PURPOSE: To provide a seal with which a lid material, having enough strength for surely closing the container, is easily removable from a container without being left on the edge of a container.

CONSTITUTION: A container 2 is provided with a top opening set by a lip 4 extending outward. The container is closed by a lid material 6, and the lid material 6 is provided with a substrate layer 8 and a sealant layer 10 supported by the substrate layer for sealing the lip of the container. A heating platen 12 is used outwardly contacting the lid material to give an example of a heat source for forming a seal between the surfaces of sealant 10 and the lip 4 of the container. The sealant layer 10 is a blend of ethylene/unsaturated ester tercopolymer and erylene/unsaturated acid/unsaturated ester polymer.