A system for controlling in a feedback control mode the air-fuel ratio of an air-fuel mixture to be supplied to an internal combustion engine in response to the deviation from a reference value, of an output of an exhaust sensor for sensing the concentration of a component of engine exhaust gas. The reference value is varied even during stopping of the feedback control due to an lowered engine temperature, thereby preventing an erroneous control. Additionally, the reference value and the value of a current flow to the exhaust sensor are varied immediately when the feedback control is stopped due to lowering in the engine temperature, thereby quickening the initiation of the feedback control.