In recent years, the robot has been in the field of medical scene and human life. These robots operate in an environment close to human, it is necessary to achieve the coordination of work and ensure the safety of people, when these robots contact with humans. Therefore, these robots be flexible and lightweight, it is desirable to have the characteristics a close to human behavior. For this reason, we have studied and developed an artificial muscle actuator using straight-fiber-type artificial muscles derived from the McKibben-type muscles. In this study, we propose a new method of joint stiffness control to make a delicate operation on the artificial muscles manipulator, consider the position control and stiffness control separately.