[Object] Behavior of a flow of foam or foaming agent ejected to a gypsum slurry can be stabilized, and a relatively large amount of foam or foaming agent can be homogeneously or uniformly dispersed in the slurry.
[Solution] A mixer (10) has a mixing area (10a) for preparing gypsum slurry(3), a slurry delivery section (4) for delivering the slurry from the mixing area, and a feeding port (60) for feeding foam (M) or a foaming agent to the slurry in the mixing area and/or the slurry delivery section under pressure. The slurry having the foam mixed therein is supplied to a production line (1) for forming gypsum boards or gypsum-based boards. The feeding port is provided with a partition member (62,64,65) dividing an ejecting region (61,61'). The ejecting region is divided into a plurality of openings (63), which simultaneously eject the foam or forming agent to the slurry.