A method of detecting the condition of combustion in each cylinder of an internal combustion engine while monitoring a variation in rotation of a crankshaft in the engine for each cylinder, in which the degree of irregular combustion is judged based on the magnitude of the rotational variation, and/or in which the condition of combustion in a succeeding cylinder is judged based on the difference between rotational variations within one combustion period, of adjacent cylinders, and/or in which the rotational ascending rate within one combustion period, of each cylinder is detected to judge the condition of combustion in the cylinder based on the dropping state of the rotational ascending rate, and/or in which the rotational angular acceleration within one combustion period, of each cylinder is detected to thereby judge that a predetermined cylinder is in a condition of irregular combustion when the quantity of the variation in rotational angular acceleration between adjacent cylinders exceeds a predetermined value.