To hold a good negative pressure responsiveness for a long time in an elastic ring fitted in a ring groove of a negative pressure operated valve by setting a radius of the outer periphery on the side of free end to be almost equal to a radius of a negative pressure cylinder hole and setting a radius of continuous end larger than a radius of the negative cylinder hole.

In a constant vacuum carburetor a negative pressure operated valve guiding cylinder 9 is opened in a negative pressure cylinder hole 8. In the cylinder 9 a negative pressure operated part 11A is fitted. At an upper part of the part 11A an annular part 11B is formed, and an elastic ring 12 is installed in a ring groove 11C of the part 11B. In the ring 12 a specified circular-arc length part on an opposing opening end side is cut in a flat shape so that they are overlupped with each other to form a stepped shape, and the stepped part serves as an abutment part. In this case, in a free state, the radius of outer periphery on the side of free end in the vicinity of the abutment part is set almost equal to the radius of the hole 8 while the radius of continuous side outer periphery is set larger than the radius of the hole 8.