PURPOSE: To stabilize idle operation, and improve fuel consumption by preventing the occurrence of a run-on phenomenon by a wall surface flow, and reducing a change width of engine rotating speed at idle operation in a variable venturi type carburetor.

CONSTITUTION: A bypass port 16 is formed in a bypass passage 14 so as to be positioned in the downstream vicinity of a throttle valve 11 at idle operation time. A branch to the bypass passage 14 from a main fuel passage 6 is branched off into plural tributary passages 14a and 14b from the main fuel passage 6. The downstream sides of these respective tributary passages 14a and 14b are joined together, and are formed as a single bypass passage 14.