The objective of the invention is to provide a plant activator with superior resistance-inducing activity and low toxicity and soil contamination. A plant activator comprising, as an active ingredient, an oxo fatty acid derivative of general formula (I): HOOC—(R1)—C═C—C(═O)—R2  (I) (wherein, R1 is a straight or branched alkyl group with 6 to 12 carbon atoms, and optionally comprises one or more double bonds, R2 is an alkyl group with 2 to 8 carbon atoms, and optionally comprises one or more branches and/or double bonds) or a salt or an ester thereof.