Disclosed is a process for the continuous modification of dihydrate gypsum. The process includes a hemihydration step of calcining the dihydrate gypsum as a raw material into hemihydrate gypsum and a recrystallization step of hydrating and recrystallizing the hemihydrate gypsum in an aqueous slurry to convert the hemihydrate gypsum into modified dihydrate gypsum of a crystalline form different from the dihydrate gypsum as the raw material. In the recrystallization step, the aqueous slurry in a recrystallization reaction tank is maintained at a constant temperature under stirring such that the aqueous slurry becomes uniform, and a feed rate of the hemihydrate gypsum to the recrystallization reaction tank and a discharge rate of the recrystallized and modified dihydrate gypsum from the recrystallization reaction tank are controlled such that the feed rate and the discharge rate become substantially equal to each other, whereby the feeding of the hemihydrate gypsum and the discharging of the recrystallized and modified dihydrate gypsum are conducted continuously or intermittently. According to the process, 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 although such modified dihydrate gypsum is not available in general.