flash device.

This DC-DC converter is equipped with a transformer, a primary switching element, a secondary rectifying circuit, a first switching circuit, and a second switching circuit. The secondary rectifying circuit prevents a current from flowing to secondary, winding thereby accumulating energy in the transformer; while the primary current is flowing through the primary winding, and discharges this energy as a secondary current, while the primary current is not flowing. The second switching circuit switches on the primary switching element, in synchronization with the finish of the outflow of the secondary current, and the first switching circuit switches off the primary switching element, when the absolute value of the primary current reaches its upper limit value.