To provide flexibility in a binder agent film so as to give flexibility to an electrode, and to improve adhesiveness of electrode group, and to improve the battery performance by providing fine cavity holes communicated with each other in a binder agent part existing in an active material.

Foaming agent for generating gas with heating is dissolved in the solution, which is composed of binder agent. This binder solution and the active material powder are mixed so as to obtain the paste, and this paste is coated on a conductive core material, and dried. At the time of drying, solvent in the binder agent solution is vaporized, and the binder agent is formed into a film. An active material is connected to each other and the active material and the conductive core material are connected to each other by the binding force of this film. Drying and heating are continued, and the foaming agent taken into the binder agent is foamed. With this foaming, fine cavity holes communicated with each other are provided in the binder agent film. With this structure, since utilization factor can be improved without lowering quantity of the binder agent, lowering of the binding force due to the lowering of the binder agent can be prevented, and fall of the active material in the electrode is prevented so as to prolong the lifetime.