To obtain an architectural material from which a colored slurry layer is hardly peelable by supplying a colored slurry to the surface of a hydraulic slurry before completion of a hydraulic slurry dehydration step.

A hydraulic slurry S is fed onto a transfer belt 1a from a flow box F and then is molded into a plate form. A green panel 4 for an outer wall equipped with a colored decorative layer 4b is molded by supplying a colored slurry T to the surface of the hydraulic slurry S before completion of a step to dehydrate the hydraulic slurry S. Both colored slurry T and hydraulic slurry S are mixed in such a state that they are very easily miscible with each other. In addition, the colored decorative layer 4b and a base layer 4a are integrally bound tightly together by sucking action to them. Consequently, a clearly defined interface between both layers is hardly created and thus it is possible to form an outer wall panel which is almost free from the generation of freezing damage due to a water seepage into the laminar boundary.