IrO2 cocatalysts for water splitting were photodeposited from aqueous solution of (NH4)(2)[IrCl6] or Na-3[IrCl6] in the presence of nitrate ions on La-doped NaTaO3 (denoted as NaTaO3:La) photocatalysts, while metallic iridium particles were deposited in the absence of nitrate ions. The IrO2-loaded NaTaO3:La showed the activity for water splitting 2.5 times higher than the nonloaded NaTaO3:La. In contrast, the steady activity was not obtained for the metallic iridium-loaded photocatalyst, because the back reaction quickly proceeded.