Photocatalytic activities of Ga_2−xIn_xO₃ solid solutions consisting of β-Ga₂O₃ with a wide band gap and In₂O₃ with a medium band gap were studied in order to develop new photocatalysts based on oxide solid solutions. The Ga_2−xIn_xO₃ solid solutions were characterized by X-ray diffraction, surface area, diffuse reflection spectra, photoluminescent and photoelectrochemical measurements. As the ratio of In was increased from x = 0 to 1 in Ga_2−xIn_xO₃, the lattice parameters were increased, indicating the formation of solid solutions as has previously been reported. Corresponding to this, the absorption, excitation and emission spectra were red-shifted, showing a decrease in the band gaps, and the flat band potentials were shifted positively. Among the solid solutions, GaInO₃ (or Ga_1.14In_0.86O₃) showed the highest photocatalytic activity for H₂ evolution from aqueous methanol solutions and for O₂ evolution from aqueous silver nitrate solutions. The dependency of the photocatalytic activities on the composition of the solid solutions was explained by taking the number of available photons and the flat band potentials into consideration.