Whether a motion state of leg bodies (2) is a single stance state or a double stance state is sequentially determined and a total reaction force (F) is estimated on the basis of an equation of motion for a center of gravity (G0) of a bipedal walking body (1). If the motion state is the single stance state, then the estimated value of the total floor reaction force (F) is directly used as an estimated value of the floor reaction force of the leg body (2). If the motion state is the double stance state, then a floor reaction force (Fr) of the leg body (2) at the rear side is determined, using measurement data of elapsed time of the double stance state and moving speed of a bipedal walking body (1) and pre-established characteristic data, and the floor reaction force (Fr) is subtracted from the total floor reaction force (F) to determine a floor reaction force (Ff) of the leg body (2) at the front side. Thus, it is possible to provide a method that allows floor reaction forces acting on leg bodies of a bipedal walking body, such as a human being, and moments acting on joints of the leg bodies to be determined in real time with high accuracy by a relatively simple technique.