Adsorption isotherms of methane on high-silica zeolites at ambient temperature and up to high pressure were experimentally measured. The isotherms were analyzed on the basis of thermodynamics and statistical mechanics, and the relationship between microscopic properties and the macroscopic adsorption behavior was investigated. A comparison between the confined and unconfined phases revealed that molecular motion is restricted in the pores. As a result, the adsorbed phase is entropically destabilized, which cannot be neglected in comparison with the energetical stabilization that occurs as a result of the solid-molecule interaction. Our findings also indicate that the smaller slope ( d rho/d ln p) of the adsorption isotherms compared to that of the isotherm of the bulk at the same density is due to the smaller intermolecular interaction in the pores. The pore-size dependence is indicated not only in solid-molecule interactions but also in intermolecular interactions and molecular motion. Of these, the solid-molecule interactions strongly influence the adsorption behaviors in pores of different sizes. The origin of the restriction of molecular motion in the pores is well-explained by the one-dimensional transition and two-dimensional vibration ( 1D-trans, 2D-vib) model.