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Reducing the oxides of nitrogen (NOx) and particulate matter (PM) from Diesel engine exhaust gas has become very important problem in view of the protection of environment and the saying energy. But it is difficult to reduce both emissions significantly at the same time by engine modifications and operating condition control. Because, their characteristics have a trade off relation. Therefore many investigators are studying the aftertreatment technique of Diesel engine emissions such as a PM trap filter, EGR, etc. The aftertreatment technique is expected that engine operating condition limits will be loose and engine efficiency will be improved. A few years ago, Iwamoto showed that Zeolite catalyst could reduce the NOx with hydrocarbon reducing agents under oxygen rich condition[1]*. After that, many researchers have been investigating Zeolite catalysts[2]-[4].