Provided is a biomarker that enables easy and rapid detection of oxidative stress on a living organism and enables prevention of tissue damage or cell necrosis by drug administration, and which is a powerful marker for the study of toxicity and pharmacokinetics of various agents. Oxidative stress is determined by measuring blood concentration of ophthalmic acid, which is a substance that varies in blood depending on the variation of reduced glutathione (GSH) concentration in a biological sample with the use of an analyzer such as a capillary electrophoresis-mass spectrometer. Further, an anti-oxidative stress agent is screened by administering an anti-oxidative stress candidate agent to a non-human animal under oxidative stress conditions, measuring blood concentration of ophthalmic acid, and evaluating the degree of decrease in the ophthalmic acid concentration.