An apparatus for measuring a concentration of carbon dioxide in water, comprising: a measurement cell including a carbon dioxide permeable section that allows permeation of carbon dioxide but that does not allow permeation of water when the measurement cell is immersed in water; a pH indicator solution with which the measurement cell is filled and the pH indicator solution establishing an equilibrium of: |Chemical| formula 7| HL - ⇄ H + + L 2- if the pH indicator solution is represented as the general formula H 2 L; a spectrum measuring apparatus adapted to measure an absorption spectrum of light absorbed by the pH indicator solution; an absorbance computing section adapted to compute from the absorption spectrum a first absorbance A 1 at a first peak absorption wavelength λ 1 which is equivalent to a concentration of the HL - in the pH indicator solution, a second absorbance A 2 at a second peak absorbance wavelength λ 2 which is equivalent to a concentration of the L 2- in the pH indicator solution, an isobestic point absorbance A 0 at an isobestic point wavelength λ 0 and a non-light-absorbing absorbance A b at a non-light-absorbing wavelength λ b; a pH value computing section adapted to compute a pH value of the pH indicator solution using a basic pH computation expression for computing the pH value of the pH indicator solution on the basis of a ratio (A 1 - A b) / (A 2 - A b) of a difference (A 1 - A b) between the first absorbance A 1 and the non-light-absorbing absorbance A b to a difference (A 2 - A b) between the second absorbance A 2 and the non-light-absorbing absorbance A b; a carbon dioxide concentration determining section adapted to determine the concentration of carbon dioxide in water from the computed pH value; and a deterioration determining section adapted to compute a change rate ΔA 0 of the isosbestic point absorbance A 0 and to determine that that the pH indicator solution is deteriorated when the change rate ΔA 0 is reduced by a predetermined reduction rate or more, wherein after the deterioration determining section determines deterioration of the pH indicator solution, the pH value computing section computes the pH value of the pH indicator solution using a corrected pH computation expression for correcting a deterioration of the pH value of the pH indicator solution on the basis of the change rate ΔA 0. a measurement cell including a carbon dioxide permeable section that allows permeation of carbon dioxide but that does not allow permeation of water when the measurement cell is immersed in water; a pH indicator solution with which the measurement cell is filled and the pH indicator solution establishing an equilibrium of: |Chemical| formula 7| HL - ⇄ H + + L 2- if the pH indicator solution is represented as the general formula H 2 L; a spectrum measuring apparatus adapted to measure an absorption spectrum of light absorbed by the pH indicator solution; an absorbance computing section adapted to compute from the absorption spectrum a first absorbance A 1 at a first peak absorption wavelength λ 1 which is equivalent to a concentration of the HL - in the pH indicator solution, a second absorbance A 2 at a second peak absorbance wavelength λ 2 which is equivalent to a concentration of the L 2- in the pH indicator solution, an isobestic point absorbance A 0 at an isobestic point wavelength λ 0 and a non-light-absorbing absorbance A b at a non-light-absorbing wavelength λ b; a pH value computing section adapted to compute a pH value of the pH indicator solution using a basic pH computation expression for computing the pH value of the pH indicator solution on the basis of a ratio (A 1 - A b) / (A 2 - A b) of a difference (A 1 - A b) between the first absorbance A 1 and the non-light-absorbing absorbance A b to a difference (A 2 - A b) between the second absorbance A 2 and the non-light-absorbing absorbance A b; a carbon dioxide concentration determining section adapted to determine the concentration of carbon dioxide in water from the computed pH value; and a deterioration determining section adapted to compute a change rate ΔA 0 of the isosbestic point absorbance A 0 and to determine that that the pH indicator solution is deteriorated when the change rate ΔA 0 is reduced by a predetermined reduction rate or more, wherein after the deterioration determining section determines deterioration of the pH indicator solution, the pH value computing section computes the pH value of the pH indicator solution using a corrected pH computation expression for correcting a deterioration of the pH value of the pH indicator solution on the basis of the change rate ΔA 0.