An oxidation-reduction substance equilibrium potential estimating method is provided, the method including: applying a voltage to an electrode contacting a sample containing an oxidation-reduction substance and sweeping the voltage; measuring a current flowing through the electrode; if an integrated value of the current becomes a value within a reference range, determining whether to sweep the voltage in an opposite direction to a sweep direction in the previous sweeping or to terminate sweeping of the voltage; if it is determined to terminate sweeping of the voltage, estimating an oxidation-reduction substance equilibrium potential at a value of the voltage; and if it is determined to sweep the voltage, sweeping the voltage in an opposite direction to a sweep direction in the previous sweeping.
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