A process for continuously modifying dihydrate gypsum, said process including: (a) a hemihydration step of subjecting the dihydrate gypsum as a starting raw material to wet calcination or dry calcination at a calcination temperature from 100°C to 250°C to convert the dihydrate gypsum into hemihydrate gypsum, (b) a charging step of feeding the hemihydrate gypsum to an aqueous slurry in a recrystallization reaction tank, (c) a recrystallization step of hydrating and recrystallizing the hemihydrate gypsum into modified dihydrate gypsum of a crystalline form different from the dihydrate gypsum as the raw material, and (d) a discharge step of discharging a portion of the modified dihydrate gypsum from the recrystallization reaction tank, wherein: at least the steps (b) to (d) are conducted continuously or intermittently, a feed rate of the hemihydrate gypsum in the charging step (b) and a discharge rate of the modified dihydrate gypsum in the discharge step (d) are substantially equal to each other and are each not more than 20 % of a total amount of gypsum in the recrystallization reaction tank per hour, and in the recrystallization step (c), the aqueous slurry in the recrystallization reaction tank is maintained at a constant temperature not higher than 90°C under stirring such that the aqueous slurry becomes uniform to avoid solid-liquid separation, and the solid concentration of the slurry is from 10 to 50 wt%. (a) a hemihydration step of subjecting the dihydrate gypsum as a starting raw material to wet calcination or dry calcination at a calcination temperature from 100°C to 250°C to convert the dihydrate gypsum into hemihydrate gypsum, (b) a charging step of feeding the hemihydrate gypsum to an aqueous slurry in a recrystallization reaction tank, (c) a recrystallization step of hydrating and recrystallizing the hemihydrate gypsum into modified dihydrate gypsum of a crystalline form different from the dihydrate gypsum as the raw material, and (d) a discharge step of discharging a portion of the modified dihydrate gypsum from the recrystallization reaction tank, wherein: at least the steps (b) to (d) are conducted continuously or intermittently, a feed rate of the hemihydrate gypsum in the charging step (b) and a discharge rate of the modified dihydrate gypsum in the discharge step (d) are substantially equal to each other and are each not more than 20 % of a total amount of gypsum in the recrystallization reaction tank per hour, and in the recrystallization step (c), the aqueous slurry in the recrystallization reaction tank is maintained at a constant temperature not higher than 90°C under stirring such that the aqueous slurry becomes uniform to avoid solid-liquid separation, and the solid concentration of the slurry is from 10 to 50 wt%. at least the steps (b) to (d) are conducted continuously or intermittently, a feed rate of the hemihydrate gypsum in the charging step (b) and a discharge rate of the modified dihydrate gypsum in the discharge step (d) are substantially equal to each other and are each not more than 20 % of a total amount of gypsum in the recrystallization reaction tank per hour, and in the recrystallization step (c), the aqueous slurry in the recrystallization reaction tank is maintained at a constant temperature not higher than 90°C under stirring such that the aqueous slurry becomes uniform to avoid solid-liquid separation, and the solid concentration of the slurry is from 10 to 50 wt%.