To upgrade the surface smoothness of a final panel by heating a diluted slurry containing pulverized gypsum and host particles of a fiber reinforcement under pressure, then forming a beamlike calcium sulfate hemihydrate crystal and stacking the diluted slurry to the dehydrated into a filter cake.

A diluted slurry containing 70 wt.% fluid is prepared by adding pulverized gypsum and host particles of a fiber reinforcement, and this diluted slurry is equally supplied dispersedly onto a molding table by means of a head box 12. Next dehydration and baking treatments are performed in a vacuum box 14 to form a mat containing about 28-41 wet % moisture and the fluid is ejected from a plurality of nozzles of a fluid atomizing device 50, in the process of mat formation, to smooth the surface of the mat. On the molding table, a side dam retaining a slurry pond inside and a conveyor for moving the mat are provided. The mat is sent to a main press device 16 and a follow-up press device 18 and is shaped into a flat form with the removal of residual water.