NaTaO3 with 4.0 eV band gap showed a high activity for the decomposition of pure water into H-2 and O-2 without co-catalysts when it was prepared in the presence of excess Na in order to prevent the formation of defects by the volatilization of Na. When NiO was supported on the NaTaO3 powder, the activity was drastically increased. The quantum yield of the NiO(0.05 wt%)/NaTaO3 photocatalyst was 28% for the water splitting at 270 nm.