PURPOSE: To detect and correct the unevenness of the combustion conditions of an engine, and to set the air-fuel ratio according to the respective object, by providing a function to correct in order to improve the combustion condition of the engine, and a function to correct in order to mininize the exhaust NOX component in the exhaust gas.

CONSTITUTION: Parameters to detect the combustion condition of an engine are made possible to be used for discriminating the limit of the operating property at the lowest fuel consumption, and for discriminating the limit of the engine stability to minimize the NOX exhaust amount. Object stability parameters ΔS, ΔS1, and ΔS2, for example, are found beforehand so as to respond according to the operating condition (Steps 103, 108, and 109). And when it is discriminated to be in the area B of Step 106, it is in the fuel consumption emphasis area, and the combustion stability parameter ΔS1 is calculated according to the engine operating condition. And when it is discriminated to be in the area C of Step 107, it is in the NOX emphasis area, and the combustion stability paramerer ΔS2 is calculated according to the engine operating condition. By such a way, a desired combustion condition is realized.