PURPOSE: To stabilize operation and restrict the emission harmful substances, by a method wherein an air-fuel ratio feedback control is released when an idling condition is continued for at least a predetermined period of time and either the engine revolution number is not higher than a set point or said revolution number is not lower than a set point and the variation of said revolution number is not lower than a set point.

CONSTITUTION: After the predetermined period of time is elapsed from the moment an idle switch 13 connected to a throttle valve 4 is turned ON, the engine revolution number N is read from a sensor 15 for the revolution number and is compared to a preset engine revolution number Y. When N<Y, the air-fuel ratio feedback control is released. When N>Y, the variation DN of the engine revolution number N is determined and is compared to the set point Z. When DN>Z, the feedback control is released.