We prepared titanium dioxide (TiO₂) photocatalysts sensitive to visible light, namely copper(II) [Cu(II)]-grafted tungsten and gallium co-doped TiO₂ (Ti_1−3xW_xGa_2xO₂, x value is up to 0.12), based on the concept of narrowing the band gap of TiO₂ by positively shifting its conduction band (CB) edge to a lower energy level and the catalytic multi-electron reduction of oxygen by Cu(I) ions [H. Yu, H. Irie, K. Hashimoto, J. Am. Chem. Soc., 132, 6898 (2010)]. Using this approach, the optical band-gap energy of TiO₂ was decreased to ~2.8 eV, and band-gap narrowing was confirmed by measuring the action spectrum for oxygen evolution from water in the presence of iron ions [Fe(III), from FeCl₃]. The Cu(II)-grafted Ti_1−3xW_xGa_2xO₂ (x = 0.12) photocatalyst effectively decomposed 2-propanol to carbon dioxide (CO₂) via acetone under visible light (400–530 nm, 1 mW/cm²) with a CO₂-generation rate of 0.30 µmol/h. Grafting Cu(II) ions after the hydrochloric acid treatment of Ti_1−3xW_xGa_2xO₂ (x = 0.12) increased the CO₂-generation rate to 0.40 µmol/h.