The present invention provides a method for quantitatively determining a reducing substance, which comprises reacting a reducing substance in a test specimen with iron (III) ions, reacting iron (II) ions formed by reduction of the iron (III) ions or residual iron (III) ions with a metal indicator which is capable of reacting specifically with the iron (II) ions or the residual iron (III) ions to undergo color development, and carrying out quantitative determination by measuring the degree of color development, wherein a chelating agent which is specific to copper ions is added to the test specimen before the reaction of the reducing substance with the iron (III) ions; and a reagent used for it.