An internal combustion engine is disclosed which includes active and inactive cylinders, a load detector adapted to provide a low load indicative signal when the engine load is below a predetermined value, first means responsive to the low load indicative signal for cutting off the flow of air to the inactive cylinders, and second means for supplying a controlled amount of fuel into the active and inactive cylinders so as to achieve a somewhat lean air-fuel mixture therein. The second means is responsive to the low load indicative signal for cutting off the supply of fuel to the inactive cylinders and increasing the amount of fuel supplied to the active cylinders so as to achieve a somewhat rich air-fuel mixture therein. Third means is provided for monitoring the oxygen content of the exhaust from the engine to control the second means so that the fuel supplied to the engine is correct to maintain the stoichiometric air/fuel ratio.