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A tendon-driven multijoint robot is a humanlike robot which is driven by a tendon-driven actuator. One of the problems is that the complexity of the tendon-driven multijoint robot body structure. The complexity makes it difficult to control body by commonly used methods which are based on a dynamics computation of a physical robot model. We proposed a appropriate control method for tendon-driven actuators. In this control method, we defined an ideal reaction model that is based on a physical motion equation. This equation includes an elastic term, inertia term and a friction term. The control method then calculates target rotational velocity of the actuator from the motion equation. Finally, the motor driver of the actuator outputs voltage to follow the target velocity through a basic PID control. Our control method enables the tendon-driven actuator to act as a mechanical spring. Through this, the tendon-driven multijoint robot can attain the flexibility to adapt to external forces applied by collision with objects without using a precise dynamics computation. Moreover, our control method enables the tendon-driven multijoint robot to adjust its stiffness and flexibility depending on the purpose of its given tasks. We tested this control method by using KOJIRO, and showed the control method works as declared.