A material (hereinafter referred to as “positive electrode material”) including sodium manganate powder as a positive electrode active material, carbon black powder as a conductive agent, and polytetrafluoroethylene as a binder is prepared. The positive electrode material is mixed in an N-methylpyrrolidone solution to produce slurry as a positive electrode mixture. A working electrode is produced by applying the slurry on a positive electrode collector. A negative electrode containing tin or germanium is produced. The non-aqueous electrolyte is produced by adding sodium hexafluorophosphate as an electrolyte salt in a non-aqueous solvent produced by mixing ethylenecarbonate and diethyl carbonate.