Interest in the earth environmental problem has been rising year after year. Air pollutants such as NOx, SOx are contained in exhaust gases from car engines and electric power generators and they are one of the causes of the environmental disruption. From now on, countermeasures have to be taken especially for exhaust gas from diesel engines, because high concentration of NOx, higher remaining oxygen and soot make the development of selective catalyzer difficult. A diesel engine generator of 2.4kVA was selected as a target of exhaust gas cleaning, and the non-thermal plasma technology was applied to reduce NOx in the exhaust gas. A system composed of gas cooler, flow lines, discharge reactor of silent discharge type and scrubber was constructed and NOx removal experiments were carried out under various conditions. The system was operated successfully and the maximum NOx removal rate of 60% at the half engine load was attained. The NOx removal efficiency, the effect of hydrocarbon injection and the practical problems experienced in these experiments were described.