To completely eliminate fuel in a float chamber provided with a fuel chamber recessed part of a holding bolt with means of a single drain operation for loosening a drain screw by arranging, in a drain passage, the drain screw for opening/closing the drain passage.

A holding bolt 1 is detachably arranged by screwing a male screw arranged on an outer periphery of the upper end 1A of the holding bolt 1 around a female screw arranged on a bottom part of a float chamber main body 14. A fuel chamber recessed part 1B which is opened to the upper end 1A as well as facing to a fuel control jet J, is arranged in the holding bolt 1, and is connected to an atmosphere through a drain passage 1C. A drain screw 2 for opening/closing the drain passage 1C is arranged in the drain passage 1C. When the drain screw is loosened and the drain passage 1C is opened, fuel reserved in the float chamber 15 is eliminated from the fuel chamber recessed part 1B of the holding bolt 1 to the atmosphere through the drain passage 1C.