An external force control system for controlling an external force applied to an animal through an orthosis attached to the animal that makes a movement along with the activities of muscle fibers, the system comprising: myoelectric potential measurement means for measuring a myoelectric potential x that occurs in the body of the animal; external force setting means for setting a value of an external force f applied to the animal by a motor through the orthosis according to an external force function f(x)- with the myoelectric potential x as a variable on the basis of the measured value of the myoelectric potential x measured by the myoelectric potential measurement means; motion variable measurement means for measuring a motion variable y varying with the motion of the animal under the condition of the external force applied through the orthosis; factor setting means for setting a value of a factor γ according to a factor function γ(f, y) with the external force f and the motion variable y as variables on the basis of the set value of the external force f set by the external force setting means and the measured value of the motion variable y measured by the motion variable measurement means; determination means for determining whether a deviation δ between the set value of the factor γ set by the factor setting means and target value γ t thereof is less than a reference value ε; external force function setting means for setting a new external force function f(x) in such a way that the set value of the factor γ approaches the target value γ t if the deviation δ is determined to be equal to or greater than the reference value ε by the determination means; and motor current control means for controlling the external force applied to the animal by controlling supply current from a battery to the motor according to set value of the external force f set by the external force setting means. myoelectric potential measurement means for measuring a myoelectric potential x that occurs in the body of the animal; external force setting means for setting a value of an external force f applied to the animal by a motor through the orthosis according to an external force function f(x)- with the myoelectric potential x as a variable on the basis of the measured value of the myoelectric potential x measured by the myoelectric potential measurement means; motion variable measurement means for measuring a motion variable y varying with the motion of the animal under the condition of the external force applied through the orthosis; factor setting means for setting a value of a factor γ according to a factor function γ(f, y) with the external force f and the motion variable y as variables on the basis of the set value of the external force f set by the external force setting means and the measured value of the motion variable y measured by the motion variable measurement means; determination means for determining whether a deviation δ between the set value of the factor γ set by the factor setting means and target value γ t thereof is less than a reference value ε; external force function setting means for setting a new external force function f(x) in such a way that the set value of the factor γ approaches the target value γ t if the deviation δ is determined to be equal to or greater than the reference value ε by the determination means; and motor current control means for controlling the external force applied to the animal by controlling supply current from a battery to the motor according to set value of the external force f set by the external force setting means.