PURPOSE: To obtain the title compound under mild reaction conditions in high selectivity by subjecting hexafluoropropene oxide containing hexafluoropropene to vapor-liquid contact with a specific tert. amine solvent in the tert. amine solvent to isomerize hexafluoropropene.

CONSTITUTION: Hexafluoropropene oxide (wherein especially hexafluoropropene oxide containing hexafluoropropene may be used as a raw material) is subjected to vapor-liquid contact in a tert. amine (e.g. triethylamine, 2,6-dimethylpyridine or 2-methylquinoline) solvent. Thereby isomerization reaction of hexafluoropropene oxide is carried out to provide perfluoropropionic acid fluoride. According to the above-mentioned method, perfluoropropionic acid fluoride is obtained in high selectivity, good yield and good conversion even by using hexafluoropropene oxide containing hexafluoropropene as a raw material.