The surface of a positive-electrode active material layer or a negative-electrode an active material layer is coated with a gel electrolyte composition to form a gel electrolyte layer. The positive and negative electrodes are laminated such that the gel electrolyte layer is sandwiched. At this time, the gel electrolyte composition is formed into a sol form so as to be applied to the surface of the positive-electrode active material layer or the negative-electrode active material layer. The viscosity of the gel electrolyte composition formed into the sol form is 1 cp to 50 cp. The gel electrolyte composition is formed into the sol form by heating the gel electrolyte composition or diluting the gel electrolyte composition with nonaqueous solvent. When dilution with the nonaqueous solvent is performed, solvent having a high boiling point and solvent having a low boiling point are mixed with each other. After a coating operation has been completed, the solvent having the low boiling point is removed by vaporization. It is effective to heat the positive electrode or the negative electrode when the gel electrolyte composition formed into the sol is applied. Since the gel electrolyte composition formed into the sol form is applied, the electrolytic solution is able to quickly penetrate the active material layer.