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This paper presents an external force observer that estimates the external force acting on a biped humanoid robot, such as a collision force with a human or with an object. Since biped humanoid robots balance themselves on a limited area with foot soles, the detection of external force is important to realize for a stable balance controller for humanoid robots working in a real environment. In the proposed observer, the external force is estimated using inertial sensors and foot force sensors based on simple but efficient modeling of the forces applied to the robot. This paper also shows the experiments of the proposed external force observer using a real humanoid robot HRP-2. The experimental results show that the proposed observer has satisfactory performance to estimate the external force with sufficient response and accuracy.