Abstract: A leg structure for a walking robot includes first and second input links rotatably attached to a robot body, a connecting link connected with the second input link at a point whose position can be varied, and an output link connected at one end with the distal end of the connecting link and connected at an intermediate point thereof with the first input link. When the leg structure is to be converted for use as an arm mechanism, the connecting link and the output link are passed beyond the singular point where they lie on a straight line, whereby the angle between the connecting link and the output link moves through the 180.degree. boundary between the leg and arm operating states and the driving mode of the output link is changed from low speed and high torque to high speed and low torque.