To improve durability of a throttle valve supporting shaft dust-proof seal for a turning throttle valve type carburetor.

The dust-proof seal 31 protects a clearance between a supporting shaft 24 of a turning valve body 3 and a shaft supporting part 23a for supporting the shaft from dust. The dust-proof seal 31 is formed to be a cylindrical shape whose diameter is enlarged at the intermediate part, and an outward flange part 23b is provided at an intermediate part of the shaft supporting part 23a. Thereby, as the dust-proof seal can be engaged with and mounted to the turning support shaft and the shaft supporting part at three points, its both ends and its intermediate diameter-enlarged part 31a, it can retain a dust-proof performance more securely than that engaging only at its both ends can. The diameter of the both ends of the dust-proof seal is made smaller than that of the intermediate part, which can prevent an opening end of the shaft supporting part side from moving over a radial direction outwardly projecting part so as to make the clearance between the turning support shaft and the shaft supporting part always covered with the dust-proof seal. As the durability of the dust-proof seal is improved as well as the dust-proof function, the seal need not to be changed.