Rock mass contains many discontinuities such as joints and faults. In this paper, the effectiveness of FESM, which is an analysis method of discontinuous medium considering elemental deformations and also slips and openings at interface boundaries, was verified by means of the stress transfer in a photo-elastic experiment of discontinuous rock mass models, and by means of the deformation and fracture strength in bi-axial compressive tests of plaster samples including joints. This paper clarifies that the use of a equivalent rigidity for interface boundaries results in highly precise solutions for an analysis using polygonal shaped elements.