The present paper develops for determination theory of initial stresses in isotropic or anisotropic elastic rock masses utilizing stress concentration of two neighboring boreholes, and describes a method determining five stress components (σ^∞_x, σ^∞_y, τ^∞_xy, τ^∞_xz, and τ^∞_yz) of in-plane and out-of-plane against the borehole axis (z-axis) among six components that are included in three-dimensional initial stress condition. Furthermore, some numerical results of high accuracy of sensitivity coefficients along the wall surface of the boreholes are shown and the concrete calculating method of initial stress components using these results is given.