PURPOSE: To improve the preservability, the insect-proof property or the like by including a bromized organic compound having at least one of an epoxy group and a vinyl group in a molecule as a functional group or/and a bromized phenol derivative- formaldehyde resin, or the initial condensate thereof as the essential components.

CONSTITUTION: A bromized organic compound or/and bromized phenol derivative- formaldehyde resin, or the initial condensate thereof are included as the essential components. Any bromized organic compound such as the cyclic, chained and alicyclic bromized organic compound will do so long as it comprises at least one of an epoxy group and a vinyl group in a molecular as a functional group. The number of bromines in the molecule is not especially limited, but the fire-resistance efficiency of the obtained modifier can be improved when the content thereof is high. The bromized phenol derivative-formaldehyde resin or the initial condensate thereof is a resin prepared by the additional condensation of the bromized phenol derivative through formaldehyde, or the initial condensate thereof. The modifying treatment of the wood material is performed by impregnating the wood material with a solution of the modifier having the above-mentioned components, and polymerizing or hardening the same by a proper method.