PROBLEM TO BE SOLVED: To provide a knee joint which does not need an external power, is light and small, and generates extension torque with a simple structure.SOLUTION: A knee joint 100 is provided which comprises a thigh side link 10 fixed to the thigh side, a lower thigh side link 20 fixed to the lower thigh side, and a shaft connecting the thigh side link and the lower thigh side link. The knee joint has an input cylinder 40 arranged on a sole position or a lower thigh position, an output cylinder 50 arranged between the thigh side link and the lower thigh side link, and a hydraulic circuit 60 connecting the input cylinder and the output cylinder. The input cylinder is compressed by floor reaction force during walking movement. Such an input is transmitted via the hydraulic circuit to the output cylinder. Force output by the output cylinder generates an extension torque at the thigh side link.
