While a biped walking mobile body is in a motion, including level-ground
walking, the position of the center of gravity (G0) of the biped walking
mobile body, the position of an ankle joint (12) of each leg (2), and the
position of a metatarsophalangeal joint (13a) of a foot (13) are
successively grasped, and the horizontal position of a floor reaction
force acting point of the leg (2) in contact with the ground is estimated
on the basis of the relative positional relationship among the aforesaid
positions. Depending on whether the center of gravity (G0) is behind the
ankle joint (12), between the ankle joint (12) and the
metatarsophalangeal joint (13a), or before the metatarsophalangeal joint
(13a) with respect to the advancing direction of the biped walking mobile
body, the horizontal position of the ankle joint (12), the center of
gravity (G0), or the metatarsophalangeal joint (13a) is defined as the
horizontal position of a floor reaction force acting point. The vertical
position of the floor reaction force acting point is estimated on the
basis of the vertical distance from the ankle joint (12) to a ground
contact surface.