Describes a biped walking control system based on the reactive force interaction control at the foothold: 1) robust control of reactive force/torque interaction at the foothold based on Cartesian space motion controller: 2) posture control considering the physical constraints of the reactive force/torque at the foothold by quadratic programming. The proposed approach realizes robust biped locomotion because the environmental interaction is directly controlled. The control is applied to the 20 axes simulation model, and the stable biped locomotion is realized even if an unknown small slope exists. Stable attitude control is confirmed by 14-axis biped robot experiments.