We developed an electrochemical reactor that decomposes and purifies NO_x in diesel engine exhaust gas at a low temperature with high efficiency. Nano-structured catalytic electrodes of the reactor consist of a framework of GDC (Gd doped CeO₂) particles and nano-fibers of an electro-conductive material tangled over the framework. Electro-conductive particles about 3 nm in diameter were self-assembled to form the nano-fibers by an electro-chemical treatment. This electrochemical reactor decomposes NO_x in diesel engine exhaust gas containing high concentrations (about 20 %) of oxygen at a lower temperature than 250 °C, and no reactant is necessary. It also improves the fuel efficiency because of the reduction in energy required to clean exhaust gas. This technology enables the construction of a system that combines air quality conservation with a reduction in CO₂ emission. The reactor will replace current exhaust purification systems for diesel engines.