PURPOSE: To improve the extent of combustibility by constituting a part of combustion air flow counted as bleed air so as to be used in a slow zone, and maintaining an air-fuel characteristic of mixture so well, while making the bleed air in proper pressure chargeable at each zone.

CONSTITUTION: In a slow zone, a slow system bleed air passage 28 and a bleed air passage 27 are connected to each other via a notch 26a of a throttle shaft 26, and a part of air flow in negative pressure P2 measured to fuel being discharged to the bleed air passage 27 via a fuel discharge passage 19 after passing through an opening 2a of an air valve 2 is led into the slow system bleed air passage 28 from an intake port 28b as bleed air, and it is made so as to flow into the bleed air passage 27. In a stage of transition to a main zone where a throttle valve 6 is further rotated, both slow and main bleed air passages 28, 30 are connected to the bleed air passage 27 by rotation of the notch 26a.