PURPOSE: To guide a negative-pressure operating valve into a carburetor body in slidable ways exceedingly certainly and easily by arranging the negative- pressure operating valve through a guide part.

CONSTITUTION: A guide member 7 having a Venturi forming part guide groove 7A, plate-shaped guide groove 7B, and a penetration hole 7C is inserted into a guide groove 6 formed on the concaved part of the atmosphere chamber of a carburetor body 1. Therefore, the Venturi forming part 17A and the plate- shaped guide part 17C of a negative-pressure operating valve 17 are guided and held in slidable ways. Particularly, a plate-shaped guide groove 7E which can be hardly casted with the carburetor body 1 is formed on the guide member 7, and it is enough only to form a plate-shaped guide groove 6B having the increased plate width in comparison with that of the plate-shaped guide groove 7E on the carburetor body 1, and thus casting of the plate-shaped guide groove 6B with the carburetor body 1 is exceedingly facilitated.