To form high-purity gypsum by restoration to a normal slurry state within a short time when an unreacted slurry is formed.

In forming gypsum by forming calcium sulfite from sulfur oxides contained in a flue gas discharged from a boiler 1 in, e.g., a thermal power station by treatment with a limestone slurry and oxidizing the calcium sulfite formed in an absorption tower 3 in an oxidation tower 4, a normal slurry state is restored by preventing the formation of an unreacted slurry by charging a limestone slurry into the absorption tower 3 while controlling the pH in the absorption tower 3, feeding oxygen into the oxidation tower 4 so as to optimize the gypsum slurry concentration in the oxidation tower 4, and altering the starting conditions of a gypsum concentration tank 5 and of a centrifuge 13.