A pretreatment mixing and stirring device, arranged at a stage prior to a slurry formation mixing and stirring device which forms a gypsum slurry by kneading a calcined gypsum and water, and performing a pretreatment by mixing a calcined gypsum with one or more kinds of additives selected from liquids and powders, comprising: 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 the calcined gypsum and the 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 plate or the bottom plate of the housing, and coupled to the rotating plate; a calcined gypsum supply port, arranged in the top plate, and configured to supply the calcined gypsum to the mixing and stirring region; and an additive supply port, arranged in one of or both of the top plate and the side plate, and configured to supply the additive to the mixing and stirring region, is provided.