PURPOSE: To supply mixed air close to the theoretical air-fuel ratio by providing the control valve in the flow passage to conduct air to the suction system at the downstream from the carburetor and controlling the control valve corresponding to the output from the sensor which generates step signal in relation to the oxygen concentration in the exhaust system.

CONSTITUTION: The air cleaner 12 and the port 13 of the suction branch pipe 2 are connected by the air flow passage 11 in the middle of which the air control valve 14 of linear solenoid type is provided. The air control valve 14 generates magnetic force by the electricity supplied to the solenoid 18 to control air flow by sliding the cylindrical member 17 to the direction toward which the window 15 of the pipe 16 is opened, and by appropriately selecting the shape of the window 15, the relation between the feeding current I to solenoid and the opening area S of the window 15 is set as shown by the diagram. The air control valve 14 is controlled, through the electronic control device 25, by the output from the O2 sensor 24 of which output power changes in step condition with the theoretical air-fuel ratio as the border line.