In an inverter generator having an engine generator unit generating alternating current, a converter converting the alternating current to direct current, an inverter that converts the direct current to alternating current, an inverter driver that drives switching elements with a PWM signal generated using a reference sine wave of a desired output voltage waveform and a carrier and makes the alternating current to the alternating current of a predetermined frequency, the alternating current supplied to an electrical load is detected, gains Gn (n: 2, 3, 4, 5, 6, . . . m) of n-th harmonics of the reference sine wave and amplitudes An are calculated from the detected current, and the reference sine wave is corrected by a sum obtained by the calculated gains and amplitudes. With this, the reference sine wave is accurately corrected to reliably remove harmonic distortion components from the output voltage waveform.