The large sized, multi-walled coaxial cylindrical tank has been studied in recent years because various kinds of oil can be stored simultaneously in one tank. In the design of this tank, the bifurcation buckling of the ring-stiffened inner shell plate under external liquid pressure becomes an important problem. The diameter/thickness ratio of the oil storage tank is considerably greater than that of the other cylindrical shells, and the ring-stiffened cylinders of such diameter/thickness ratio under the large external pressure have never been used so far. This paper presents the bifurcation buckling analysis of the inner shell plate by the finite element method. For the estimation of the minimum required number of ring stiffeners to prevent the buckling, the location where rings should be attached to the shell is determined considering the buckling mode.