To make it possible to raise the AC output voltage of a converter to a maximum value without increasing a DC voltage by a method wherein a means by which harmonics at a tertiary integral multiple are superposed is installed at the output of a current-controlled amplifier to which the deviation between a sine-wave current command value and an AC input current detection value from a rectification part is input.

A control device is provided with a rectification part 1 which converts the AC voltage of an AC power supply into a DC voltage and with an inversion part 2 which converts the DC voltage of the rectification part 1 into an AC voltage. Then, the deviation between the DC voltage of the rectification part 1 and a DC setting voltage is input to a voltage-controlled amplifier 10, the output of the voltage- controlled amplifier 10 is multiplied by a signal which is synchronized with the AC power supply, and a sine-wave current command value is created. The deviation between the sine-wave current command value and an AC-input-current detection value from the rectification part 1 is input to a current-controlled amplifier, and a means by which harmonics at a tertiary integral multiple are superposed is installed at the output of the current-controlled amplifier 13. Alternatively, the output of the current- controlled amplifier 13 is added to a power-supply voltage feedfoward so as to be zero-phase-modulated by a zero-phase modulator 20.