The “imbalance index value” representing the magnitude of the difference among the cylinder-by-cylinder air-fuel ratios (imbalance of air-fuel ratios among cylinders) is acquired based on the output value of the air-fuel ratio sensor disposed above the catalyst. The execution of the emission reduction control is “limited” when the magnitude of the imbalance of air-fuel ratios among cylinders represented by the imbalance index value is the first degree or more, and less than the second degree which is larger than the first degree, and is “prohibited” when it is the second degree or more. As the emission reduction control, a purge control, an EGR control, an Al incremental control, a cold VVT control, a catalyst warm-up delay angle control, an SCV control, and the like can be exemplified. Misfire due to the execution of emission reduction control are suppressed, when the imbalance of air-fuel ratios among cylinders occurs.