PURPOSE: To simply compose a negative voltage generator for driving a transistor of an inverter by connecting a second capacitor in parallel with a series circuit of a first capacitor and a first diode, a series circuit of a Zener diode and a second diode, and a Zener diode.

CONSTITUTION: When an IGBT 11 is turned ON, a charging current flows from a power source 15 through the IGBT11, a first capacitor 49 and a first diode 51, and the capacitor 49 is charged to a voltage VB of the power source 15. When the IGBT11 is turned OFF and the IGBT12 is turned ON from this state, a current i2 for charging a second capacitor 55 with a part of the charges in the capacitor 49 flows in a closed circuit of the capacitor 49, the IGBT12, the capacitor 55, a resistor 53, a second diode 52 and the capacitor 49. The voltage of the capacitor 44 generated at this time is negative to the emitter of the IGBT12, and this negative voltage -VC is used as a negative voltage for driving the gate of the IGBT12.