The method of the present disclosure includes: preparing a solution containing a water-soluble polymer dissolved in a mixed solvent containing water admixed with a first solvent having a higher boiling point than that of the water; applying the solution in a film form to a surface of an electrode to form a coating made of the solution on the surface of the electrode; and removing the mixed solvent from the coating by vaporization such that a porous separator layer made of the water-soluble polymer is formed on the surface of the electrode while a plurality of pores are formed in an inside of the coating due to removal of the first solvent. The solubility of the water-soluble polymer in the first solvent is lower than that of the water-soluble polymer in the water.