PURPOSE: To suppress the peak value of a surge voltage and to reduce the loss, of a voltage clamp snubber circuit, which is generated in a discharge resistor by forming the snubber circuit which clamps a voltage across a capacitor to a half the voltage between the positive side and the negative side of a DC voltage source with reference to a first switching element and a fourth switching element.

CONSTITUTION: When a transistor 13 is turned off, a main circuit current as a load current is shifted to a series circuit which consists of a capacitor 24 and a diode 25. A surge voltage is absorbed by the capacitor 24, and the peak value of a voltage applied to the transistor 13 is suppressed. Then, a voltage when the transistor 13 is turned off is discharged via a resistor 26, and the voltage across the capacitor 24 is clamped to the voltage across a capacitor 11. Thereby, energy which is consumed by the resistor 26 is only the one corresponding to the voltage increment of the capacitor 24 when the transistor is turned off, a loss is reduced even when a conversion frequency is increased, a device can be made small and lightweight, and its efficiency can be increased.