TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Process: The thermally decomposed gas is burned to remove flammable components and a fluorine compound from the thermal decomposition of the mixed gas is removed by washing the mixed gas thus burnt with water or subjecting to chemical adsorption.The resulting gas is burned to remove flammable components.The exhaust gas discharged from the manufacturing equipment is washed with water before thermal decomposition in the non-oxidizing atmosphere.The thermal decomposition of the mixed gas is performed at 600-1200degrees C.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Gas: The gas mixed with the exhaust gas for the thermal decomposition is selected from methane, ethane, propane, butane and ammonia gas.    TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Components: The apparatus further comprises an adsorber tower loaded with granular calcium oxide (CaO) or calcium carbonate. EXAMPLE - A mixed gas containing methane at 1 % and nitrogen at 99 % was passed at a flow rate of 30 l/minute through a front water scrubber into a gas decomposer room of decomposer-burner tower coated with an alumina-based castable fire resistance material. A gas mixture containing propane at 90 % and n-butane at 10 % was supplied at a flow rate of 0.3 l/minute into the decomposer room. Heater surface temperature of decomposer room was maintained at 1100degrees C and cleaned exhaust gas containing perfluorocarbon was discomposed by a mixed gas under reducing or non-oxidizing atmosphere. The decomposed gas was introduced into a burner room at 500degrees C. The processed flammable gas containing residual propane and carbon as soot was burnt and oxidized in the presence of air. The burnt gas was passed through a rear water scrubber to absorb and to dissolve the byproducts of fluorine and hydrogen fluoride in water. The flow rate of water was 10 l/minute. The methane concentration in processed gas was 50 ppm and the methane removal ratio was 99.5 %.