A combustion exhaust gas processing device comprises: a dust collector collecting dust in a cement kiln combustion exhaust gas; a wet dust collector using sodium hypochlorite as a catalyst-poisoning-substance stripper for removing a catalyst-poisoning substance and mercury from a combustion exhaust gas which passed the dust collector; heaters downstream of the wet dust collector for heating the combustion exhaust gas from the wet dust collector; and a catalyst device from which NOx, a persistent organic pollutant, etc. in the heated combustion exhaust gas, are removed. An upstream side of the catalyst device is installed with an oxide catalyst such as titanium-vanadium and a downstream side is installed with a platinum catalyst as a noble-metal catalyst. The temperature of the combustion exhaust gas after the wet dust collector is increased up to 140° C. or more with the heaters to prevent decline in denitration efficiency of and the decomposition efficiency of a volatile organic compound. A mercury adsorbing tower is provided for adsorbing the mercury in liquid exhausted from the wet dust collector and wastewater from the liquid is used to generate sodium hypochlorite for use in the wet dust collector.