An air flow amount adjusting system produces an air-fuel mixture of a desirable air-fuel ratio by controlling the direction of movement of a bypass valve mounted in an additional air supply pipe adapted to supply additional air into either the intake system or exhaust system of an internal combustion engine in accordance with an output signal produced from a gas sensor mounted in the exhaust system of the engine and indicative of the air-fuel ratio of the mixture supplied to the engine. A two-level mode signal whose threshold corresponds to the desired air-fuel ratio is produced in accordance with the output signal of the gas detector. After the two-level mode signal has changed from one level to the other level, the movement of the bypass valve is stopped for a certain period of time. If desired, the stopping period of the bypass valve may be changed in accordance with the temperature of the engine.