PURPOSE: To prevent a float from being reversely incorporated, by providing at least one permanent magnet to the float, and as well by creating a magnetic field which exerts a predetermined magnetic force on both sides of the float in the moving direction thereof.

CONSTITUTION: Permanent magnets 32, 33 are arranged integrally onto both end parts of a float 25 in the moving direction thereof. The above-mentioned float 25 is fitted into a recess 11 in an upper body 1 which is set with a lower body 2 in a pair disposed in press dies 34, 35. Since one of magnets 32, 33 tends to be attracted onto a surface 40 of the press die 34, the float 25 is magnetically secured. At this time even if the fitting direction of the float 25 is reversed, the float 25 is surely secured irrespective of the direction of the float 25 since the other magnet 32 tends to attract the surface 40 of the press die 34. Thereafter, fusing sections in the opening ends of both upper and lower bodies 1, 2 are heated and melted by means of a heating plate 41, and is fused together by means of the press dies 34, 35.