A gait generating system of a legged mobile robot is provided with a device for determining a desired trajectory of an external force to be applied to a robot 1, a device for determining a parameter of a desired gait (current time gait) for a predetermined period on the basis of a desired trajectory of an external force or the like, a device for determining a parameter of a virtual cyclic gait that follows the current time gait on the basis of the desired trajectory of the external force or the like, a device for correcting the current time gait parameter such that a body motion trajectory of the robot 1 of the current time gait converges to a body motion trajectory of the cyclic gait, and a device for sequentially determining an instantaneous value of the current time gait on the basis of the corrected current time gait parameter. With this arrangement, in an environment wherein an external force acts on a robot as necessary, a desired gait that allows continual stability of the robot to be secured can be generated even if the external force suddenly changes.