The uninterruptible power supply includes a converter (5) for converting AC power into DC power, an inverter (10) supplied to a load by converting DC power into AC power (24), the converter (5) and an inverter ( comprising a control device for controlling the 10) and (18). Control device (18) includes a sine wave output mode providing waveform distortion free sinusoidal AC voltage (VO) to the load (24), the load AC voltage having a waveform distortion in the allowed range taking in (24) (VO) that perform the selected mode of the waveform distortion generation mode to be supplied to the load (24).