We evaluated Cs M4,5 near edge X-ray absorption fine structure (NEXAFS) of Cs halides and clay minerals containing Cs using DFT calculation with the NEXAFS of the clay minerals measured. The evaluation of Cs halides indicated that the core-hole strength (CHS) in the NEXAFS measurement was reduced to 0.6, taking into account the strength interaction of the core-hole with the valence-band electrons. On the basis of this result, Cs M_<4,5> NEXAFS of the clay mineral was well reproduced including the dominant peak and tail structures using DFT calculation. Moreover the calculation showed that the NEXAFS clearly changed with the displacement of Si atoms to Al in the six-membered ring of clay minerals and little did with the coexist of K atoms with Cs between layers of the clay mineral. In addition, we examined the electronic state of CsCl and the clay mineral containing Cs. This examination indicated that interactions between Cs and the nearest atom were significant for the clay mineral and small for CsCl. In addition, the charge density distribution indicated that the interaction between Cs p and O p was a bonding state and that the interaction between Cs s and O s is an anti-bonding state.