Rhodium-doped gallium oxide (Ga2O3:Rh) that was prepared by a simple coprecipitation method exhibited activity for the photocatalytic conversion of CO2 by H2O under photoirradiation with light of >300 nm wavelength. Although Ga2O3 is a wide-band-gap photocatalyst that is active only under photoirradiation with <300 nm light, the absorption edge shifted to >300 nm as a result of Rh doping. Ag–Cr-loaded Ga2O3:Rh (0.7 mol %) showed activity for the production of CO (3.9 μmol h–1) as a reduction product of CO2. The formation of a stoichiometric amount of O2 indicated that H2O acts as an electron donor for the photocatalytic conversion of CO2. Characterization using several techniques such as X-ray absorption spectroscopy (XAS) revealed that after doping in Ga2O3 the trivalent Rh species substituted not for the tetrahedral Ga site but for the octahedral Ga site of β-Ga2O3. From the result of DFT calculations, the new energy level owing to Rh3+ was within the Ga2O3 band gap. It was concluded that the new absorption due to the Rh d t2g orbital to the conduction band contributed to the activity of photocatalytic conversion of CO2 by H2O under photoirradiation with light of >300 nm wavelength.