To suppress the lowering of intake efficiency and excessive concentration in air-fuel mixture concentration by disposing a collision buffering member formed out of a material of low hardness at a contact part between a cover and a negative pressure operation valve at the time of the maximum opening of the negative pressure operation valve.

A negative pressure operation valve guiding cylinder 3 is formed continuously at an intake passage 2 which penetrates a carburetor body 1. A negative pressure operation valve 6 which conducts variable control of the area of a venturi part V is disposed at the negative pressure operational valve guiding cylinder 3 so as to move freely. The valve part 6A, the retainer 6D, and the cover 8 of the negative pressure operation valve 6 are formed by synthetic resin material which contains glass fiber. A collision buffering member 10 is disposed at a contact part between the upper part of the negative pressure operation valve 6, and the bottom part of the cover 8 facing it, at the time of the maximum opening where the negative pressure operation valve 6 keeps the intake passage 2 in a fully opened condition. The collision buffering member 10 is formed out of a material of lower hardness than the synthetic resin material which forms the negative pressure operation valve 6 and the cover 8. It is thus possible to suppress the lowering of intake efficiency, and excessive concentration phenomenon in air-fuel mixture.