PURPOSE: To restrain a decrease in pressure difference utilized for the introduction of auxiliary air such as bleed air to reduce the turning of mixture into excessive richness in the case of increase in the air feed resistance of an intake passage by adopting a means for giving the whole pressure not influenced from pressure loss in the intake passage to an intake port for the auxiliary air.

CONSTITUTION: A thin gap structure 7 serving as an air cleaner is provided in a intake passage 5, and a carburetor 9 is connected to its downstream side. A fuel passage 14 which is opened to an intake barrel 8 at one end and to a fuel reserve well 10 at the other end is provided in the carburetor 9. A fixed oil face 11 is formed in the fuel reserve well 10, and a fixed oil face upper chamber passage 13 is connected to a fixed oil face upper chamber 12 above the fixed oil face 11. The fixed oil face upper chamber passage 13 is connected to the portion of a car body 4 on which the summed up pressure of ram pressure and static pressure acts through the whole pressure introducing passage 21 for a fixed oil face upper chamber. In addition to that, an auxiliary air passage 18 for introducing the auxiliary air is connected to the fuel passage 14, and moreover connected to the portion of the car body 14 on which the summed up pressure of the ram pressure and static pressure acts through the whole pressure introducing passage 19 for auxiliary air.