The core discing, which is observed on the boring core collected from rock mass in underground, has attracted interest with respect to the estimate of stress conditions in the rock mass.In order to investigate the occurrence of the core discing on the soft rock, the laboratory tests are conducted on the blocks of cement-mortar at various compressive strength under the various confining pressures. From the test, the discing phenomenon is observed on the core collected from the block of cement-mortar and the relation between the confining pressure (Pc) required to produce discing and the compressive strength of the block (Sc) is formulated as follows:Sc<0.56×PC+20It is then found that the thickness of disc is thiner and that the recovery percentage of the core is smaller, as the confining pressure increases.The same boring tests as was stated previously are carried on the blocks of KIMACHI sandstone. Consequently, it is clear that the confining pressure required to produce discing does not conform to above inequality.Besides, the stresses are analyzed on the simplified model of the boring test with using F. E. M. to investigate the mechanism of producing the core discing. In the stress analysis, it is considered that the failure of core on the blocks of cement-mortar occurs by the shear stress and that the failure of core on the block of KIMACHI sandstone occours by the tensile stress. In this way, it is supposed that the mechanisms of forming core discing are different with the mechanical properties of the rock mass.