A wet type flue gas desulfurization apparatus comprising: an absorber that removes sulfur oxide in a flue gas with an absorbent ; an absorbent storing unit that stores the absorbent that has absorbed the sulfur oxide; a circulation line (L 11 ) that circulates the absorbent from the absorbent storing unit ; a spouting unit that is provided in a vicinity of a middle part of the absorber and spouts the absorbent supplied by the circulation line (L 11 ) upward as a spouting liquid through a spraying unit or drops the absorbent downward as liquid droplets directly from a spraying unit ; an oxidation-reduction potential meter that measures an oxidation-reduction potential of the absorbent ; an air introducing unit that supplies air into the absorbent storing unit of the absorber to adjust oxidation-reduction potential of the absorbent to be an appropriate value; a reducing additive supplying unit that supplies at least one of sodium thiosulfate, sodium metabisulfite, and sodium dithionite as a reducing additive into the absorbent or the absorbent storing unit ; an absorbent supplying unit that supplies the absorbent to the absorbent storing unit located in the bottom of the absorber ; and a control device that controls the reducing additive supplying unit based on a measured value of the oxidation-reduction potential of the absorbent measured with the oxidation-reduction potential meter , wherein the control device controls the reducing additive supplying unit to supply the reducing additive when the measured value of the oxidation-reduction potential has exceeded an upper limit of an appropriate range of the oxidation-reduction potential even if the amount of the air supplied by the air introducing unit into the absorbent storing unit is reduced to zero. an absorber that removes sulfur oxide in a flue gas with an absorbent ; an absorbent storing unit that stores the absorbent that has absorbed the sulfur oxide; a circulation line (L 11 ) that circulates the absorbent from the absorbent storing unit ; a spouting unit that is provided in a vicinity of a middle part of the absorber and spouts the absorbent supplied by the circulation line (L 11 ) upward as a spouting liquid through a spraying unit or drops the absorbent downward as liquid droplets directly from a spraying unit ; an oxidation-reduction potential meter that measures an oxidation-reduction potential of the absorbent ; an air introducing unit that supplies air into the absorbent storing unit of the absorber to adjust oxidation-reduction potential of the absorbent to be an appropriate value; a reducing additive supplying unit that supplies at least one of sodium thiosulfate, sodium metabisulfite, and sodium dithionite as a reducing additive into the absorbent or the absorbent storing unit ; an absorbent supplying unit that supplies the absorbent to the absorbent storing unit located in the bottom of the absorber ; and a control device that controls the reducing additive supplying unit based on a measured value of the oxidation-reduction potential of the absorbent measured with the oxidation-reduction potential meter , wherein the control device controls the reducing additive supplying unit to supply the reducing additive when the measured value of the oxidation-reduction potential has exceeded an upper limit of an appropriate range of the oxidation-reduction potential even if the amount of the air supplied by the air introducing unit into the absorbent storing unit is reduced to zero. <EOS>