PURPOSE: To prevent separation of a float from a guide cylinder by a method wherein first and second half-bodies are pressed against the surface and the back of a heating plate inserted between the two half-bodies, and after a weld surface and the regulation part of the tip of the guide cylinder are simultaneously formed to the two half-bodies through thermal caulking, the weld surfaces of the first and second half-bodies are pressed against each other for welding.

CONSTITUTION: When first and second half-bodies 7 and 8 are pressed against the surface and the back of a heating plate 27, a flange part 7a to the end surface of which a weld surface 7b is formed is formed to the opening edge of the first half-body 7 through thermal caulking. A regulating part 25 is formed to the tip part of a guide cylinder 15 through thermal caulking and further, a flange part 8a to the end surface of which a weld surface 8b is formed is formed to the opening edge of the second half-body 8 through thermal caulking. In this case, by pressing the first and second half-bodies 7 and 8 against each other after the heating plate 18 separated from between both half-bodies 7 and 8, weld surfaces 7b and 8b are welded to each other to form a reservoir body 5.