A method of manufacturing a modified wood material, wherein a raw wood material is processed in a first bath of a first solution containing metallic ions high in the affinity and showing insoluble and non-flammable properties within the wood material, and then in a second bath of a second solution containing negative ions which produces an insoluble, non-flammable inorganic composition upon reaction with the metallic ions, whereby the obtained modified wood material is made to have the insoluble, non-flammable inorganic composition produced and fixed therein to allow the material to hold a good flame retardancy and excellent rotproof and mothproof properties.