The exhaust gas purifying system

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PROBLEM TO BE SOLVED : To provide a nitrogen oxides adsorption/desorption material capable of adsorbing/desorbing NOx independently of the fluctuation of the air/fuel ratio of the subject gas and to provide an exhaust gas cleaning system excellent in NOx cleaning performance in the lean region. SOLUTION : The nitrogen oxides adsorption/desorption material contains alkali metal, alkaline earth metal and rare earth elements, etc., and the material adsorbes the NOx when ambient moisture content is much in quantity or does not adsorb the NOx or release the adsorbed NOx when the ambient moisture content is little in quantity. The exhaust gas cleaning system provides the nitrogen oxides adsorption/desorption material. An exhaust gas cleaning catalyst obtained by mixing the nitrogen oxides adsorption/desorption material and noble metal such as platinum, palladium and rhodium is provided at a flow passage of the exhaust gas, and a moisture concentration varying means capable of varying the moisture content in the exhaust gas is provided at its upstream side. The moisture concentration in the exhaust gas is varied in the range of 0-20 wt.%.