PURPOSE: To prevent backfire by introducing air in a suction system to dilute air- fuel ratio of the mixed gas purposely when an engine is misfired at low speed and high load in an ignition to fire a plural cylinder of ignition plugs simultaneously.

CONSTITUTION: In a distributor 20 to supply a high voltage generated on the secondary ignition coil simultaneously to first and fourth ignition plugs 21 and then simultaneously to second and third ignition plugs 21, when, for example, the first cylinder is misfired, an exhaust gas temperature of said cylinder drops and an exhaust gas temperature in an exhaust manifold 17-1 also drops. Therefore, the output of an exhaust temperature sensor 30-1 drops lower than that of 30-2, and where the temperature difference exceeds a set point, the output is delivered through a comparater 31. An air leading-in valve 42 is opened consequently to introduce the air into a suction manifold 15 through an air passage 41, thus diluting the mixed gas.