The invention provides a control device comprising: an actual value obtaining part, obtaining a torque actual value and a velocity actual value, wherein the torque actual value represents a torque generated by the driving source and the velocity actual value represents a velocity of the motion body; an inferring part, which calculates an external force inferred value and a velocity inferred value every other operation period based on the torque actual value by using an operation formula of a predetermined model representing driving of the motion body, wherein the external force inferring value represents an external force generated by the control system; and an output part, evaluating a reliability of the external force inferred value based on the velocity inferred value calculated together if the inferring part calculates the external inferred value, and effectively outputting the external force inferred value when it is judged that there is a designated reliability.