PURPOSE: In the control method for exchanging the air-fuel ratio under specific engine running condition, to prevent hunting of engine rotation by stopping exchange of the air-fuel ratio to the lean side during the idle-up interval.

CONSTITUTION: In the air-fuel ratio control, an electronic controller 40 will decide whether the condition for executing the lean burn control is satisfied or not. If an air-conditioner switch 35 is turned on or during the idle-up interval, theoretical air-fuel ratio control is executed. If said switch 35 is turned off, such lean-burn control that the air-fuel ratio will take the value at the lean side is executed. Since exchange of air-fuel ratio to the lean side is stopped during the idle-up interval, hunting of engine rotation is prevented.