PURPOSE: To obtain high barrier properties to vaporized radioactive isotope by a method wherein polyolefin layer and/or copolymerized polyethylene terephthalate layer are laminated to at least one side of polyester layer, which is mainly made of ethylene terephthalate unit.

CONSTITUTION: As the polyester used for the polyester A layer consisting of the laminated film concerned, ethylene terephthalate unit is mainly employed and, when necessary, copolymerized polyester, which is prepared by copolymerizing said unit and other component, is empolyed. As the polyolefin polymer used for the polyester B layer, polypropylene, low density, high density, straight chain low density polyethylene, poly(4-methy pentene-1), ethylene-vinyl acetate copolymer or the like is exampled and preferably the polyolefin polymer, which can have heat sealing properties. Further, as the copolymerized polyester C layer, one, which contains copolymer component excluding ethylene terephthalate and contains respectively 6-40mol% of the copolymer component in total carboxylic acid component and in diol component, is preferable.