The invention relates to a method for producing a first perhalogenated cyclopentene represented by the general formula: C5ClBF8-B where B is an integer of from 0 to 7. The method includes a step of (a) fluorinating a second perhalogenated cyclopentene by hydrogen fluoride in a gas phase in the presence of a fluorination catalyst. The second perhalogenated cyclopentene is represented by general formula: C5ClAF8-A where A is an integer of from 1 to 8, and A is not smaller than B. With this method, the first perhalogenated cyclopentene (e.g., 1,2-dichloro-3,3,4,4,5,5-hexafluorocyclopentene, another chlorofluorinated cyclopentene, or octafluorocyclopentene) can continuously easily be produced, for example, from octachlorocyclopentene obtained by chlorination of hexachlorocyclopentadiene that is easily available. Therefore, the above method is very useful as an industrial scale production method. In case that the first perhalogenated cyclopentene is octafluorocyclopentene, the method may further include a step of (b) fluorinating a reaction product of the step (a) in a way that is different from that of the step (a), thereby to convert said reaction product of the step (a) into octafluoropentene. The step (a) may be conducted by a multistep reaction wherein there are provided "m" of reaction zones in number where m is an integer of from 2 to 10. In this case, the reaction zones are arranged in series and such that a reaction temperature of each reaction zone is independently controllable.