PURPOSE: To provide continuous operation stably, by providing a means for compensating a phase of an output voltage in accordance with a DC voltage and reducing a cross current between inverter units.

CONSTITUTION: With respect to a frequency reference signal from PLL's 11a and 11b, transient phase compensators 22a and 22b generate a phase compensating signal in accordance with DC voltages 15a and 15b and supply it to gate signal generators 12a and 12b, and the phase compensating signal is received by shifters 21a and 21b for phase compensation. When a phase of an inverter 2a falls behind that of an inverter 2b, a cross current flows from the inverter 2b to the inverter 2a, and thereby a DC voltage 15a rises above a set value. Then, the transient phase compensating unit 22a generates a signal that is determined by the excess value of the DC voltage 15a and a proportion gain (G). According to this signal, the shifter 21a advances the phase of the inverter 2a. Consequently, the cross current in power from the inverter 2b to inverter 2a is checked, and the DC voltage 15a is prevented from rising excessively.