PURPOSE: To provide more uniform air-fuel mixing by promoting the atomization and fineness of fuel while ensuring appropriate fuel discharge quantity corresponding to benturi opening D.

CONSTITUTION: The upstream pressure operating chamber 21a and the upstream side suction path 1a are communicated via the upstream pressure communication path 22, and the downstream pressure operating chamber 21b and the downstream side suction path 1b are communicated via a boost pressure communication path 23. The downstream pressure operating chamber 21b and the float chamber 8 in a float chamber 7 are communicated via a float chamber communication path 12. A communication valve 30 is provided on the float chamber communication path 12, and is composed so as to increase valve opening as well as to reduce benturi opening D while interlocking a pressure operating valve 17.