Photocatalyst supported on granulated titania was applied to the prevention of polluted algae propagation in eutrophic water. Even in the best conditions for the propagation of algae in MA medium under solar irradiation in summer, water remained almost clear for a period of a month when the photocatalyst was placed approximately 1cm below the surface of water level. Similar results were obtained in the model household waste-water. A larger amount of waste-water was also treated similarly by circulating it to the irradiating water-channel where photocatalyst was densely scattered.