A pretreatment mixing and stirring device includes a cylindrical housing having a top plate, a bottom plate arranged to oppose to the top plate, a side plate arranged between the top plate and the bottom plate, and a mixing and stirring region where a calcined gypsum and an additive are mixed and stirred in an interior surrounded by the top plate, the bottom plate, and the side plate, a disk-shaped rotating plate arranged inside the housing, a rotating drive shaft, penetrating the top or bottom plate, and coupled to the rotating plate, a calcined gypsum supply port, arranged in the top plate, and supplying the calcined gypsum to the mixing and stirring region, and an additive supply port, arranged in at least one of the top and side plates, and supplying the additive to the mixing and stirring region.