PURPOSE: To perform the detection of a misfired cylinder easily with accuracy by computing the engine fluctuation degree discrimination value on the basis of a rotational cycle, comparing this discrimination value with a slice level to detect the misfired cylinder, and in this case, setting the slice level variably and automatically according to the discrimination value.

CONSTITUTION: At the time of detecting the misfired cylinder of an internal combustion engine, the rotational cycle of the internal combustion engine is measured by a means A. On the basis of the measured rotational cycle, the engine fluctuation degree discrimination value, the value corresponding to the variation of average effective pressure in each cylinder, is computed by a means B cylinder under corresponding to each cylinder. The computed discrimination value and a slice level are further compared to discriminate the misfired cylinder by a means C. The slice level is variably set by a means D according to the above-mentioned discrimination value. In the means D at this time, the change rate of the above-mentioned discrimination value is detected by a means E, and the slice level is set by a means F on the basis of the change rate, as well as the change rate is corrected by a means G on the basis of the engine speed.