The decommissioning of the Fukushima Daiichi Nuclear Power Plants is a national urgent problem in Japan. The distribution and characteristics of the fuel debris inside the nuclear reactor must be investigated to safely retrieve them.This study describes a 10 m-long articulated manipulator for investigation inside the primary container vessel. We employed a coupled tendon-driven mechanism and a gravity compensation mechanism using synthetic fiber ropes to design a lightweight and slender articulated manipulator. After discussing the basic principle and control algorithm, we focus on the detailed mechanical design of a prototype model. We confirmed its feasibility through basic motion experiments.