PURPOSE: To permit the treatment of a lumber, used especially for outdoor use, so as to be prominent in resistance to water, antiseptic effect and the like.

CONSTITUTION: Either one of aliphatic acid treating solution, produced by melting an aliphatic acid consisting of one to two kinds or more of palmitic acid, stearic acid, oleic acid and the like into water solution like soap, or an inorganic treating solution, produced by dissolving one to two kinds or more of zinc, lead, magnesium, copper, aluminum, sodium, potassium, calcium hydroxide and the like, is poured into an evacuated vessel. Then, a lumber is treated by a method wherein the treating solution is impregnated into the lumber by effecting heating treatment together with high moisture content lumber, evacuating operation, pressurizing operation and the like, then, the other inorganic treating solution or aliphatic acid treating solution is added to cause metathetical reaction, connect the aliphatic acid with inorganic substance and make them remain in the lumber as soap-like solid or powdered solid and, an insecticide, an antiseptic, a strain preventing resin solution and the like are added into one or both of the treating solutions.