1. A method of estimating a floor reaction force acting on each of leg bodies of the bipedal walking moving body, comprising: a first step of judging whether a motion state of the leg bodies of said bipedal walking moving body is a single stance state in which only one of the leg bodies touches the ground, or a double stance state in which both leg bodies touch the ground; a second step of sequentially deriving positions of the center of gravity of said bipedal walking moving body, and sequentially deriving accelerations of said center of gravity in an absolute coordinate system fixed relative to the ground by the use of time series data about the positions of said center of gravity; a third step of sequentially deriving positions of a specific portion relative to said center of gravity at least in said double stance state, said specific portion predetermined in the neighborhood of a lower end portion of each leg body; a step of deriving estimated values of said floor reaction force acting on the leg body touching the ground sequentially, in the single stance state of said bipedal walking moving body, based on an equation of motion for said center of gravity expressed by a weight and an acceleration of gravity of the bipedal walking moving body, the acceleration of said center of gravity, and said floor reaction force acting on the leg body touching the ground; and a step of deriving estimated values of said floor reaction forces respectively acting on both leg bodies sequentially, in the double stance state of said bipedal walking moving body, based on an equation of motion for said center of gravity expressed by a weight and an acceleration of gravity of the bipedal walking moving body, the acceleration of said center of gravity, and said floor reaction forces respectively acting on both leg bodies, and an expression of relation between the position of said specific portion of each leg body relative to said center of gravity and said floor reaction force acting on said leg body, said expression of relation being determined based on assumption that said floor reaction force acting on each leg body acts from said specific portion of said leg body toward said center of gravity.