Behavior of a flow of foam ejected to a gypsum slurry can be stabilized, and a relatively large amount of foam can be homogeneously or uniformly dispersed in the slurry. A mixer has a mixing area for preparing gypsum slurry, a slurry delivery section for delivering the slurry from the mixing area, and a feeding port for feeding foam 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 for forming gypsum boards or gypsum-based boards. The feeding port is provided with a partition member dividing an ejecting region. The ejecting region is divided into a plurality of openings, which simultaneously eject the foam to the slurry.