A rotational fluctuation malfunction detection device for an internal combustion engine determines that fluctuations in output shaft rotational speed are “normal” when a rotational fluctuation value, obtained in a “disturbance fluctuation state” is below threshold L1. If rotational fluctuation value obtained in “disturbance fluctuation state” equals or exceeds L2, the rotational fluctuation malfunction detection device determines that the rotational fluctuation value is “abnormal.” However, when the rotational fluctuation value obtained in “disturbance fluctuation state” is equal to L1 or between L1 and L2, the rotational fluctuation malfunction detection device determines that the rotational fluctuation value is “normal” if the rotational fluctuation value obtained in the “non-disturbance state” is below L3, and the rotational fluctuation value is “abnormal” if the rotational fluctuation value obtained in the “non-disturbance state” equals or exceeds L3.