As illustrated in FIG. 16A, when ah external force is applied to wave shape 203 formed in the surfaces of laminate films 201 and 202 in perpendicular direction 208, the laminate films have a state of being curved as illustrated in FIG. 16B. At this time, radiuses of laminate film 201 on the upper side and laminate film 202 on the lower side are different due to the curvature such that a difference between circumferential lengths (developed lengths) obtained in the case of being curved is generated.

In other words, the curved length, (circumferential length, of the laminate film is greater than the uncurved length), and a relationship of the uncurved length of the laminate film <the curved length (circumferential length) of the laminate film is established.

The difference between the circumferential lengths indicates wrinkling or cracking (breaking) on the surface of the laminate film. The wrinkling varies in size or shape depending on the state of curvature and the cracking has a significant influence on quality of a product, such as leakage of the electrolyte.

FIGS. 17A and 17B are diagrams illustrating a wrinkling mechanism.

As illustrated in FIG. 17A, when the electrode group is interposed between laminate film 201 on the upper side and laminate film 202 on the lower side, protrusion 209 is formed by the thickness of the electrode group. At this time, L1 represents a length of the top surface of protrusion 209.