PURPOSE: To avoid erroneous determination which likely occurs when an output varying means is defective by determining a misfire when pressures at the two points nearly equal with the same crank angle both sides of a top dead center in a combustion chamber.
CONSTITUTION: An intake valve 10 and an exhaust valve 11 are opened/closed with predetermined timing by a valve actuating mechanism, wherein the valve timing can be changed by a timing changing mechanism. Signals from a sensor 12 for detecting a pressure in a combustion chamber 14 and from a sensor 14 for detecting a crank angle of a crank shaft 15 are input to a misfire detector 13 comprising a microcomputer. Determination of a misfire performed by the misfire detector 13 is basically made in such a manner that a pressure P1 of the combustion chamber 4 at a point 20 degrees before a top dead center and a pressure P2 of the combustion chamber 4 20 degrees behind the top dead center are compared and when P1 and P2 are almost equal to each other, the misfire is determined. When the valve timing changing mechanism is defective, the detection of misfires s canceled in principle and execution of the determination of misfires is suspended.