An electric power conversion device includes: a transformer including first and second windings magnetically coupled; a bridge circuit including a switch element; a resonant inductor; and a resonant capacitor, wherein the resonant inductor and the resonant capacitor constitute a resonant circuit together with an inductance of the first winding, n1≧n2, the resonant inductor is inserted into a pathway from the first connection point to the second connection point via the first winding in series with the first winding, the resonant capacitor is inserted into a pathway from the second winding to the output terminal in series with the second winding, and Cr>C1 where Cr is a capacitance of the resonant capacitor and C1 is a capacitance of a capacitance component that is in series with the first winding in the pathway from the first connection point to the second connection point via the first winding.