Very long-reach snake-like robotic arms in the range of 14 meters are expected to be used in decommissioning work inside nuclear reactor containers. We developed a tendon-driven system which has the advantage of placing electronic devices protected in the arm's base part which stays out of the reactor container, and only few expensive highly radiation hardened sensors and tools are mounted in the arm tip. Generation of large joint torque is necessary to realize such a very long arm. We applied the concept of coupled tendon-driven mechanism formerly used to generate restricted two dimensional motions, and extended its concept by proposing a new joint arrangement which makes possible three dimensional motions in a large workspace. Mechanical design, compact storage method and derivation of the arm motion control are detailed. Moreover, we built a preliminary mechanical prototype called ”Mini 3D CT-Arm”, and the experimental results demonstrated the validity of the newly proposed concept.