To provide sine waves without distortions in the output voltage waveform, even if the input voltage contains harmonics or at unbalance of power supply without using a high-speed calculation device, a harmonics detecting means of an analogue circuit or the like.

An AC-AC power converter, such as a matrix converter 110, directly converts a multi-phase AC voltage into a multi-phase AC voltage having the frequency of arbitrary intensity. A control unit 500A comprises a voltage detecting means 501; an input current instruction calculating means 502 which keeps instantaneous effective power constant, based on the detected input voltage, while calculates an input current instruction so that the instantaneous effective power agrees with the instruction; a rectifier control means 503 which controls the matrix converter 110 so that the current, corresponding to the input current instruction flows; an inverter control means 504; and a PWM pulse synthesizing means 505.