Osmotic pressure induced by semi-permeability of hardened cement mortar was studied. The apparatus composed of the two cells, one of which is filled with NaCl solution of 2mol and the other is filled with distilled water, was developed to measure osmotic pressure. Mortar disks of three levels of thickness from 5 to 15mm were adopted as test specimens, and osmotic pressure, water permeability and ion movement were measured at 30℃ for the specimens with water cement ratios such as 45,60 and 75%. The following results were obtained. 1.Osmotic pressure is basically affected by movability of water, Na^+ and Cl-Ion in cement mortar.and maximum osmotic pressure, 1.2MPa was observed. 2.Hardened cement mortar may be considered as semi-permeable material with low reflection coefficient.