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 (Tr1) connected through the first and second windings to both ends of a DC power source (Vi); a second switch (Tr2) 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 (Tr1) and including a first diode (D1) and a smoothing capacitor (C0); a second series circuit connected to both ends of the second switch (Tr2) and including a second diode (D2) and the smoothing capacitor (C0); 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 are wound around first side legs on one side of the combined E-shaped cores; and the second and third windings 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 (Tr1) connected through the first and second windings to both ends of a DC power source (Vi); a second switch (Tr2) 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 (Tr1) and including a first diode (D1) and a smoothing capacitor (C0); a second series circuit connected to both ends of the second switch (Tr2) and including a second diode (D2) and the smoothing capacitor (C0); 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 are wound around first side legs on one side of the combined E-shaped cores; and the second and third windings 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 are wound around first side legs on one side of the combined E-shaped cores; and the second and third windings are wound around second side legs on another side of the combined E-shaped cores. <EOS>