To improve the performance of semiconductor photoelectrodes for water splitting, the amount of band bending in the depletion layer of a semiconductor should be accurately ascertained, since it determines the splitting efficiency of photogenerated carriers. Band bending has been determined by X-ray photoelectron spectroscopy (XPS) from the valence band maximum (VBM), which has been calculated from the Ga 3d peak using the energy difference between VBM and Ga 3d (ΔEVBM-3d). This work validates several values for ΔEVBM-3d which have been reported previously, by analyzing the spectrum around the VBM and its distance from Ga 3d for the n-GaN(0001) surface under both ultrahigh vacuum (UHV) and ambient H2O. ΔEVBM-3d is estimated to be between 17.36 and 17.55 eV. By adopting 17.5 eV as ΔEVBM-3d, the amounts of band-bending were 0.5 eV under UHV and 0.1 eV under a relative humidity of 46%, respectively. For the latter condition, a surface photovoltage of 20 meV was observed upon Xe lamp irradiation, confirming the existence of band bending even with H2O adsorption on the surface. The origin of such band bending seems to be Fermi level pinning to the subsurface states which cannot be compensated by H2O.