Humanoid robots demand high performance on actuators, such as power-to-weight ratio, durability, occupation volume, and freedom of placement. Also, for robots to interact with unknown objects, flexibility is necessary. Backdrivability is effective when both flexibility and high output torque needs to be realized, which were often contradicting requirement. In this paper, backdrivable hydrostatic transmission is proposed to satisfy requirements above. First, development of hydrostatic actuator including conditions of backdrivability is explained. Second design of backdrivable pump, hydraulic motor and manifold design is explained. Next, design of an anthropomorphic robot hand using developed hydrostatic transmission is discussed. Finally, developed hand is presented and experiments on backdrivability, force sensing, and grasping were performed.