PURPOSE: To regenerate the snubber energy of a three-level inverter to the power supply side without increasing voltage in a device.

CONSTITUTION: Energy absorbed to clamp capacitors 31, 51 is moved to auxiliary capacitors 65, 75 by utilizing a resonance phenomenon, but voltage at a neutral point Z does not exceed inverter voltage even when energy is shifted to the neutral point because the auxiliary capacitors 65, 75 are connected so that the neutral point Z reaches reference potential. The energy of the auxiliary capacitors 65, 75 is regenerated to the power supply side by converters 66, 76 for regeneration composed of step-up choppers, but energy is regenerated to the power supply side by using a converter for regeneration, in which the input inside and the output side are insulated and which has a transformer at a transformation ratio corresponding to input voltage and output voltage, when the auxiliary capacitors 65, 75 are connected in series with main capacitors 21, 41.