This paper presents an energy-ecient biped walking method that has been implemented in a low-cost humanoid platform, PINO, for various research purposes. For biped walking robots with low torque actuators, a control method that enables biped walking with low torque is one of the most important problems. While many humanoids use highperformance motor systems to attain stable walking, such motor systems tend to be very expensive. Motors that are aordable for many researchers have only limited torque and accuracy. Development of a method that allows biped walking using low-cost components would have a major impact on the research community as well as industry. From the view point of high energy-eciency, many researchers have studied a simple planar walker without any control torque. Their walking motions, however, are decided by the relationship between a gravity potential effect and structural parameters of their robots. Thus, there is no control of walking behaviors such as speed and...