A selection unit switches between a phase θp and a phase θn different from the phase θp substantially by 180 degrees, and outputs one of them in synchronization with a carrier signal. A voltage-command generation unit generates and outputs three-phase voltage command values Vu*, Vv* and Vw* based on the phase outputted by the selection unit. A PWM-signal generation unit generates three-phase voltage command values Vu*′, Vv*′ and Vw*′ by correcting the three-phase voltage command values Vu*, Vv* and Vw* outputted by the voltage-command generation unit according to a predetermined method, and generates six drive signals corresponding to switching elements of the inverter based on the three-phase voltage command values Vu*′, Vv*′ and Vw*′ and the carrier signal. The PWM-signal generation unit outputs the generated drive signals to the corresponding switching elements of the three-phase inverter, to cause the inverter to generate a high-frequency AC voltage.