PURPOSE: To remarkably reduce the discoloration of paulownia wood by treating the paulownia wood through immersion or application of water-soluble liquid, which contains boron compound as the specified amount of boron.

CONSTITUTION: Water-soluble liquid, which contains boron compound as 0.005W1.5wt.% of boron, is used. The boron compound contains boron and is water-soluble compound and can be used in combination of one or two and more selected from, for example, boric acid, sodium borate and the like. As the water-soluble liquid, liquid mixture of one or two and more of, for example, water and/or alcohols is used. When resin, which reveals bond performance in acid state, is used as adhesive, the discoloration preventing treatment of paulownia wood must be done under the condition that the pH of the water-soluble liquid of the boron compound as discoloration inhibitor is prepared in advance to be 3W6 or on acid side. As the discoloration preventing treatment of paulownia wood, the paulownia wood is immersed and/or application of boron compound water solution in order to add boron to the paulownia wood.