A DC-DC converter comprising: a coupling transformer comprising a first winding, a second winding electrically connected with the first winding, a third winding, and a fourth winding electrically connected with the third winding; a first switch connected through the first and second windings (31a, 31b) to both ends of a DC power source (Vi); a second switch connected through the third and fourth windings (32a 32b) to the both ends of the DC power source (Vi); a first series circuit connected to both ends of the first switch and including a first diode and a smoothing capacitor; a second series circuit connected to both ends of the second switch and including a second diode and the smoothing capacitor; and a controller configured to alternately turn on the first and second switches (Tr1, Tr2) and configured to make the first and second switches (Tr1, Tr2) simultaneously on for a predetermined overlapping period on every half cycle, characterized in that: the couplina transformer comprises two E-shaped cores that are combined together in a planar direction with their center legs being faced to each other; a gap is formed between the center legs of the combined E-shaped cores; the second winding is connected in series with the first winding and the fourth winding is connected in series with the third winding; the first and fourth windings (31a, 32b) are wound around first side legs on one side of the combined E-shaped cores; and the second and third windings (31b, 32a) are wound around second side legs on another side of the combined E-shaped cores. a coupling transformer comprising a first winding, a second winding electrically connected with the first winding, a third winding, and a fourth winding electrically connected with the third winding; a first switch connected through the first and second windings (31a, 31b) to both ends of a DC power source (Vi); a second switch connected through the third and fourth windings (32a 32b) to the both ends of the DC power source (Vi); a first series circuit connected to both ends of the first switch and including a first diode and a smoothing capacitor; a second series circuit connected to both ends of the second switch and including a second diode and the smoothing capacitor; and a controller configured to alternately turn on the first and second switches (Tr1, Tr2) and configured to make the first and second switches (Tr1, Tr2) simultaneously on for a predetermined overlapping period on every half cycle, characterized in that: the couplina transformer comprises two E-shaped cores that are combined together in a planar direction with their center legs being faced to each other; a gap is formed between the center legs of the combined E-shaped cores; the second winding is connected in series with the first winding and the fourth winding is connected in series with the third winding; the first and fourth windings (31a, 32b) are wound around first side legs on one side of the combined E-shaped cores; and the second and third windings (31b, 32a) are wound around second side legs on another side of the combined E-shaped cores. the couplina transformer comprises two E-shaped cores that are combined together in a planar direction with their center legs being faced to each other; a gap is formed between the center legs of the combined E-shaped cores; the second winding is connected in series with the first winding and the fourth winding is connected in series with the third winding; the first and fourth windings (31a, 32b) are wound around first side legs on one side of the combined E-shaped cores; and the second and third windings (31b, 32a) are wound around second side legs on another side of the combined E-shaped cores.