The states of the inert gases that are encapsulated in Cs3Na9-A zeolite were investigated by Monte Carlo (MC) simulations. The interacting potential energy between the zeolite framework and the encapsulated atom was expressed as a function of polar coordinates. A cumulative distribution function was introduced for the sampling of the positions in the polar coordinate system during MC sampling. When the size of the encapsulated atom is large (Kr, Xe), the binding energy of the atoms decreases with oscillation as the number of the encapsulated atoms increases. Since the alpha-cage of the model zeolite has high symmetry, the binding energy of the encapsulated atoms that form a high-symmetry cluster is relatively larger than that of the atom in a low-symmetry cluster.