The positive electrode active material of a positive electrode includes a sodium-containing transition metal oxide (Na.sub.aLi.sub.bM.sub.xO.sub.2.+-..alpha.). The M includes at least two of manganese (Mn), iron (Fe), cobalt (Co), and nickel (Ni). For a negative electrode, a sodium metal or a metal that forms an alloy with sodium is used. A non-aqueous electrolyte produced by dissolving an electrolytic salt (sodium salt) in a non-aqueous solvent is used. Examples of the non-aqueous solvent may include a cyclic carbonate, a chain carbonate, esters, cyclic ethers, chain ethers, nitrites, amides and a combination thereof.