The whole of a conductive material layer is formed on a bonding surface of an outer film having flexibility and barrier properties to prepare a composite body in which the conductive material layer is integrated with the outer film. A current collector is located within an application region, and at least a part of an electrode terminal is located outside the application region. A positive electrode active material precursor layer, an electrolyte precursor layer, and a negative electrode active material precursor layer are added to the composite body with plane positions thereof aligned with that of the current collector. These precursor layers are subjected to a crosslinking process. The application regions are applied, and the outer films are bonded, to seal a cell. The crosslinking process may be omitted.