The Downward Compact Conical-ended Borehole Overcoring (DCCBO) technique is one of the overcoring methods. In this study, a measurement theory for application of the DCCBO technique to orthotropic rock is proposed. Numerical experiments with the use of a 3D-FEM analysis were conducted to confirm the applicability and efficiency of the proposed measurement theory. The results showed that the measurement includes a non-negligible error if we do not consider the anisotropy of rock when such anisotropy is strong. On the other hand, this measurement is both applicable and efficient if we consider the anisotropy of rock. Finally, laboratory experiments for four kinds of orthotropic rocks using a true-triaxial compressive apparatus were carried out to verify the efficiency of the proposed measurement theory. These experiments also verified that the proposed method is suitable for use in orthotropic rock.