PURPOSE: To improve the drivability of an engine and the rate of fuel consumption, by reducing a fuel injection quantity to a main chamber, while increasing the fuel injection quantity to an auxiliary chamber according to the reduced quantity, in a state of specified engine driving.

CONSTITUTION: An internal combustion engine 1 is provided with a main chamber 2 and an auxiliary chamber 3 being interconnected with each other. An electronic control unit 9 discriminates on whether a coefficient of leanness in time of idling is smaller than a value 1 or not, and if "NOT" is the case, it further discriminates on whether or not the coefficient of leanness in time of lean stoichiometric operation is smaller than the value 1. If the answer is "YES," it is so discriminated that the promotion of leanness for the air-fuel mixture to be fed to the main chamber 2 takes place whereby a richness coefficient is set to the small specified value, and on the basis of this value, valve opening time in a subinjector 12 is calculated. With this constitution, the drivability of an engine and the rate of fuel consumption are made to improve ever so better as well as a drop in emission is promoted.