To smooth the operation of an inlet piston and stabilize the flow rate of a fuel by fitting a guide member of a venturi body for the inlet piston into a slit guide groove for preventing the occurrence of a swirl flow at both sides of an inlet passage.

A carburetor 15 is separated into two bodies, a first body 31 at the upstream side and a second body 41 at the downstream side, the first body 31 being fitted into a mounting recessed portion of the second body 41. A first peripheral wall face 17A and a second peripheral wall face 17B of a guide hole 17 of the venturi body 2 and a first side face 18A and a second side face 18B of the guide groove 18 of the guide member 12 are formed on the fitted-in end face of the first body 31 and on the corner face of the mounting recessed portion, respectively. Thus, the operation of the inlet piston 1 is smoothed on a surface requiring no size correcting work even if no cast taper exist.