Air pollution caused by exhaust gases from combustion engines and systems has recently become one of the serious environmental issues on a global scale. To purify exhaust gas, monolithic ceramic honeycombs are widely used as a support of catalyst components in practical applications; however it has several disadvantages, e.g. small sideways gas diffusion and low thermal conductivity. Thus, there has been increasing interest in the new catalytic materials with micro-meter scale porous structures, which enable the effective gas diffusion during the catalytic reaction. In our previous works, we have successfully prepared the Cu/ZnO catalyst powder/inorganic fiber composites, called paper-structured catalyst, by a papermaking technique. The paper-structured catalyst possessed a unique fiber-network microstructure and demonstrated a high catalytic efficiency and durability as compared with the original catalyst powder in the methanol reforming process for hydrogen production. In this study, a novel paper-structured catalyst containing Pt/Al₂O₃ powders was prepared aiming at the effective catalytic reduction of nitrogen oxides (NO_X) in the automobile exhaust gas. Fine powders of Pt/Al₂O₃ catalyst were successfully supported on the ceramic fiber network by a papermaking technique with a dual polyelectrolyte system; the total retention of inorganic components reached up to ca. 95%. The paper composite was flexible, lightweight and easy-to-handle. The ceramic fiber network tailored in the paper-structured catalyst formed a characteristic porous microstructure (average pore size: ca. 20 µm, porosity: ca. 70%). Conversion efficiency from NO_X to N₂ by using paper-structured catalyst was superior to those by original catalyst powders, commercial honeycomb catalyst or pellet-shaped catalyst. Besides, the paper-structured catalyst demonstrated a quick response in the catalytic NO_X reduction at a rapid increasing reaction temperature. Loading of catalyst powders or pellets brought about both poor heat transfer and heterogeneous gas flow. On the other hand, paper-structured catalyst possessed a unique micro-porous structure that can promote the effective heat and gas transfer to the active surfaces, surpassing that of a commercial honeycomb catalyst. Therefore, the paper-structured Pt/Al₂O₃ catalyst is expected to be a promising catalytic material in the practical applications for the purification of automobile exhaust gas.