PURPOSE: To exhaust electrolyte remaining inside of a sealing plate perfectly to outside of a battery by washing the battery after its formation using a wash solvent having an equal or lower boiling point to/than that of the electrolyte solvents which has the lowest boiling point among those contained.

CONSTITUTION: A battery concerned is washed with a wash solvent having a lower boiling point than any electrolyte solvent, and thereby the organic solvent or solvents having higher boiling point are mixed with this wash solvent having low boiling point to form a sort of co-boiling mixture, which can be splashed to outside the battery easily along with gasification of the wash solvent. Otherwise, the battery is washed with water at a higher temp. than the boiling point of the electrolyte, and thereby the solvent is gasified to generate splash of electrolyte easily to outside of the battery. This prevents crystals of the solutes from attaching to the surface of the sealing plate.