Titanium(IV) oxide (TiO₂) has been one of the most attractive materials as a photocatalyst due to low cost, nontoxicity, photostability, and availability. Photocatalytic reactions on such semiconductor are induced by photoexcited electrons and holes, which are generated by above-bandgap irradiation. However, electrons and holes recombine each other without influencing chemical reactions. Therefore, it is important to prevent recombination in order to improve photocatalytic efficiency. In this study, various transition metal ions, such as Fe³⁺,Cu²⁺,Ni²⁺,Cr²⁺, and Rh³⁺ were prepared by a wet impregnation method. These metal ions are expected to accelerate reduction reaction by acting as electron acceptor and prevent electrons and holes from recombination. Photocatalytic activity of the sample adsorbed metal ions (Mⁿ⁺/TiO₂) were estimated with decomposition of acetaldehyde. As a result of experiment, the photocatalytic activity was improved by transition metal ions adsorbed. This result suggests that transition metal ions accelerate photocatalytic activity by acting as electron acceptor. Moreover, the Mⁿ⁺/TiO₂ sample showed photocatalytic activity even under visible light irradiation. Then, We measured the photoacoustic spectroscopy (PAS) intensities of Ti³⁺ of the Mⁿ⁺/TiO₂ samples in order to inquire the behavior of photoexcited electrons under UV irradiation and visible irradiation. We presumed the following thing from the measurement results. Under visible light irradiation, photoexcited metal-ion adsorbed on TiO₂ surface inject electrons into TiO₂, and subsequent reduction. These behaviors were discussed on the correlation with photcatalytic activity on various kinds of Mn+/TiO₂ samples.