A current collector is covered with sodium metal through: (1) applying a sodium dispersion containing sodium metal and at least one substance selected from the group consisting of an imide salt and a binder, on a current collector in an inert gas environment (with an oxygen concentration of not more than 0.01% and a dew point of not more than −10° C.), followed by heating and drying; (2) pressure bonding a piece of solid sodium metal having a surface which exhibits a metallic luster onto a current collector in the aforementioned inert gas environment; (3) vapor-depositing sodium metal on a current collector in a reduced pressure environment; or (4) immersing a current collector having a surface fired at a temperature ranging from 150 to 300° C. in molten sodium metal after removing a coating film which is generated on a surface and formed from impurities, in the aforementioned inert gas environment.