To a polyunsaturated fatty acid or its salt or ester was added an antioxidative sesame component and ascorbic acid or an ascorbyl fatty acid ester. To the polyunsaturated fatty acid or its salt or ester, or an oil or fat containing the polyunsaturated fatty acid or its salt or ester having an peroxide value (PV) of 3.0 meq/kg or less and an acid value (AV) of 1.0 or less, and no odor in terms of sensory testing was mixed with an antioxidative sesame component, ascorbic acid or an ascorbyl fatty acid ester, and an external base material, under conditions preventing oxygen from mixing, and the mixture is quickly placed in a hermetic container. The above method provides an external composition containing a polyunsaturated fatty acid or its salt or ester, exhibiting enhanced oxidative stability, and consequently preventing the generation of peculiar odor and peroxides resulting from the oxidation of the polyunsaturated fatty acid.