To provide a means to accurately stop discharge of a sodium-sulfur battery at the end of discharge even when suddenly changing discharge power.

In this method of a sodium-sulfur battery, when the formula (1): Vd≤VL is satisfied, the battery is determined to be at the end of discharge, and discharge is stopped. In the formula (1), Vd: a unit cell voltage (measured value [V]), VL: a cut voltage at the end of discharge of the unit cell, which is found by the formula (2): VL=Vddc - Rln(T) × Idl(t) - Rpl(T) × Idp(t). In the formula (2), Vddc: a designed open circuit voltage at the end of discharge of the unit cell (set value [V]), Rln: peak resistance at the temperature of T (ohmic component, set value (Ω)), Idl(t): a discharge current at the time t (measured value, [A]), Rpl(T): polarization resistance attendant on positive electrode diffusion at the temperature of T (set value (Ω)), Idp(t): a discharge current in consideration of a delay by polarization (value to which delay treatment is applied based on the discharge current measured at the time t [A]).

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