In this paper, we present a novel stiffness adjustable structure that changes its stiffness by pulling a tendon. It adopts an endoskeleton structure where rigid segments and compliant segments are alternately connected in series. The stiffness of this structure is controlled by compressing the compliant segments with an axial force. A tendon that runs through the endoskeleton and is fixed at the tip provides the axial compression force when pulled. We analyze the structure using the cylindrical isolation bearing model. The bending stiffness of the proposed structure was simulated and compared with experimental results. The structure can be stiffened approximately fifty-times the original stiffness. This stiffness adjustable structure can be used to increase the efficiency of a system that uses compliance, e.g., a robotic fish that uses a compliant fin for its propulsion system.