The sodium molten salt battery includes a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, a separator arranged between the positive electrode and the negative electrode, and a molten salt electrolyte having sodium ion conductivity, in which the positive electrode active material contains a sodium-containing transition metal oxide, the negative electrode active material contains hard carbon, and the ratio of the reversible capacity Cn of the negative electrode to the reversible capacity Cp of the positive electrode, i.e., Cn/Cp, is 0.86 to 1.2.