To pressurize a unit cell group without damaging a film by compressing and packing the unit cell group by a thermally contractible film under a condition where unit cell group pressurizing springs are arranged on its layering directional both end surfaces.

A unit cell group 2 is compressed and packed by a thermally contractible film 6 in a state of being adhered by a double coated tape by arranging unit cell group pressurizing springs 5 on its layering directional both end surfaces. At this time, a unit cell 1 is sealed by wrapping a plate group retained between a positive electrode plate and a negative electrode plate with a synthetic resin film. The springs 5 are composed of flat springs, and its central parts 5a are curved so as to project to the end surface side of the unit cell group 2, and both end parts 5b are rounded inward. In this way, the springs 5 are arranged in the layering directional both ens surfaces of the unit cell group, and in a state of being adhered by a double coated tape, these layered bodies are wrapped with the film 6, and the film 6 is heated by a hot blast, and the springs 5 are put in an energizing condition.