To mix fuel with intaken air by stopping the fuel creeping up from a high speed fuel nozzle hole along a fuel adjusting needle valve by means of an annular groove, and then splashing the fuel into an intake passage by intake air flow.

A cover 12 is connected to an upper end wall of a carburetor body 40 sandwiching a membrane 15 so as to define a negative pressure chamber 14 above the membrane 15 and an atmospheric chamber 18 below the membrane 15, respectively. A piston type throttle valve 24 is inserted into a cylinder 20 arranged so as to be perpendicular to an intake passage 30 of the carburetor body 40 and an upper end of the piston type throttle valve 24 is connected to the membrane 15. A passage 36 for introducing intake negative pressure is formed on a bottom wall of the piston type throttle valve 24 so as to lift the piston type throttle valve 24 according to the intake negative pressure acting against the force of a spring 13. A fuel adjustment needle valve 29 projecting from the bottom wall of the piston type throttle valve 24 is inserted into a fuel supply is inserted into a fuel supply pipe 6 elongating from a fuel reservoir 32 positioned at a lower portion of the carburetor body 40 toward the intake passage 30. The fuel adjustment needle valve 29 is provided with annular grooves 41, 42 which are exposed to the intake passage 30 in a state of a medium opening of the piston type throttle valve 24.