A system for controlling the air-fuel ratio in which the amount of fuel injected into the engine is controlled in accordance with the flow rate of air passing through an air intake pipe including a throttle valve for introducing the air into the engine and the engine operating conditions. A bypass of the air intake pipe is opened or closed to change the air-fuel ratio of the mixture to slightly richer and leaner sides of a basic air-fuel ratio, and the change of the engine output conditions based on the change of the air-fuel ratio is used to correct the air-fuel ratio in an optimum direction. On the basis of a parameter indicating the engine operating conditions such as the detection value of the intake pipe pressure, a control is made to introduce the air which passed through the bypass to one of the upstream or downstream portion of the throttle valve.