As the material for forming the expansible member 3, those having a certain extent of flexibility are preferred and there can be used, for example, thermoplastic resin such as polyolefin such as polyethylene, polypropylene, ethylene-propylene copolymer, ethylene-vinyl acetate copolymer and cross-linked ethylene-vinyl acetate copolymer, polyvinyl chloride, polyamide elastomer, polyester and polyure