The molecular structure of pore water in the montmorillonite-water system is studied by means of the Fourier-transform infrared spectroscopy. In the near-infrared region, absorption bands observed at around 5,088 cm^-1 (very broad) and 5,245 cm^-1 (relatively sharp) are attributed to combination tone of bending and vibrational mode of water molecule (H₂O) in the intelamellar space of montmorillonite, while the 4,518 cm^-1 peak is due to combination tone of structural hydroxyls at the octahedral layers. These facts indicate that the water molecule at the interlamellar space is in the two different vibrational states depending on the hydrogen bonding. The former band at around 5,088 cm^-1 is related to the hydrogen bonding between water molecules, while the latter band at 5,245cm^-1 is to that between montmorillonite-water interface. Although, considering their frequencies, the hydrogen bonding between the montmorillonite-water interface is weaker than that between water molecules, the activation energy of self-diffusion of water at the water-montmorillonite interface is higher than the bulk water. This discrepancy seems to be due to the preferred orientation of water molecule in the neighborhood of the montmorillonite surface.