In a portable inverter power generation apparatus, a rectifying circuit converts AC electric power generated by an electric generator into DC electric power, and an inverter circuit converts the DC electric power into AC electric power. A current sensor detects an AC output current from the inverter circuit. In a first case where the AC output current detected by the current sensor increases from a value lower than a first value to a value higher than the first value, a computer sets a target rotational speed to a value lower than a resonance rotational speed at which an engine resonates until the AC output current detected by the current sensor increases to a second value higher than the first value, and sets the target rotational speed to a value higher than the resonance rotational speed when the detected AC output current increases to the second value. In a second case where the AC output current detected by the current sensor decreases from a value higher than the second value to a value lower than the second value, the computer unit sets the target rotational speed to a value higher than the resonance rotational speed until the AC output current detected by the current sensor decreases to the first value, and sets the target rotational speed to a value lower than the resonance rotational speed when the detected AC output current decreases to the first value.