[en] The X-ray power dependence of the charge-up shift of a photoelectron line was examined for metal films insulated from a spectrometer. As the X-ray power increased, the observed line position shifted to a higher binding energy, and approached an asymptotic value. This charge-up behavior was treated based on the balance between the photoelectron emission from the sample surface and electron injection from an aluminum window of the X-ray source and from other electron sources. It was concluded that, for insulated metals, the vacuum level of the sample is aligned to that of the spectrometer when the X-ray power is extrapolated to zero, i.e., the magnitude of the charge-up is equal to zero. The work function of the spectrometer was successfully determined with samples having a known work function. A new method to obtain the binding energy relative to the vacuum level for insulators is proposed. (author)