To obtain a fluorine-containing compound useful as a surface-modifying agent in a high purity and in a high yield in a safe work environment by reacting a specific hexafluoropropene dimer with a specific compound together with a fluorinated alkylammonium salt in a fluorinated solvent.

This method for producing a compound of formula IV such as 3,3,4,4,5,5,5-heptafluoro-2,2-bis(trifluoromethyl)pentylbenzene is obtained by reacting hexafluoropropene dimer of formula I with a compound of formula II [X is a releasing group; Y is a group of formula III (R1 is H or an active substituent), OR2 (R2 is an inactive substituent), COOR3 (R3 is an inactive substituent), or CH=CH] together with a fluorinated alkyl ammonium salt in a fluorinated solvent such as 1,4-(trifluoromethyl)benzene.