In this paper, we develop a novel robotic joint mechanism that has a polyurethane round-belt and a linear spring placed on each side of the joint. This mechanism is based on antagonistic mechanical configuration by the round-belt activated by twisting motion. This mechanism enables the robotic joint to move around its axis according to the contraction force generated by the twisting. In addition, antagonist side of the joint generates passive tension by a spring, which may reduce the range of motion of the joint. In order to improve that problem, this study newly modified the joint mechanism so that the tensile force can be eliminated by maintaining natural length of the spring.