There are disclosed a decompositionally treating agent for fluorocarbons which comprises an aluminum compound and a lanthanoid compound as effective ingredients; a decompositionally treating agent for fluorocarbons which comprises an aluminum compound, a lanthanoid compound and an alkaline earth metal compound as effective ingredients; and a decompositionally treating method for fluorocarbons which comprises decomposing a fluorocarbon by bringing a fluorocarbon-containing gas into contact under heating with a decompositionally treating agent mentioned above or by bringing the above gas into contact under heating with a decompositionally treating agent comprising aluminum oxide as an effective ingredient and thereafter with a decompositionally treating agent comprising a lanthanoid oxide and an alkaline earth metal oxide. It is made possible by the above agent and method to decompose the fluorocarbons contained in an exhaust gas exhausted from a semiconductor manufacturing industry and the like at a decomposition rate of at least 99.9% at a relatively low decomposition temperature of 1000 C. or lower without deactivating the agent in a short time, or exhausting a corrosive gas such as hydrogen fluoride in the atmosphere.