To prevent extensive change of a fuel flow in a fully opened position of a rotary throttle valve of a carburetor even with uneven hole diameters of a fuel jet.

The rotary throttle valve 5 having a throttle hole 6 is fit in a cylindrical valve chamber 4 perpendicular to an intake passage 3 horizontally penetrating a carburetor body 2. A needle 16 supported on the rotary throttle valve 5 is fit in a fuel feeding pipe 7 protruding toward the throttle hole 6 from a constant pressure fuel chamber 29 in a bottom part of the carburetor body 2. It is composed so that an opening of the throttle hole 7a to the intake passage 3 is increased by rotation of the rotary throttle valve 5 and an opening of a fuel nozzle hole 7a is increased by lifting of the rotary throttle valve 5. A baffle plate 8 is provided facing the fuel nozzle hole 7a.