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In this paper, We develop three-dimensional quasi-passive walker with flat feet driven by an antagonistic pneumatic artificial muscle. An antagonistic mechanism is constituted by a pair of McKibben muscles. And an antagonistic pneumatic system is used as joint actuators of linkage mechanisms which control the torque, joint stiffness and position simultaneously. However, it is more difficult to control the pneumatic actuator than a usual electric motor. Therefore, we also examined the structure of an ankle and the control method of an antagonistic pneumatic system. As a result, a three-dimensional quasi-passive walking is realizable by means of adjusting the stiffness of an ankle joint. From experimental results of quasi-passive walking, the average gait were obtained with the travel distance 1350 mm, the stride of a step 90 mm, and the number of steps 15 steps. It was demonstrated that the quasi-passive walking is stabilized when both the robotic motions in a lateral plane and in a sagittal plane are synchronized.