An assist device that applies an auxiliary driving force to a joint in parallel with a driving force of a joint actuator between a thigh portion and a crus portion, which are a pair of link members of a leg. The assist device generates the auxiliary driving force by use of spring device, such as a solid spring or an air spring. A member supporting a rod member connected to the spring device is provided with a device for transmitting a bending and stretching motion of the leg at the joint (a relative displacement motion between the thigh portion and the crus portion) to the spring device to generate an elastic force of the spring device, and for discontinuing the transmission of the bending and stretching motion to the spring device. This transmitting device is controlled in accordance with a gait of a robot. Thus, a burden on the joint actuator is reduced where necessary and favorable utilization efficiency of energy can be stably ensured.