PURPOSE: To generate a uniform tension continuously over the while length of film joining end, and greatly enhance the sealing performance.

CONSTITUTION: A film 30 is formed by joining together a plurality of film pieces 30a, which is performed as straddling each joining end of film pieces 30a through a first and a second holding member 33, 34 arranged on both film surfaces. These holding members 32, 34 are formed being continued in the direction of film joining. A main fastener 40 to couple them together is installed between the central parts of their mating surfaces 32a, 34a. An aux. fastener 42 to couple these holding members 32, 34 with the film 30 is installed continued between the ends of the holding member mating surfaces 32a, 34a and the two surfaces of each film joining end pinched by these two ends of mating surfaces.