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Skin aging is classified into chronological aging and photoaging, involving ultraviolet radiation (UV), visible light, and others. UVA and UVA-photosensitizers (involving photocatalysis) contribute to the production of chronically induced skin damage that results in photoaging, especially wrinkles that are associated with histopathological actinic elastosis in the dermis. Hydrogen peroxide produced by the photosensitization involving photocatalysis, such as flavin, has been proposed as a risk factor for photoaging. It was also revealed that hydrogen peroxide production by UVA is amplified through the following reactions. The photosensitization of type I and type II by riboflavin as an initiator oxidizes coexisted amino acids and vitamins. The oxidized amino acids and vitamins produce reactive oxygen species (ROS), including hydrogen peroxide, through secondary UVA-photosensitization. Finally, we proposed a screening method for detecting the effects of antioxidants on UVA-photosensitization. In our previous study, histidine and other antioxidants did not inhibit UVA-photosensitized by riboflavin, even though they have been reported to scavenge singlet oxygen and superoxide. In contrast, we demonstrated that ergothioneine suppressed the production of hydrogen peroxide by UVA-photosensitization. The purpose of this report is to provide new findings for the prevention of photoaging by discussing the characteristics of UVA-photocatalysts in the skin.