PURPOSE: To prevent erroneous detection of misfire by judging rough road running while not judging misfire when an engine speed fluctuation rate exceeds a second judgment value which is smaller than a first judgment value to be used for misfire judgment by specified times or more, and thereby accurately distinguishing continuous misfire from rough road running.

CONSTITUTION: An internal combustion engine 2 has a rotational angle sensor 10, a cylinder judgment sensor 16 and the like for detecting an operation condition. Detection signals from the sensors are input to an electronic control unit(ECU) 20 which performs lighting of an alarm lamp or the like based on misfire detection result through misfire detection process. In the case that an engine speed fluctuation rate of the internal combustion engine 2 is larger than a first judgment value, misfire occurrence is judged. In the case that the engine speed fluctuation rate exists between the first judgment value and a second judgment value, existence times are counted for a specified period. When the value of the counter is larger than the specified value, the judgment result of the misfire judgment means is voided. Misfire detection is carried out with further accuracy.