Provided is a novel active oxygen-resisting agent which has an excellent active oxygen resistance and which exhibits a good solubility in an oil solvent. An active oxygen-resisting agent containing as an active ingredient an amino acid derivative represented by the following formula (I) ##STR1## wherein Ar represents a substituted or substituted 2-hydroxyphenyl group or a pyridyl group, said substitution being selected from the group consisting of a halogen atom, a C.sub.1-6 alkyl group, a hydroxyl group, a hydroxy C.sub.1-6 alkyl group, a nitro group, a C.sub.1-6 alkoxyl group or a carboxyl group, R.sup.1 represents a side chain of an amino acid, X represents --O-- or --NH--, R.sup.2 represents a C.sub.8-22 alkyl group, and n represents 0 or 1, or its salt.