To prevent deformation of a venturi forming part guide hole, a plate type guide groove and an air intake passage hole formed on a guide block at the time of forming the guide block of a synthetic resin material and fitting it in a guide block fitting hole of a carburetor main body.

A guide block G is formed of a synthetic resin material and is furnished with a venturi forming part guide hole 4, a plate part guide groove and an air intake passage hole 6. A first engagement part 11 orthogonal with a longitudinal groove with direction Y-Y of the plate part guide groove 5 is provided on one side outside surface 7A of an upper end part 7 of the guide block G facing the venturi forming part guide hole 4 and the plate part guide groove 5 and the other side outside surface 7B, and a second engagement part 12 is provided on a groove bottom part 5A of the plate part guide groove 5. The guide block G is fit in a guide block fitting hole 24, and the first engagement part 11 and the second engagement part 12 formed on the guide block G are inserted and arranged into a first engaged part 25 and a second engaged part formed on a carburetor main body corresponding to them.