PROBLEM TO BE SOLVED: To provide an unsaturated fatty acid containing a hydroxyl group as a factor reaching to show useful pharmacological activity such as an antioxidative effect in an animal such as a human being.SOLUTION: An octadecenoic compound represented by a formula (1) (R, R' are any one of hydrogen or a hydroxyl group but at least any one is a hydroxyl group) and, among these, an octadecenoic acid compound in which R' is hydrogen and R is 11-hydroxy-9(Z)-octadecenoic acid can solve the problem.SELECTED DRAWING: Figure 1