A method for removing efficiently and economically harmful materials from a waste gas exhausted in a process for producing acrylonitrile by ammoxidation of propylene, which comprises dividing the waste gas exhausted after the absorption of acrylonitrile into two flows, mixing the first flow gas with air in an amount necessary for combustion of the combustible gas contained in the whole waste gas, subjecting the main flow gas of the mixed gas thus obtained to heat-exchange with the gas of the outlet of the first catalyst layer, combining the main flow gas with the remainder of the mixed gas, preheating thereby the mixed gas at a temperature of 200.degree. to 450.degree. C, passing the preheated mixed gas through the first catalyst layer wherein at least one noble metal is dispersed on an alumina carrier, and therein burning the gas at a temperature of 650.degree. to 750.degree. C in the presence of an excess amount of oxygen to give a combustion gas, combining the combustion gas with the second flow gas which is preheated by subjecting the main flow gas of the second flow gas to heat-exchange with the gas of the outlet of the second catalyst layer and combining the main flow gas with the remainder of the second flow gas, and thereby controlling the temperature of the combustion gas to 250.degree. to 450.degree. C, passing the combustion gas combined with the second flow gas through the second catalyst layer, and therein burning the gas at a temperature of the outlet of 600.degree. to 750.degree. C in a concentration of the remaining oxygen of 0 to 0.5 % by volume.