Biped robots require substantial amounts of power on each leg alternately while walking, hopping, and running. However, it is difficult to adopt large high-power electrical motors in conventional mechanical transmission systems owing to spatial limitations. To address this problem, a hydraulic interlocking drive system that incorporates two hydraulic direct-drive systems is proposed for biped humanoid robots. The hydraulic interlocking drive system connects the two hydraulic direct-drive systems and concentrates the pump output on one side cylinder. The other side cylinder meter-in flow rate is controlled by the meter-out flow rate from the cylinder on which the pump is concentrated. Good position tracking and excellent energy saving are achieved with the proposed system. A performance comparison with a single hydraulic direct-drive system shows that the motor power of the hip pitch joint is reduced by 27.3% for walking patterns. This result shows that the rated output of the motor can be reduced, and smaller and lighter motors can be installed in biped robots.