The electronic structure and photocatalytic activities of TiO2 and metalnanoparticle-modified TiO2 were investigated by far-ultraviolet and deep-ultraviolet spectroscopy and photodegradation reaction of methylene blue. First, spectra of naked anatase TiO2 (Sect. 6.2) and metal (Au, Pd, Pt)-nanoparticle-modified TiO2 (Sect. 6.3) were measured. The naked TiO2 spectrum corresponded well with the previously reported reflection spectrum and theoretical calculations. Then, the deposition of metal nanoparticles substantially changed the spectral shape, which indicates changes in the electronic states of TiO2, and the degree of spectral changes strongly depends on the work function of the modified metal. In addition, consistent changes of photocatalytic activities were also observed. Next, two crystalline types of TiO2 (anatase and rutile) were compared (Sect. 6.4), and a larger enhancement of the photocatalytic activity of rutile TiO2 upon Pt nanoparticle deposition was revealed. Subsequently, size effects of modified Au nanoparticle on electronic structures and photocatalytic activities of TiO2 were discussed (Sect. 6.5), and it was made clear that the smaller Au nanoparticle induced the larger electronicstate changes and the higher photocatalytic-activity enhancements. These results demonstrated that the novel far-ultraviolet and deep-ultraviolet spectroscopy is a considerable promising method to investigate the electronic states of materials, leading to the development of high-efficiency optical materials such as photocatalysts and solar cells.