Tow forces can be measured only by providing one force sensor in a device structure, and a drive system is driven so as to assist an external force by detecting it. A value at a force sensor (114) at the moment when a foot switch (121) is set as the original point (detection reference) of a force to be assisted. Variations of the force after the original point setting is measured by the force sensor (114) the difference between the measured value and the assist original point is obtained, and a force to be assisted is detected. Depending on the magnitude of the force to be assisted, assist operation is performed by causing a drive force to act on a foot such that the force to be assisted is reduced. When the force is assisted reaches zero, the assist operation is completed. Then, two forces, or "a force acting on a patients foot" and "a force applied to the foot" from the outside, are detected by the one force sensor (114).
