The present invention provides an apparatus and a method for fractionating gypsum slurry which can surely control the density of the gypsum slurry fractionated from the mixer (fractionated slurry), which can restrict change of the flow rate of the fractionated slurry, and which can reduce the consumption of foam or foaming agent. The fractionation apparatus (30) fractionates the gypsum slurry from a mixer (4) for mixing calcined gypsum and water. The mixer has a hollow connector section (50), which allows the gypsum slurry to flow from a mixing area inside of the mixer into a chute section (5), and the chute section (5) discharging the inflow of gypsum slurry through its slurry discharge port onto a gypsum board liner paper. The fractionation apparatus has a slurry fractionation port (33) opening to the chute section or the hollow connector section, and causes a part of the gypsum slurry in the chute or hollow connector section to be delivered to a slurry delivery conduit (11,13,19).