Modification of Ga2O3 photocatalysts with rare earth elements is known to enhance the activity and selectivity of CO formation during the photocatalytic conversion of CO2 by H2O as an electron donor, but the role of the rare earth species requires clarification. Photocurrent measurements revealed that modification with Yb species via hydrothermal treatment enhanced the anodic photocurrent of the Ga2O3 photoelectrode in CO2-saturated NaHCO3 aqueous solution while maintaining the onset potential of the anodic photocurrent. Impedance measurements revealed that the band structure of Ga2O3 was maintained following Yb modification, suggesting that the enhanced anodic photocurrent was due to the surface properties. The anodic photocurrent values of the Yb-modified Ga2O3 photoelectrode increased with an increase in the surface adsorption ability of CO2. The positive relationship between the anodic photocurrent value and the concentration of HCO3- species, as measured in two-component NaHCO3 and Na3PO4 electrolytes, indicated that the concentration of HCO3- species near the surface strongly influenced the anodic O-2-formation activity of the Yb-modified Ga2O3 photoelectrode. We concluded that Yb modification improved the oxidation activity of H2O on the surface of Ga2O3 by enhancing the affinity for intermediate HCO3- species, resulting in a high activity for the photocatalytic conversion of CO2 by H2O.