PURPOSE: To make it possible to adjust the air-fuel ratio in accordance with the condition of driving by operating bypass air supply corresponding to the amount of suction air at a carburetor in which the bypass air path which opens in the lower stream of a throttle valve is feedback-controlled by the exhaust gas concentration.

CONSTITUTION: Air which has passed through air cleaner A is mixed with fuel in carburetor C, then it is supplied to engine E via suction pipe MS and is burned. Exhaust gas is discharged through exhaust pipe ME. Here, in addition to exhaust gas concentration detecting element 1 inserted into exhaust pipe ME, elements 2W7 are provided which respectively detect the opening of the throttle valve, air temperature, engine temperature, engine suction negative pressure, and air density. Output signals issued by these elements are sent to control circuit 8, which issues proper control signals. Carburetor C is provided with bleed air control valve 9 and bypass air control valve 10. In accordance with the control signals issued by control circuit 8, two-step on-off operations are performed separately.