PURPOSE: To provide an air fuel ratio adjusting device by which an air fuel ratio can be adjusted to an ideal air fuel ratio even if there exists an individual difference in engine performance or carburetor performance.

CONSTITUTION: An air-fuel mixture passage 9 and an auxiliary air passage 10 are formed in an air intake pipe 4 connected to an air intake port 2 of a spark-ignition type engine, and the air-fuel mixture passage 9 is communicated with an air cleaner 6 through a carburetor, and one end of the auxiliary air passage 10 is communicated with the air-fuel mixture passage 9 and the other end is communicated with the air cleaner 6, respectively, and a passage opening/ closing control valve 12 is arranged in the auxiliary air passage 10. A boost sensor 13 is arranged closer on the air intake port 2 side than this passage opening/closing control valve 12, and an oxygen sensor 14 arranged in an air exhaust pipe 7 derived from this boost sensor 13, an engine speed detecting sensor 17 and the engine is connected to an air fuel ratio control mechanism 16, and an opening of the passage opening/closing control valve 12 arranged in the auxiliary air passage 10 is adjusted according to an output signal from the air fuel ratio control mechanism 16.