A coil spring 14 is provided in a leg link 3. In the case where an extent of flexion of the leg link 3 is not more than a predetermined extent, the coil spring 14 applies, to a third joint 8, a first biasing force for preventing the extent of flexion of the leg link 3 from decreasing due to gravity acting on a body weight support device A. In the case where the extent of flexion of the leg link 3 exceeds the predetermined extent, the coil spring 14 applies, to the third joint 8, a second biasing force for decreasing the extent of flexion, where the second biasing force is larger than the first biasing force and increases with an increase of the extent of flexion. This enables a sufficient support force to be generated when standing up from a half-sitting posture or a crouching posture, without significantly interfering with walking.