Misfiring in a cylinder of an engine can be detected with high reliability using a statistical treatment even if the level of ion current generated by a spark plug upon ignition thereof is affected by noise. An ion current, which is generated when a spark plug for a cylinder discharges, is detected to provide a corresponding voltage which is then integrated or peak held to produce a current component signal which is generated for a predetermined time from the time of discharge of the spark plug. The current component signal is used as misfiring information and compared with a prescribed threshold so as to determine whether there is misfiring in the cylinder. Misfirings thus detected are counted so as to evaluate the state or extent of misfiring in the cylinder. If the state of misfiring is serious, the fuel supply to the cylinder is stopped. If, however, it is light, an appropriate measure for optimizing the operation of the misfiring cylinder is taken to remedy such a situation and recover normal combustion therein.