PURPOSE: To improve a sensitivity of air breeding and make it uniform by a method wherein a downstream side of the air breed is made to have such a diameter as enabling an ignoring of a throttle effect.

CONSTITUTION: Air breed inlet port 12a is arranged near an inlet port of the air intake passage 3 and air breed outlet port 12b is arranged downstream near the jet part 7a of the fuel jet 7. Air breed 7b in the fuel jet 7 containing an air breed outlet port 12b constitutes a throttle part and its diameter is made slightly larger than a diameter of the throttle in the jet part 7a. Annular throttle area formed by the air breed part 7b and the metering needle 11 is the narrowest one when an idling operation of the engine is performed. The value of area is set to a larger value than a throttling area of the air breed passage 12 to be controlled to a value required for controlling an air fuel ratio by the actuator 15. The fuel jet 7 does not generate any unstable phenomena in the mixture of the air and fuel flow due to the fact that no throttle is arranged downstream of the air breed 7b.