An object of the invention is to carry out the flux method with improved work efficiency while maintaining the purity of flux at high level and saving flux material cost. The sodium-purifying apparatus includes a sodium-holding-and-management apparatus for maintaining purified sodium (Na) in a liquid state. Liquid sodium is supplied into a sodium-holding-and-management apparatus through a liquid-sodium supply piping maintained at 100° C. to 200° C. The sodium-holding-and-management apparatus further has an argon-gas-purifying apparatus for controlling the condition of argon (Ar) gas that fills the internal space thereof. Thus, by opening and closing a faucet at desired timing, purified liquid sodium (Na) supplied from the sodium-purifying apparatus can be introduced into a crucible as appropriate via the liquid-sodium supply piping, the sodium-holding-and-management apparatus, and the piping.