A control device configured for controlling a control system which contains a motion body and a driving source driving the motion body, comprising: an actual value obtaining part (12, 14), configured for obtaining a torque actual value and a velocity actual value, wherein the torque actual value represents a torque generated by the driving source for driving the motion body and the velocity actual value represents a velocity of the motion body; and an disturbance observer, comprising: an inferring part, which is configured to calculate an external force inferred value and a velocity inferred value every 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, the velocity inferred value corresponds to the external force inferred value and represents a velocity of the motion body, and the operation formula contains items for updating the velocity inferred value and the external force inferred value such that the velocity actual value and the velocity inferred value are consistent to converge the operation formula with a convergence parameter determined according to stability and convergence; the control device being characterized in that the disturbance observer further comprises: an output part, configured for evaluating a reliability of the external force inferred value based on the velocity inferred value calculated together if the inferring part calculates the external force inferred value, the output part comprises a switching block, wherein the switching block is configured to judge that there is a designated reliability when a difference between the velocity inferred value and the corresponding velocity actual value is smaller than a threshold predetermined according to the convergence parameter, and the output part is configured to effectively output the external force inferred value when there is the designated reliability judged by the switching block, the external force reflects a friction force generated between the motion body and an object. an actual value obtaining part (12, 14), configured for obtaining a torque actual value and a velocity actual value, wherein the torque actual value represents a torque generated by the driving source for driving the motion body and the velocity actual value represents a velocity of the motion body; and an disturbance observer, comprising: an inferring part, which is configured to calculate an external force inferred value and a velocity inferred value every 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, the velocity inferred value corresponds to the external force inferred value and represents a velocity of the motion body, and the operation formula contains items for updating the velocity inferred value and the external force inferred value such that the velocity actual value and the velocity inferred value are consistent to converge the operation formula with a convergence parameter determined according to stability and convergence; the control device being characterized in that the disturbance observer further comprises: an output part, configured for evaluating a reliability of the external force inferred value based on the velocity inferred value calculated together if the inferring part calculates the external force inferred value, the output part comprises a switching block, wherein the switching block is configured to judge that there is a designated reliability when a difference between the velocity inferred value and the corresponding velocity actual value is smaller than a threshold predetermined according to the convergence parameter, and the output part is configured to effectively output the external force inferred value when there is the designated reliability judged by the switching block, the external force reflects a friction force generated between the motion body and an object. an output part, configured for evaluating a reliability of the external force inferred value based on the velocity inferred value calculated together if the inferring part calculates the external force inferred value, the output part comprises a switching block, wherein the switching block is configured to judge that there is a designated reliability when a difference between the velocity inferred value and the corresponding velocity actual value is smaller than a threshold predetermined according to the convergence parameter, and the output part is configured to effectively output the external force inferred value when there is the designated reliability judged by the switching block, the external force reflects a friction force generated between the motion body and an object.