Many research groups have already made significant progress in the field of water splitting to produce pure H₂ and O₂; in contrast, studies on the photocatalytic conversion of CO₂ using H₂O as a reductant are very few. This is because in such systems, overall water splitting occurs together with the reduction of CO₂, resulting in H₂ being the main product. Thus, it is very important to control the reduction of protons in the presence of both CO₂ and H₂O in order to achieve the reduction of CO₂ by H₂O. In this study, we found that Ag/ZnGa₂O₄/Ga₂O₃ exhibits a higher conversion of CO₂ as compared to several photocatalysts reported previously. CO was evolved as the main product during the photocatalytic conversion of CO₂ over Ag/ZnGa₂O₄/Ga₂O₃ under UV irradiation. H₂ was simultaneously generated as the reduction product similar to CO, although the amount of H₂ evolved was tiny as compared to that of CO. Expectedly, the evolution of O₂ was observed together with those of CO and H₂.