A multistep NO_x reduction system that set up the catalysts and the plasma equipment is developed to purify the lean-burn engine exhaust gas. The plasma equipment converts the unburnt fuel in exhaust gas into the aldehyde, and converts the nitric monoxide into the nitrogen dioxide. NO_x conversion efficiency of the NO_x selective reduction catalyst is improved by the effect of plasma equipment. Ag catalyst is used for the NO_x selection reduction catalyst, and Pt catalyst is used to purify the hydrocarbon(HC) and the carbon monoxide(CO). The conversion efficiency of the exhaust gas is obtained 75% or more in NO_x and 99% in HC and CO as the result of using these catalysts with the plasma equipment.