The main hydrate of cement is calcium silicate hydrate (CSH). Such a cement-based material is essential for constructing the geological disposal system of TRU radioactive wastes including I-129 in Japan. So far, the sorption behavior of iodine on CSH gel has been examined by using the CSH samples dried once. However, the Japan's repository would be constructed under water table. Therefore, we must focus on also the interaction of altered cementitious material and iodine under the condition saturated with saline groundwater. In this study, the sorption behavior of iodide ions into CSH gel, formed without dried processes, was examined in imitated saline groundwater. Ca/Si ratio was set to 0.4, 0.8, 1.2 and 1.6, and NaCl concentration of each sample also was set to 0.6 M, 0.06 M or 0.006 M. These samples were synthesized with CaO, SiO 2 (fumed silica), and distilled water in a given combination of 20 ml/g in liquid/solid ratio. A NaI solution was added after curing the CSH gel (hereinafter referred to as the "Surface sorption sample") for 7 days, setting the initial concentration of NaI to 0.5 mM in sample tube. The values of Eh and pH of each sample showed iodide ions as the chemical species of iodine in the sample tube. Furthermore, this study prepared the "Co-precipitation sample" of CSH gel with iodide ions. Here, the NaI solution was added before curing the CSH gel. For all samples, the contact time-period of the CSH gel with iodide ions was set to 7 days. After each contact time-period, each sample for analyses was separated into the solid and the liquid phases by 0.20 μm membrane filter. In the liquid phase, the concentrations of Ca, I, Si and Na ions in the liquid phase were measured by ICP-AES. Besides, the Raman spectra were obtained from the solid phases of the surface sorption sample and the co-precipitation sample without dried process. The results showed that the sorption of iodide ions into CSH gel strongly depends on the amount of water included in the CSH gel. Such a sorption behavior was confirmed in both co-precipitation samples and the surface sorption samples, even if the Ca/Si ratio is low. This means that iodide ions can be easily immobilized through the water-molecular of CSH gel. Besides, Na concentration did not so much affect the sorption behavior of iodide ions into CSH gel. In addition, the Raman spectra showed that the degree of polymerization of SiO4 tetrahedrons in CSH gel was unaffected with increasing Na ions concentration. These results suggest that the CSH gel saturated with groundwater would retard the migration of iodide ions, even if the groundwater includes salinity.