When the sole of the foot of one of the leg sections of the robot leaves or touch the floor, the angular velocity of the joint angle .theta. of each of the joints of the leg section of the robot becomes infinitely large and excessive loads are applied to the respective joints if the leg section is stretched out or the robot takes some other peculiar attitude. Therefore, when the sole of the foot section of a leg section leaves or touches the floor, the constraining conditions are defined for the foot section relative to the floor surface that the foot section touches according to the degree of operability of the leg section in order to reduce the loads that are applied to the respective joints of the leg section. Additionally, the attitude of the foot section is determined so as to minimize or reduce the potential energy attributable to a minute quantity of positional changes of the joints of the leg section.