To form the guide groove of a plate-form slide valve through integral casting of a carburetor body.

A piston comprises a cylinder part 20 having a diameter smaller than that of an intake passage; plate-form parts 21 (21a and 21b) protruding from a side extending along the slide direction of the cylinder part 20. The cylinder part 20 of the suction piston is guided in a contact state by an insertion hole 31 formed in the upper part of the intake passage. The guide surface 30b in a spot situated downstream of a plate-form part guide groove 30 is formed in parallel to the slide direction of the plate-form parts 21 (21a and 21b) and a gap between the guide surface and the surface 21b in a spot situated downstream of the plate-form parts 21 (21a and 21b) is formed in a manner to be decreased as much as possible. Further, the guide surface 30a in a spot situated upper stream of the plate-form part guide groove 30 is formed in a state that the guide surface makes no contact with the surface 21a situated upper stream of the plate-form parts 21 (21a and 21b) and forms a slope having a draft θ inclining in an upper stream direction toward above in the slide direction of the plate-form parts 21 (21a and 21b). A suction piston is formed such that the peripheral surface of the cylinder part 20 and only the surface 21b, situated downstream, of the plate-form parts 21 (21a and 21b) are brought into slide contact with a carburetor body to effect vertical movement and the suction piston effects smooth slide in a cast body.