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The fire resistance of structural wooden walls covered with fire-resisting boards was investigated. A 49-mm-thick gypsum board (No. 1 and No. 2), a 24-mm-thick diatomite (No. 3), and a combination of gypsum and diatomite board (No. 4 and No. 5) were theoretically designed for 2-h fire resistance. As a result, when the gypsum board was fixed to the wood studs with nails (No. 1), there was no damage after 2 h, as predicted. When diatomite boards were used on the surface to face the fire (No. 3 and No. 4), the diatomite boards were destroyed after approximately 50 min because of the cracks of the diatomite board. However, when the diatomite board (6 mm) was placed inside the gypsum board (30 mm), the specimen (No. 5) showed a fire resistance of more than 2 h. Therefore, it is possible to reduce the thickness of the gypsum board using the diatomite board by avoiding placement of the diatomite board in direct contact with heat.