Device for eliminating N2O within exhaust gas and method for eliminating N2O

By use of an N 2 O removing system comprising a plurality of gas flow passages 1 and catalyst packed layers 2 packed with N 2 O decomposition catalysts, which are alternately arranged, wherein the gas flow passages 1 next to each other via the catalyst packed layers 2 comprise a gas introduction passage 11 and a gas discharge passage 12, and wherein the respective gas flow passages are provided with valves 3 sequentially opened along with the deterioration of N 2 O catalysts due to their time-dependent changes to cumulatively increase the number of the catalyst packed layers used, the valves 31 and 32 are opened along with time-dependent changes after the beginning of N 2 O removal treatment, thereby cumulatively increasing the number of the catalyst packed layers 2 used.