Fluorine compounds such as C2F6, CF4, CHF3, SF6 and NF3, are made in contact with a fluorine compound decomposition catalyst and a catalyst the decomposition oft least one of CO, SO2F2 and N2O in the presence of water or in the presence of water and oxygen. The catalyst the decomposition oft least one of CO, SO2F2 and N2O preferably contains at least one selected from Pd, Pt, Cu, Mn, Fe, Co, Rh, Ir and Au in the form of a metal or an oxide. According to the invention, the fluorine compound can be converted to HF, which is liable to be absorbed by water or an alkaline aqueous solution, and a substance, such as CO, SO2F2 and N2O, formed by decomposition of the fluorine compound can also be decomposed.