PROBLEM TO BE SOLVED: To provide a free residual chlorine measurement device which makes it possible to take appropriate measure for a measurement error based on combined chlorine or electrode abnormality without forcibly imposing excessive burdens on a user.SOLUTION: When the ratio R(=ΔI/ΔV) of the absolute value (ΔI) of the amount of change in an oxidation-reduction current to the absolute value (ΔV) of the amount of change in an applied voltage in a plateau region is outside a prescribed range Kto K, the efficiency of polishing a detection pole by a polishing member is increased by raising the number of revolutions of a motor that rotates or drives the detection pole before polishing is performed. When the ratio R'(=ΔI'/ΔV') of the absolute value (ΔI') of the amount of change in the oxidation-reduction current to the absolute value (ΔV') of the amount of change in the applied voltage after polishing with increased efficiency is outside a prescribed range K' to K', abnormality information is generated.SELECTED DRAWING: Figure 2
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