A DC-DC converter includes a coupling transformer that has windings 11 and 12, switches (Tr1, Tr2) that are connected through the windings to both ends of a DC power source Vi, a series circuit that is connected to both ends of each of the switches and includes a diode and a smoothing capacitor, and a controller 100 that alternately turns on the switches Tr1 and Tr2 and simultaneously turns on the switches Tr1 and Tr2 for a predetermined overlapping period on every half cycle. The coupling transformer 1 includes an I-shaped core 4, two E-shaped cores 2 and 3 holding the I-shaped core 4 between them, and a gap 5 formed between each of center legs 2a and 3a of the E-shaped cores 2 and 3 and the I-shaped core 4. The windings 11 and 12 are wound around the I-shaped core 4.