We have performed Co 2p X-ray absorption spectroscopy (XAS) on a mixed valence hexagonal layered system NaCo2O4, which shows large thermopower and metallic conductivity. We also measured XAS for SrCoO3 and LaCoO3 as examples of typical Co ionic states. The Co ions of the last two materials are octahedrally surrounded by oxygen atoms. A Co atom in NaCo2O4 also has octahedral coordination with oxygen atoms but it is distorted slightly. The Co ions in SrCoO3 and LaCoO3 are in the d(5) and d(6) configuration respectively. The spectrum of NaCo2O4 resembles that of LaCoO3 rather than that of SrCoO3; the Co ions in NaCo2O4 can be regarded as the d(6) configuration. Also, we have tried to calculate the XAS spectra by using the charge-transfer multiplet model (CT-M). Although we could not find out the best-fitting parameters, the results suggested the itinerancy of the Co 3d states.