To realize the manufacturing of a favorable quality decorative plate with fine external appearance finish with good productivity and without using decorative aggregate by a method wherein one color slurry is poured in lumps or in lines in the other color slurry at the placing of two or more slurrys different in color formed of specified powders and aqueous solution in a mold for hardening.

One color slurry A and the other color slurry B, both of which are respectively formed of SiO2-Al2O3 powder and the aqueous solution of an alkali metal silicate, are placed in a vessel and agitatingly mixed with each other. At the pouring of both the slurrys A and B mixed with each other in a forming mold 11 for hardening, the fluidity of one color slurry A is made different from that of the other color slurry B so as to not completely mix with each other, resulting in allowing to pour one color slurry A having smaller fluidity into the other color slurry B having larger fluidity in lines. Accordingly, a polychrome decorative plate, on the surface of which a polychrome pattern is applied with no mixing of colors, can by manufactured with good productivity by simple facilities.