1. A gait generation device for generating a desired gait for a legged mobile robot that moves by operating a plurality of legs extending from a body, comprising: allowable range setting means for setting an allowable range of a limitation-target quantity, the limitation-target quantity being at least any of a horizontal component of a translation floor reaction force or a floor-surface-parallel component of a translation floor reaction force applied to the robot or a horizontal component of a total center-of-gravity acceleration or a floor-surface-parallel component of a total center-of-gravity acceleration of the robot and a horizontal component of a floor reaction force moment or a ZMP; provisional instantaneous value determining means for determining provisional instantaneous values of a desired movement and a desired floor reaction force of said desired gait; model calculating means that inputs at least the provisional instantaneous value of said desired movement to a dynamics model to determine a model floor reaction force instantaneous value as an output of the dynamics model, the dynamics model representing a relationship between a movement and a floor reaction force of said robot; and desired instantaneous value determining means for correcting the provisional instantaneous value of said desired movement to determine a desired instantaneous value of the desired movement based on at least the difference between the model floor reaction force instantaneous value and the provisional instantaneous value of said desired floor reaction force and said allowable range, wherein the desired instantaneous value determining means corrects the provisional instantaneous value of said desired movement in such a manner that said limitation-target quantity which corresponds to the instantaneous value of the floor reaction force substantially in balance with, in said dynamics model, the resultant force of an inertial force and gravity applied to the robot caused by the desired instantaneous value of the desired movement falls within said allowable range.