The relationship between the electronic structures of ITO thin films and their optical and electric properties has been investigated by using X-ray photoelectron spectroscopy (XPS) and bremsstrahlung Isochromat spectroscopy (BIS). The spectra of the state densities of valence and conduction bands obtained from XPS and BIS are consistent with those calculated by DV-X molecular orbital method. The state density of the conduction band obtained from BIS increased gradually above Ef. This is related to high carrier mobility and high transparency in the short wavelength region of the ITO film. Curve fitting of the O1s XPS spectra revealed a peak at 531.1 eV embedded in the central part of the spectra, We propose that the oxygen atoms having the binding energy of 531.1 eV to the tin ions which produce carrier electrons in the conduction bands.