PURPOSE: To provide an air intake system for internal combustion engine, in which main carburetor and high-output carburetor are provided respectively in a first and a second intake passages, and fuel supply to additional fuel supplying means provided in said main carburetor is effected from said high-output carburetor, so that it is prevented that vapor-lock or the like is caused in the high-output carburetor.

CONSTITUTION: When throttle valve 21 is opened for shifting engine operation from idling to normal operation, fuel from acceleration pump 41 communicated to float chamber 15a of high-output carburetor 15 is passed through passage 46 and introduced from port 48 into a first intake passage 14 of main carbutetor 13. When the throttle valve 21 is opened more than a certain angle, control valve 22 and high- output throttle valve 54 are opened by means of linkage connected to the throttle valve 21. Resultantly, both of mixture passed through the valves 22, 54 and secondary mixture passed through a subsidiary intake passage (not shown) are introduced into combustion chamber 4. With such an arrangement, most of engine operation can be conducted while closing the high-output throttle valve 54, so that problems of vapor-lock or the like can be solved.