The structural decay of wood (using "Obi" Japanese cedar) was studied by focusing on the effect of flame retardancy. The various flame retardants, including boron compound, used as one of the most popular flame retardants, were applied in this study. Thermogravimetric analysis, pyrolysis/gas chromatography/mass spectrometry, and elemental analysis were used to analyze the structural decay and thermal degradation. Flammability was observed via a vertical combustion test and a cone calorimeter. Woods to be alive was opened of pit for through the water. However, woods are independent and isolated each other by blockade of the pit at the end of their lives. Therefore, woods was prepared and destroy the pit under several conditions so as to be blended with flame retardants. After the flame retardants were impregnated, several thermal degradation tests were achieved. Boron showed an excellent effect on the flame retardancy and fluorocarbon acid salt and organic phosphate also showed a remarkable effect.