A pneumatic actuator outputs large power in proportion to weight and can be installed in an inexpensive, light and compact configuration. From the standpoint of controllability, however, it is weak due to nonlinearity by compression and delayed transfer of air, it has a problem of poor accuracy in position control. To solve such problem, we propose a hydraulic/pneumatic hybrid actuator that has a pneumatic cylinder and hydraulic cylinder in parallel and uses the hydraulic cylinder as damper. We demonstrate that it can be used in practice and structurally applying it to a manipulator with 2-DOF. It also demonstrates that high-speed and stable control can be achieved by applying optimal control method.