A process for the continuous modification of dihydrate gypsum includes calcining dihydrate gypsum into hemihydrate gypsum and recrystallizing the hemihydrate gypsum in an aqueous slurry to convert the hemihydrate gypsum into modified dihydrate gypsum of different crystalline form. The aqueous slurry in a recrystallization reaction tank is maintained at a constant temperature under stirring, and a feed rate of the hemihydrate gypsum to the recrystallization reaction tank and a discharge rate of the recrystallized dihydrate gypsum are controlled to substantially equal. Dihydrate gypsum as a starting raw material can be converted into high-purity, modified dihydrate gypsum of large crystals by recrystallizing the starting dihydrate gypsum after it is once calcined into hemihydrate gypsum.