In order to avoid large problems regarding peering of the titania layer coated on the substrate, we developed an epoch-making “strong titania fiber” consisting of photoactive surface layer with a nanometer-scale compositional gradient, which can effectively oxidize any kind of organic materials. An effective water-purification system using this fiber has been also developed. The basis of this technology is to incorporate a selected low-molecular-mass additive (Ti(OC4H9)4) into a precursor polymer from which the ceramic forms. After melt-spinning the resulting precursor polymer, thermal treatment of the spun fiber leads to controlled phase separation (“bleed-out”) of the additive; subsequent calcination stabilizes the compositionally changed surface region, generating a functional surface layer. This fiber consists of the silica-based core-structure and the gradient-like surface titania layer, which are strongly sintered. We also developed a water-purifier using this fiber (felt material). Any bacteria (common bacterium, legionera pneumophila, colon bacillus, heterotrophic bacteria, and so forth) and organic chemicals (dioxin, PCB, and so forth) were effectively decomposed into CO2 and H2O passing through the above purifier.