A legged mobile robot gives up a normal walking motion and starts a tumbling motion when an excessively high external force or external moment is applied thereto and a behavior plan of a foot part thereof is disabled. At this time, the variation amount .DELTA.S/.DELTA.t of the area S of a support polygon of the body per time t is minimized and the support polygon when the body drops onto a floor is maximized to distribute an impact which acts upon the body from the floor when the body drops onto the floor to the whole body to suppress the damage to the body to the minimum. Further, the legged mobile robot autonomously restores a standing up posture from an on-floor posture thereof such as a supine posture or a prone posture.