To stably maintain a function of accurately controlling fuel flow rate by moving a throttle valve in a predetermined axis direction without receiving influence by engine heat with a cam mechanism disposed outside a carburetor body having a throttle valve cover made of synthetic resin.

A metal cam 22 is formed in a surface of a metal throttle lever 8 facing the carburetor body 1, and a metal driven piece 24 is projected on the metal carburetor body 1 and is brought into contact with a cam surface 23. By disposing the metal cam mechanism 21 in a metal component of the carburetor, the fuel flow rate control by a metering needle 10 and a nozzle opening 17 of a fuel nozzle 16 can be accurately performed without concerning deformation by the engine heat.