PURPOSE: To enable to form a grain pattern which is the same as natural lumber, by applying sanding treatment to resin containing a cellulose series powdery aggregate.

CONSTITUTION: An aggregate (a) is made into fine powder of 80W200 mesh and water content is made so as to fall the same within 3%. After a molded resin synthetic material A has been heated for 24 hours in a temperature region of 60W130°C it is cooled naturally. As for hardening of the resin, it is so constituted that residual stress existing within the synthetic material A and contraction strain to be generated through density of a texture part are drawn out, a part corresponding to an (x) phase such as a surface skin layer which has been constituted so as to prevent warping and twisting of a ligneous synthetic material resulting from aged contraction is removed through sanding treatment and a (y) part, which is positioned to an inner part than a skin layer and possesses comparatively few residual inner stress, is exposed directly as the external surface. In addition to the above, as a scratch (g) is provided beforehand, a grain pattern which can not be discriminated with that of natural lumber is obtained.