An assist device is provided with a spring device that generates an auxiliary driving force, by an elastic energy, acting on a joint of a leg. The spring device includes a gas spring having a cylinder and a piston. Bending/stretching motion between link members and at the joint is transmitted to the spring device through a motion transmission device that includes a link arm. The spring device is provided so that the auxiliary driving force increases as a knee bending angle .theta. increases until the knee bending angle .theta. reaches a predetermined angle, and so that, once the knee bending angle .theta. exceeds the predetermined angle, the auxiliary driving force becomes smaller than or equal to the driving force obtaining at the predetermined angle, effectively reducing a burden on an actuator of the legged mobile robot joint.