To easily and effectively perform various treatment of antibacterial treatment, antifungal treatment, deodorization, stain decomposition, stain prevention and sliminess prevention of warm water or the like used in bathtubs of general households, various kinds of facilities, hotels, and country inn, etc., and to solve the problem that injurious substances are generated without being noticed or bacteria or the like are reproduced and infected or the like and cases of getting sick and getting infected often occur since the same warm water is used for a long period of time especially in the case of the bathtub of a circulation system of circulating, filtering, cleaning and repeatedly using the warm water or the like used once.

In a strainer cap to be used by being freely detachably attached to the supply port or circulation port or the like of the warm water or the like of the bathtub, dark place hybrid photocatalytic titanium oxide is coated on the entire surface of the strainer cap, and the coating is formed. Also, for the strainer cap of the bathtub, an insulating material is interposed and both or one of a silver plate and a copper plate is arranged inside the strainer cap.