An unhardened inorganic board is shaped by press clamping the surface of an unhardened inorganic mat having a colored surface layer using an embossing template. A semi-hardened inorganic board is obtained by performing primary curing of the unhardened inorganic board, and coating the surface of the semi-hardened inorganic board with a primary sealer. The semi-hardened inorganic board coated with the primary sealer is autoclave cured to obtain a hardened inorganic board. The surface of the hardened inorganic board is coated with a powdery or granular material fixing sealer and, thereafter, an ornamental powdery or granular material is dispersed onto the surface of the hardened inorganic board. The surface of the hardened inorganic board on which the ornamental powdery or granular material has been dispersed is coated with a finishing transparent paint.