PURPOSE: To prevent an increase in the turn-on time due to the mutual interference of individual elements for an IGBT which constitutes the main circuit conversion part of a voltage-type inverter by a bridge connection.

CONSTITUTION: A DC power supply which is composed of diodes 1, 2, of a constant-voltage diode 7, of a resistance 5 and of capacitors 9, 10 is installed independently for a gate signal formation park, with reference to an IGBT 35, which is composed of transistors 21, 22, 27, of a switch 17, of a resistance 13 and of the like. In addition, the DC power supply which is composed of the diodes 1, 2, of the constant-voltage diode 7, of the resistance 5 and of the capacitors 9, 10 is installed independently for a gate signal formation part, with reference to an IGBT 36, which is composed of transistors 23, 24, 28, of a switch 18, of a resistance 14 and of the like. Thereby, DC power supplies for both signal formation parts are separated, the mutual interference of both signal formation parts is prevented via the wiring inductance of a common bus between both DC power supplies.