To provide a method for predicting a deactivation phenomenon in a flue-gas desulfurization unit to prevent the occurrence of the deactivation phenomenon before it happens.There is provided a method for preventing the occurrence of a deactivation phenomenon in a flue-gas desulfurization unit that treats flue gas of a coal-fired boiler, the method includes calculating a deactivation potential as an index of the deactivation phenomenon based on alkaline components such as Na, Ca, Mg, and K contained in ash in the flue gas, and performing an operation management, such as adjustment of set value of a pH control system, on the flue-gas desulfurization unit depending on change of the deactivation potential.