To stabilize fuel flow rate in a single fuel system for feeding fuel controlled by a measuring needle interlocking with a throttle valve to an intake passage by mixing bleed air with the fuel.

Effective area of a measuring hole 16 is regulated by the measured needle 23 interlocking with the throttle valve 3, and the fuel entering a mixing chamber 19 from a constant fuel chamber 9 after passing a measuring hole 16 for a flow rate control of the fuel is mixed with bleed air and is fed from a nozzle port 17 to the intake passage 2. The mixing chamber 19 has capacity to absorb and mitigate fluctuation of negative pressure applied to the nozzle port 17, sucks the fuel by stable negative pressure and supplies air- fuel mixture by a prescribed air-fuel ratio over the entire operation region of an engine.