An assist device 11 is equipped with a spring means 21 (gas spring), and a piston 24 in a cylinder 23 moves upward or downward according to a relative displacement motion (flexing or stretching motion) of a thigh 4 and a crus 5 at a knee joint 8 of a leg 3 of a robot. Air chambers 25 and 26 above and below the piston 24 are filled with gases. If a flexing degree at the knee joint 8 is a predetermined value or less, then the air chambers 25 and 26 are brought into communication through a groove 28 in the cylinder 23, and the spring means 21 does not generate an elastic force, but if the flexing degree exceeds the predetermined value, then the air chambers 25 and 26 are hermetically sealed from each other and the spring means 21 produces an elastic force, the elastic force acting on the knee joint 8 as assisting driving force. A burden on a joint actuator of a leg can be reduced, while reducing energy consumption of the robot by using a small and simple construction. Moreover, a change in the characteristics of an assisting driving force of the spring means 21 in response to a flexing degree of the knee joint 8 can be restrained.