[en] Tightness of the underground storage cavern was evaluated and long-tern structural stability of the storage cavern was analyzed by the geological data. Detailed analysis for the hydrogeological data near the underground storage cavern was performed and the hydrogeological concept model for the cavern site was constructed. Then, the numerical model was used to evaluate tightness in the storage cavern and predict the condition when it operates. As results of this research, additional measures for improving hydrogeological stability of the storage cavern cannot be necessary because the hydraulic barrier can secure desirable stability, though some vulnerable locations to excavation of the storage cavern were identified. However, regular monitoring for hydraulic heads is required. It can be determined that there is no problem for the structural stability by the geological data deduced from the field investigations. Additional data analysis would be required in order to check the expected lineament