Fluorine compounds such as C.sub.2F.sub.6, CF.sub.4, CHF.sub.3, SF.sub.6 and NF.sub.3, are made in contact with a fluorine compound decomposition catalyst and a catalyst the decomposition oft least one of CO, SO.sub.2F.sub.2 and N.sub.2O in the presence of water or in the presence of water and oxygen. The catalyst the decomposition oft least one of CO, SO.sub.2F.sub.2 and N.sub.2O 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, SO.sub.2F.sub.2 and N.sub.2O, formed by decomposition of the fluorine compound can also be decomposed.