The endoskeleton humanoid robot needs an artificial muscle that is comparable with a human muscle from a viewpoint of form, flexibility, output force, output displacement and the responses. For this realizing, we are studying a Shape Memory Alloy (SMA) actuator that protected by a rolled film tube have the high heat resistance and the high flexibility. In this study, this actuator is inserted into a natural rubber tube for mobilizing and experimented about its antagonistic stiffness control.