To improve the reliability of a sheet material by controlling the elongation of the material.

In a weld part, a welding tape 12 is arranged between a resin layer 36 and a gas barrier sheet 10. In a welding process, the gas barrier sheet 10 is cut on a welding surface by the heat and pressure of the resin layer which is softened into gel, one portion of the cut sheet 10 is extruded to the peripheral part of a container by the softened resin layer 36, and the portion connected with a main body part is extruded to the inside part of the container together with the unwelded tape 12. In this way, the gas barrier sheets 10 are welded together on the welding surface, at the same time, the resin layers 36 are welded similarly.