Abstract: As opening/closing device to be provided at a mouth of a container such as a paper container, a bottle, a can, etc., in which a lid is provided via a hinge on the inside of a cylindrical pouring mouth made of synthetic resin and a gap space is formed between the pouring mouth and the lid. A metallic film consisting of a metal foil having synthetic resin layers coated on its both surfaces is bonded so as to straddle both the upper surface of the lid and the upper surface of the pouring mouth. Opening of the lid is effected by peeling of the metallic film, and upon closing the lid, the metallic film is pressed from its outer surface so that it may be stuffed in the gap space between the lid and the pouring mouth. Upon making the opening/closing device, an annular protrusion is formed on the top end surface of the pouring mouth, the metallic film is bonded to this annular protrusion and the lid so that the metallic film may be easily peeled off.