  We investigate on-site Coulomb interaction energy between two 3p holes U(Ni
3p) of ultrathin NiO films on Ag(001) by both x-ray photoelectron spectroscopy
and Auger electron spectroscopy. As the film becomes thin, U(Ni 3p)
monotonically decreases, and the difference of U(Ni 3p) for 1 monolayer (ML)
film from that of bulk-like thick film \delta U(Ni 3p) reaches ~ -2.2 eV. The
observed \delta U(Ni 3p) for 1 ML film is well reproduced by the differences of
both the image potential and polarization energies between 1 ML film and the
bulk-like thick film. Hence, the present results provide an evidence for the
picture originally proposed by Duffy et al. [J. Phys. C: Solid State Phys., 16,
4087 (1983)] and Altieri et al. [Phys. Rev. B 59, R2517 (1999)]
