Wood was treated by double-diffusion method to form the precipitates of metaborates in wood. Their leachability in running water was evaluated. The precipitates of these metaborates in the wood proved to be insoluble or hardly soluble in water by the leaching test, and the best leachability for that by double-diffusion treatment with Pb2+ solution. Their flame retardancy was evaluated by limiting oxygen index (LOI) and cone calorimeter. The resultant data showed that their flame retardancy and smoke suspension were increased by the double-diffusion treatment and the best for that by the treatment with Zn2+ solution.