We experimentally and theoretically evaluated the effect of Ti substitution in hydroxyapatite (HAP) on the band gap. For samples, we used 10 mol% of Ti-substituted HAP (Ti-HAP) and normal HAP powder, synthesized by the co-precipitation method, with typical anatase-TiO2 photocatalytic powder. The Ti-HAP and HAP powder showed a hexagonal apatite structure by X-ray diffraction. The experimentally obtained optical band gap energies of Ti-HAP. HAP and TiO2 powder measured by diffuse reflectance spectroscopy were 3.65 eV, > 6 eV, and 3.27 eV, respectively. Depending on total energy evaluation and structure optimization by the first principle density functional calculation, the Ti position in the apatite structure was predicted to be at columnar, Ca(I), site with a Ca(I) site deficiency. In Ti-HAP, Ti 3d orbital hybridized with 0 2p orbital and formed an internal state in the HAP band gap. It caused absorption-edge lowering of Ti-HAP. Based on the band structure, we proposed a photocatalytic model of Ti-HAP as a two-step excitation model. Moreover, acetaldehyde gas decomposition of Ti-HAP by UV with VIS irradiation appeared to be enhanced compared with when UV irradiation alone was used. We confirmed the validity of the proposed model. (C) 2011 Elsevier B.V. All rights reserved.