PURPOSE: To prevent impregnated resin from being pushed out by heating, hollow from developing in the block concerned, the strength of the block from lowering and moisture from being absorbed in the block by a method wherein lumber is heat- treated so as to be brought into the state in which no moisture presents at all, or absolute dry state before the impregnation with the resin.

CONSTITUTION: In drying process 10, lumber, which is cut in the predetermined length, is housed in a drying chamber and heated intensely under the air-isolated state for a long period of time so as to be brought into the state in which no moisture presents at all ar absolute dry state. In impregnating process 20, the dried lumber is housed in a tank so as to discharge air in tissue under reduced pressure and, after that, immerse the lumber in an unsaturated polyester resin-filled vessel in order to infiltrate the resin in the tissue by bringing the pressure in the tank again to normal pressure. In hardening process 30, the lumber 1 is taken out of the tank and housed in a high temperature chamber, the temperature in which is 150-180°C, for about 30min in order to thermoset the unsaturated polyester resin. Thus, in the process 30, the moisture left in the lumber tissue is vaporized, resulting in preventing impregnated resin from being pushed out of the limber, hollow from developing in the wood block, the strength of the block from lowering, moisture from being absorbed in the block and the block from being rotten.