PURPOSE: To prevent the output lowering of an engine at the time of the full admission of a slottle valve while keeping the gradient function of an engine in high.

CONSTITUTION: The upstream part of the venturi part 4 of a venturi tube 1 and the float chamber 8 of a carbureter in an engine are communicated with each other by an air vent passage 16. An inlet pipe 20 formed in a downward gradient is communicated with the upstream part of the venturi part 4 of the venturi tube 1. The opening end 16a of the air vent passage 16 is opened in the lower half wall part 16 of thd venturi tube 1 in the vicinity of the downward gradient terminal part of the inlet pipe 20, and also the halfway par 16b of the air vent passage 16 is formed on the upper half wall part 1a upstream of the venturi part 4 of the venture tube 1.