PURPOSE: To improve the fire resistance, flame retardancy, strength and heat resistance of the wood concerned by a method wherein the wood, which is treated under heat and pressure so as to have specified equilibrium moisture content and anti-swell efficiency, is impregnated with alkyl silicate compound solution for hardening and polymerizing.

CONSTITUTION: Wood is treated under heat and pressure so as to lower the absolute value of the equilibrium moisture content (%) of initial wood by 2-4% and to make the anti-swell efficiency of the wood to 20-60%. After that, the treated wood is immersed in alkyl silicate compound solution so as to harden and polymerize the alkyl silicate compound in the woody material. In order to impregnate the woody material with the alkyl silicate compound solution, either one of vacuum impregnation, pressurized impregnation or thermal impregnation, which is performed by immersing said woody material in solution heated up to 60-90°C, or their combination may be adopted. The impregnated alkyl silicate compound is hardened by heating, hydrolyzing or the like so as to be turned into inorganic polymer in order to be fixed in the woody material.