h shows the relationship between the welding stroke and welding area, and the welding stroke and unsealing force exerted by the rotation of the cap with respect to the welding portion represented in FIGS. 9A, 9B and 9C.FIG. 11 is a view which shows the arrangement of the welding portions of the cap represented in FIGS. 3A and 3B.FIGS. 12A and 12B are the upper surface views which illustrate the st