A method of controlling an external force applied to an animal to achieve as a target relation the relation between the external force and a motion variable varying with the motion of the animal. A value of an external force f applied to a human leg according to an external force function f(x) on the basis of a measured value of a myoelectric potential x that occurs in the human leg, current I of a motor 220 is controlled according to the set value, and the external force f is applied to the leg through an orthosis 222. A resultant force (the sum of an internal torque and an external torque around a knee joint) F is measured as “a motion variable.” Moreover, a value of a factor γ is set according to a factor function γ(f, F) on the basis of the set value of the external force f and the measured value of the resultant force F. If a deviation δ between the set value of the factor γ and target value γt thereof is equal to or greater than a reference value ε, a new external force function f(x) is set in such a way that the set value of the factor γ approaches the target value γt.