This paper proposes a new servo control method referred to as "passive dynamic control (PDC)," which is based on the analysis of human walking mechanism. The PDC uses the passive balancing method jointly with the active control, and realizes the purpose control and the disturbance suppression control with much less energy and much more safety than conventional control methods. The servo system, which uses balancing operations, such as mechanical positioning control, is referred to as "balance servo." In this paper, the balance servo by the PDC is described, and its application for the positioning control using the spring balancer and the Magneto-Rheological brake is shown.