The reduction reaction of selenate ions in an aqueous solution by TiO2 photocatalyst is investigated. The conduction band of TiO2 is much more positive than the standard redox potential of the reduction reaction of selenium to selenide. However, the selenate ions are reduced to hydrogen selenide via amorphous selenium, by the TiO2 photocatalyst. After all selenate ions and selenite ions are reduced to amorphous selenium, no chemicals in the solution can capture the electrons from the TiO2 photocatalyst. Under this condition, photogenerated electrons accumulate in the TiO2 particles, and they raise the conduction band of the TiO2. All the experimental results can be explained by this mechanism.