To improve a tensile strength, a flexural strength, retention of a nail and a screw, and impact resistance of a plasterboard or a building product.

This method comprises a step for adding methylenediphenyl diisocyanate(MDI) as a stable emulsion under a condition capable of maintaining a slurry when adding a diisocyanate, especially the MDI to the aqueous slurry comprising a calcium sulfate material and a cellulose fiber. The method further comprises a step for forming a filter cake by passing the slurry containing the MDI through a flat and porous forming face, a step for removing a large amount of water from the filter cake through the porous forming face, a step for pressing the filter cake to form a board and further to remove the water content, and a step for drying the resultant board to remove the remaining free water.