It is necessary to explore the bottom of the RPV in order to accelerate the decommissioning of the Fukushima Daiichi Nuclear Power Plant. A coupled tendon-driven articulated robot arm, Super Dragon, has been developed to deploy the investigation equipment into the containment vessel. In this paper, we developed an extended Super Dragon with a length of 15.4 m by modifying and integrating the telescopic boom so that it can be mounted on the Super Dragon. Experiments were conducted to raise the telescopic boom vertically and to lift and hold the deployed telescopic boom horizontally. We confirmed the basic movements required to explore the bottom of the RPV.