Free radicals including active oxygen species attack various cells by destroying lipids, proteins and enzymes, and often result in cell death. Recently, free radicals have been found to participate in the mechanism of anticancer therapy such as irradiation, chemotherapy, laser and immunotherapy. Since tumor cells contain fewer scavengers of radicals, it would be reasonable to look for a way to produce a large amount of free radicals in tumor tissues for future anticancer therapy. It should be noted that a variety of side effects induced by anticancer therapy are often related to the production of free radicals in normal organs.