Hg 4{ital f} core levels of HgBa{sub 2}(Ca{sub 0.86}Sr{sub 0.14}){sub 2}Cu{sub 3}O{sub 8+{delta}} (Hg-1223) and HgO were measured by x-ray photoelectron spectroscopy in order to demonstrate the oxidation state of mercury ions in the superconducting oxide. The binding energy of the Hg 4{ital f}{sub 7/2} level is found to be at 99.9 eV in HgO and 101.0 and 103.4 eV in Hg-1223. This result reveals that the oxidation state of Hg is definitely more than +2 and it exists in a mixed-valent state. The creation of charge carriers in Hg-1223 is discussed in terms of a charge transfer between Hg-O{sub {delta}} and Cu-O layers.