TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Process: The waste gas is contacted with the latch material which removes sulfur oxide and nitrogen oxide and then is contacted with the methane oxidation catalyst. The waste gas of lean atmosphere is contacted with oxidation component more surplus than reduction component and then is contacted with the oxidation catalyst. The atmosphere is alternately switched to lean atmosphere with oxidation component, stoichiometric atmosphere, or rich atmosphere with reduction component more surplus than oxidation component. Preferred Material: The latch material is NOx which is an occlusion reduced group catalyst. EXAMPLE - Alumina powder was mixed with alumina binder and water, and a slurry was prepared. The slurry was coated on a test piece honey-comb base material and dried at 500degreesC for 1 hour, and a coat layer was formed. The coat layer was made to absorb palladium nitrate aqueous solution and was baked at 300degreesC for 3 hours, and palladium was supported, and a methane oxidation catalyst was prepared. A nitrogen oxide occlusion reduced type catalyst (35 cc) was prepared and was placed in upstream side of an evaluation apparatus. The prepared methane oxidation catalyst (10 cc) was placed in downstream side of the apparatus. A lean gas containing methane (0.14%), oxygen (5.5%), nitrogen oxide (600 ppm), carbon monoxide (0.025%), carbon dioxide (11%) and water (3%) and balance of nitrogen, and a rich gas containing methane (0.14%), nitrogen oxide (600 ppm), carbon monoxide (3%), hydrogen (1%), carbon dioxide (11%), water (3%) and remainder of nitrogen, were passed alternately through the evaluation apparatus at 400degreesC and at 450degreesC. The gas ejected was analyzed and was found that the purification rate of nitrogen oxide was 98.7% and 99.1% at 400degreesC and 450degreesC, respectively, rate of purification of methane was 59.5% and 88.9% at 400degreesC and 450degreesC, respectively and purification rate of carbon monoxide was 99.9% and 99.9% at 400degreesC and 450degreesC, respectively.