Paulownia wood (Pauloumia tomentosa) is a special kind of wood material in that it has especially excellent flame retardancy. Using this property, it has been commonly used to make clothing wardrobes for a long time in Japan. In this research, the flame retardancy of paulownia wood has been verified by heating experiments and cone calorimeter testing. The structure and tissue of the material have been analyzed by scanning electron microscope and other methods. Moreover, the mechanism of the flame retardancy of paulownia wood was analyzed by model experiments and FEM analysis. The result shows that the cell tissue of paulownia wood is very porous and similar to the structure of a honeycomb. It can be easily carbonized when heated. Since paulownia wood contains few lignins, it generates very little combustible gas when heated. Furthermore, when viewed from the radial section, the vessel structure of paulownia wood is very large and independent, compared to cedar wood (Cryptomeria japonica), which has a thin and continuous tracheids structure. Oxygen is not sufficiently supplied in this type of structure found in paulownia wood. Thus, it is difficult to ignite, and only carbonized when heated. Generally speaking, the thermal conductivity of the carbonization layer is lower than that of the wood material, which is the reason why wardrobes made of paulownia wood are able to protect kimonos from fire.