The reduction in human-induced emissions of CO2 from automobiles, factories, power stations, etc., over the next 15 years is currently one of the most important issues facing the planet. We should therefore attempt to develop industrial processes using CO2 as a feedstock in order to build a sustainable society in the near future. These days, we found that Ag-modified ZnGa2O4/Ga2O3, LiTi2O7, ZnGa2O4, and SrO/Ta2O5 exhibits activity and good selectivity toward CO evolution for the photocatalytic conversion of CO2 by H2O as an electron donor. An isotope experiment using 13CO2 and mass spectrometry clarified that the carbon source of the evolved CO is not the residual carbon species on the photocatalyst surface, but the CO2 introduced in the gas phase. In addition, stoichiometric amounts of evolved O2 were generated together with CO. In this study, we focus on how to suppress the H2 evolution derived from overall water splitting to enhance the selectivity toward CO evolution.