Changes in the electronic structure of Na doped CuInS2 thin films with Ga incorporation raising up the energy-conversion efficiency (eta) of n-CdS/p-CuInS2 solar cells from 10.6 to 11.2 were examined by ultraviolet-visible (UV-VIS) diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy (XPS). The optical absorption energy of the CuInS2:Na film increased from 1.40 to 1.43 eV with the Ga incorporation. though the valence band maximum (VBM) remained at constant. The enhanced open-circuit voltage (V-oc) from 0.76 to 0.80 eV by the Ga incorporation can be ascribed to the observed widening of the optical band gap (E-g) by 0.03 eV.