The direction of a component vector on a plane perpendicular to a predetermined axis of a floor reaction force acting on landing legs of a two-legged walking mobile body is determined such that the direction of a moment generated around a center-of-gravity of the body by a total sum of the component vectors on the plane is coincident with the direction of the component around the predetermined axis of a total rate of change of angular momentum by calculating the total rate of change of angular momentum, and the total floor reaction force acting on the body by sensing joint displacement, acceleration, angular velocity, and using a rigid link model. An estimated value of the component vector is found based on the determined direction and the obtained total floor reaction force. This enables finding an estimated value of the floor reaction force showing a smooth and continuous change.