To stabilize idle running and to improve fuel consumption by preventing the occurrence of a run on phenomenon caused by a wall surface flow and greatly reducing the fluctuating width of an engine revolution number during idle running in a variable Venturi type carburetor.

A bypass port 16 positioned in the vicinity of the downstream side of a throttle valve 11 during idle running is formed in a bypass passage 14. A tube 17 is provided in the bypass passage 14 so as to be positioned in the vicinity of the upstream side of the bypass port 16. Flow passages 17c and 17d for discharging fuel in the upstream side from the side of the tube 17 are formed in the tube 17.