It was found for the first time that controlling the charge density in oxide semiconductors with an acceptor was effective for improving the activity to photolysis of H2O. Although the photodecomposition activity of NiO supported on nondoped KTaO3 was negligibly small, doping small amount of accepters such as tri- or tetravalent cations to KTaO3 increased the formation rate of H-2 and O-2. In particular, it was found that NiO supported on KTaO3 doped with 8 mol % Zr4+ exhibits higher activity to the photocatalytic decomposition of H2O than that of a well-known photocatalyst of Pt/TiO2