A method for preventing occurrence of a deactivation phenomenon caused by formation of an inert film on a surface of limestone contained in a flue-gas desulfurization unit that uses a wet limestone-gypsum method and treats flue gas of a coal-fired boiler, characterised in that the method comprises: calculating a deactivation potential as an index of the deactivation phenomenon based on the content of Al, Si and alkaline components including Ca, Mg, Na and/or K contained in ash in the flue gas; and performing an operation management of the flue-gas desulfurization unit depending on change of dissolution rate of Al or Si estimated by the deactivation potential, wherein the inert film is generated by ash, wherein the operation management is to make an adjustment of at least any of a pH control system, a wastewater control system, and a chemical addition control system of the flue-gas desulfurization unit, wherein the dissolution rate of Al in the ash is estimated by the deactivation potential as represented by the following formula 2: Dissolution rate of Al ∝ diffusion coefficient × specific surface area × deactivation potential wherein the deactivation potential is calculated by the following formula 3: Deactivation potential = Al content of ash × quantitative factor × qualitative factor wherein the quantitative factor is a vitrification ratio of Al, and the qualitative factor is an atomic arrangement structure of glass of Al, wherein the quantitative factor and the qualitative factor correlates with an MI defined by the following formula 5: MI = 2 Ca + 1.5 Mg + 2.04 Na + 2.67 K / Al wherein Ca, Mg, Na, K, and Al each represent its molarity in fly ash, wherein when MI is in the range of 0 to 1, the quantitative factor is proportional to MI, and when MI exceeds 1, the quantitative factor remains 100%, wherein when MI is 1 or higher, the value of MI is directly adopted as the qualitative factor, and when MI is less than 1, 1 is uniformly adopted as the qualitative factor regardless of the value of MI, wherein the dissolution rate of Si in the ash is estimated by the deactivation potential as represented by the following formula 6: Dissolution rate of Si ∝ diffusion coefficient × specific surface area × deactivation potential × F MI wherein the coefficient F reflects the difference of the relationship between MI and the vitrification ratio of Al from the relationship between MI and the vitrification ratio of Si. wherein Ca, Mg, Na, K, and Al each represent its molarity in fly ash, wherein when MI is in the range of 0 to 1, the quantitative factor is proportional to MI, and when MI exceeds 1, the quantitative factor remains 100%, wherein when MI is 1 or higher, the value of MI is directly adopted as the qualitative factor, and when MI is less than 1, 1 is uniformly adopted as the qualitative factor regardless of the value of MI, wherein the dissolution rate of Si in the ash is estimated by the deactivation potential as represented by the following formula 6: Dissolution rate of Si ∝ diffusion coefficient × specific surface area × deactivation potential × F MI wherein the coefficient F reflects the difference of the relationship between MI and the vitrification ratio of Al from the relationship between MI and the vitrification ratio of Si.