PURPOSE: To obtain the subject compound useful for preventing and treating thrombosis, inflammatory diseases and cancers by rectifying fatty acids from decosahexaenoic acids-containing natural fats and oils in vacuum by plural distillation columns and subjecting the distillate to reverse-phase partition-based column chromatography.

CONSTITUTION: Before a raw material A comprising mixture of fatty acids obtained from natural fats and oils containing decosahexaenoic acids extracted from liver of Ommastrephes bartrami, etc., or its derivative or their esters is sent to a first distillation column 1, the raw material is treated in a flash tank 17 kept under reduced pressure to remove impurities such as air and water. The raw material is successively and precisely distilled by plural distillation columns 1-4 independently equipped with vacuum systems 5-8, condensation systems 9-12 and reboilers 13-16, respectively to collect a fraction consisting essentially of 22C fatty acids or esters. Then the fraction is subjected to reverse- phase partition-based column chromatography and subjected to fractional purification to give high-purity decosahexaenoic acid or its ester useful for preventing and treating thrombosis, inflammatory diseases and cancers.