To provide a cam part for turning a turn valve element being a molded product and displacing it in the direction of axial line inside a carburetor main body.

A rib 3b in the radial direction for preventing molding distortion of a recessed part 3f formed by thinning is formed in a part where a cam body 21 in the turn valve element 3 is assembled. Centering of the cam body is performed from a protruding part for centering provided on an inner peripheral face of an outer peripheral wall in the recessed part and the cam body is positioned in the direction of axial line by an end in the direction of protrusion of the rib. Since the cam body can be assembled on a lower face of the turn valve element, the cam body can be arranged on an inside side of the carburetor of the turn valve element even in a structure provided with thining and the rib for preventing molding distortion of the turn valve element made of a synthetic resin. For this reason, a contact part of a cam face of the cam body and a cam slide-contact piece is positioned in the carburetor main body to improve durability without nipping dust.