An electric power conversion device (10; 10a) comprising: a transistor (16; 16a, 16b) for electric power conversion; a diode (18; 17a, 17b) connected in series with the transistor (16; 16a, 16b); a smoothing capacitor connected in parallel with the transistor (16; 16a, 16b) and the diode (18; 17a, 17b), the smoothing capacitor being configured to suppress ripples generated by the transistor (16; 16a, 16b); and a snubber circuit including a snubber capacitor, an inductor element and a resistor, the snubber capacitor, the inductor element and the resistor being connected to each other in series, the snubber circuit being connected in parallel with the smoothing capacitor, capacitance of the snubber capacitor, inductance of the inductor element and magnitude of the resistor being set such that a resonance frequency of the snubber circuit coincides with a ringing frequency of the transistor (16; 16a, 16b), and being set such that impedance of a first loop at the resonance frequency becomes smaller than impedance of a second loop at the resonance frequency, wherein the first loop includes the snubber circuit and the smoothing capacitor, and the second loop includes the transistor (16; 16a, 16b) and the smoothing capacitor, characterized in that: the capacitance of the snubber capacitor is set so as to become equal to inter-terminal capacitance of the transistor (16; 16a, 16b); the inductance of the inductor element is set such that the inductance of the first loop becomes equal to the inductance of the second loop; and the magnitude of the resistor is set such that the impedance of the first loop at the resonance frequency becomes smaller than the impedance of the second loop at the resonance frequency. a transistor (16; 16a, 16b) for electric power conversion; a diode (18; 17a, 17b) connected in series with the transistor (16; 16a, 16b); a smoothing capacitor connected in parallel with the transistor (16; 16a, 16b) and the diode (18; 17a, 17b), the smoothing capacitor being configured to suppress ripples generated by the transistor (16; 16a, 16b); and a snubber circuit including a snubber capacitor, an inductor element and a resistor, the snubber capacitor, the inductor element and the resistor being connected to each other in series, the snubber circuit being connected in parallel with the smoothing capacitor, capacitance of the snubber capacitor, inductance of the inductor element and magnitude of the resistor being set such that a resonance frequency of the snubber circuit coincides with a ringing frequency of the transistor (16; 16a, 16b), and being set such that impedance of a first loop at the resonance frequency becomes smaller than impedance of a second loop at the resonance frequency, wherein the first loop includes the snubber circuit and the smoothing capacitor, and the second loop includes the transistor (16; 16a, 16b) and the smoothing capacitor, characterized in that: the capacitance of the snubber capacitor is set so as to become equal to inter-terminal capacitance of the transistor (16; 16a, 16b); the inductance of the inductor element is set such that the inductance of the first loop becomes equal to the inductance of the second loop; and the magnitude of the resistor is set such that the impedance of the first loop at the resonance frequency becomes smaller than the impedance of the second loop at the resonance frequency. wherein the first loop includes the snubber circuit and the smoothing capacitor, and the second loop includes the transistor (16; 16a, 16b) and the smoothing capacitor, characterized in that: the capacitance of the snubber capacitor is set so as to become equal to inter-terminal capacitance of the transistor (16; 16a, 16b); the inductance of the inductor element is set such that the inductance of the first loop becomes equal to the inductance of the second loop; and the magnitude of the resistor is set such that the impedance of the first loop at the resonance frequency becomes smaller than the impedance of the second loop at the resonance frequency. the capacitance of the snubber capacitor is set so as to become equal to inter-terminal capacitance of the transistor (16; 16a, 16b); the inductance of the inductor element is set such that the inductance of the first loop becomes equal to the inductance of the second loop; and the magnitude of the resistor is set such that the impedance of the first loop at the resonance frequency becomes smaller than the impedance of the second loop at the resonance frequency.