1. A gait generating device having instantaneous gait generating means for sequentially generating an instantaneous desired gait composed of an instantaneous desired motion of a mobile robot and an instantaneous desired floor reaction force, comprising: first provisional corrected motion determining means for determining a first provisional corrected instantaneous desired motion obtained by provisionally correcting the position and the posture of a predetermined part of the mobile robot from the instantaneous desired motion; second provisional corrected motion determining means for determining a second provisional corrected instantaneous desired motion obtained by provisionally correcting the position of the predetermined part from the instantaneous desired motion while maintaining the posture of the predetermined part to be the same as the posture in the instantaneous desired motion; and desired motion correcting means for determining a corrected instantaneous desired motion obtained by executing a true correction of the position and the posture of the predetermined part in the instantaneous desired motion on the basis of the first provisional corrected instantaneous desired motion and the second provisional corrected instantaneous desired motion, wherein, if all or a part of the mobile robot is expressed in terms of a model constructed of a plurality of elements, the elements being at least either rigid bodies having inertia or mass points, the placement of elements of the model determined according to a predetermined first geometric restrictive condition, which specifies the relationship between an instantaneous motion of the mobile robot and the placement of the elements of the model, from an instantaneous desired motion generated by the instantaneous gait generating means is defined as a first placement, the placement of the elements of the model determined according to a predetermined second geometric restrictive condition, which specifies the relationship between an instantaneous motion of the mobile robot and the placement of the elements of the model, from the first provisional corrected instantaneous desired motion determined by the first provisional corrected motion determining means is defined as a second placement, and the placement of the elements of the model determined according to the second geometric restrictive condition from the second provisional corrected instantaneous desired motion determined by the second provisional corrected motion determining means is defined as a third placement, then the first provisional corrected motion determining means determines the first provisional corrected instantaneous desired motion such that the translational force component of the resultant force of the inertial forces of the elements calculated by treating the difference in the placement of the elements of the model between the second placement and the first placement as acceleration becomes substantially zero and also the moment component generated about a predetermined point by the resultant force takes substantially a predetermined value, the second provisional corrected motion determining means determines the second provisional corrected instantaneous desired motion such that the moment component generated about the predetermined point by the resultant force of the inertial forces of the elements calculated by treating the difference in the placement of the elements of the model between the third placement and the first placement as acceleration takes substantially the predetermined value, and the desired motion correcting means determines the sum of the result obtained by multiplying the posture of the predetermined part in the first provisional corrected instantaneous desired motion by a predetermined weight w 1 and the result obtained by multiplying the posture of the predetermined part in the second provisional corrected instantaneous desired motion by a predetermined weight w 2 as the instantaneous desired posture of the predetermined part in the corrected instantaneous desired motion, and also determines the sum of the result obtained by multiplying the position of the predetermined part in the first provisional corrected instantaneous desired motion by the predetermined weight w 1 and the result obtained by multiplying the position of the predetermined part in the second provisional corrected instantaneous desired motion by the predetermined weight w 2 as the instantaneous desired position of the predetermined part in the corrected instantaneous desired motion.