PURPOSE: To facilitate specification of the cause of misfire judgement and reduce a work time and a work man-hour by stopping a difference and misfire monitor in which either one of misfire detecting stop conditions is set up, and also eliminating misfire detecting stop condition which is used to be a misfire cause at the time of miscarriage.

CONSTITUTION: A control function is provided in a control means 16 for inputting output signals such as an air flow meter 26, a front and rear each O2 sensor 40, 46, a throttle sensor 96, a crank angle sensor 108, a knock sensor 114 so as to carry out misfire monitor for judging misfire by fluctuation of operating condition of an internal combustion engine 2. A function is provided to stop a misfire monitor when either one of misfire detecting stop conditions such as at least on regulating an ignition timing, on regulating a throttle bypass air, on regulating an ISC valve flow amount, on regulating a combustion feed back correcting amount, at the time of abnormal condition in a fuel system is set up. It is thus possible to facilitate causative specification when misfire is judged, and reduce a work time for specifying misfire cause.