The present invention provides a production method of oxo fatty acid, as well as rare fatty acids such as conjugated fatty acid, hydroxylated fatty acid, partially saturated fatty acid and the like, which uses 4 kinds of enzymes (fatty acid-hydratase, hydroxylated fatty acid-dehydrogenase, oxo fatty acid-isomerase, oxo fatty acid-enone reductase) derived from Lactobacillus plantarum including lactic acid bacteria and the like. Furthermore, the present invention also provides a more efficient production method of oxo fatty acid and the like, which partly uses a chemical oxidation reaction in combination.