[en] The long-term stability of near-field rock mass influenced by swelling pressure of buffer material and expansion pressure caused by the corrosion of overpack was studied in this report to improve conventional credibility of evaluation methods for the long-term dynamical behavior of near-field rock mass which do not take account of these pressures. Swelling pressure of buffer material was modelled as temperature load and expansion pressure caused by the corrosion of overpack was modelled as forced displacement. Because swelling pressure of buffer material and expansion pressure caused by the corrosion of overpack acted on near-field rock mass as inner pressure, it was estimated that long-term stability of near-field rock mass could be improved compared with conventional evaluation which gave safe-side results. As an exception, when stiffness property of buffer material is large and initial stress of rock mass is small, tensile fractures in near field rock mass might be generated. In such a case, because buffer material may extrude into generated fractures, it is necessary to be evaluated the performance of the engineered barrier in detail. (author)