PURPOSE: To prevent the contact of a coil-like return spring with the circumferential surface of a turning shaft and to prevent the generation of errors occurring in the frictional resistance and tilting that accompany with said contact by extending a bearing to the outer side of the end face of a casing and inserting said return spring onto the extended part thereof.

CONSTITUTION: A detector is designed to be fitted to the fitting part 1a on the body side of a carburetor 1 in the flange part 3a on the outside circumference at the forward end of a casing 3 made of a plastic. The stem 2a of a throttle valve 2 is extended to the side of the part 1a and a lever 2b of approximately an L shape is fixed in the end part thereof. A bearing 10 according to said invertion is extended forward from the surface at the forward end of the casing 3, and a coil-like return spring 9 is inserted onto the extended part 10a. The contact of the return spring with the circumferential surface of the turning shaft is prevented with such constitution and the generation of errors occurring in the frictional resistance and tilting that accompany with the contact is prevented.