Transparent TiO2 thin film photocatalysts were prepared on transparent porous Vycor glass (PVG) by the ionized cluster beam (ICB) method. In order to improve the photocatalytic performance of these thin films under visible light irradiation, transition metal ions such as Cr and V were implanted into the deep bulk inside of the films using an advanced metal-ion-implantation technique. The UV-vis absorption spectra of these metal-ion-implanted TiO2 thin films were found to shift smoothly toward visible light regions, its extent depending on the amount and kinds of metal ions implanted. Using these metal-ion-implanted TiO2 thin films as photocatalysts, the photocatalytic decomposition of NOx into N-2 and O-2 was successfully carried out under visible light (lambda > 450 nm) irradiation at 275 K.