This study aims at clarifying the effects of internal force on the structural rotational stiffness of antagonized wire drive mechanism. First, we use the wire drive model to analyze the structural stiffness due to internal force, and measure the structural stiffness as a function of the internal force. Through numerical calculation and experiment results, it is revealed that the tensile internal forces can effectively change the structural rotational stiffness of the antagonized wire drive mechanism. Moreover, it is clarified that the structural stiffness due to internal force is strongly affected by the mechanical parameters such as the length of arm and the position of actuators. Next, the effective ways to generate structural stiffness of antagonized wire drive mechanism by using internal force are proposed in this paper.