Human-robot systems including interaction between human operators and robots should be designed with careful consideration for dynamic property and control ability of the human operator. In this paper, tracking tests of the human-robot system using an impedance-controlled robot are performed in order to clarify the control ability of human operators, and changes of the tracking ability and the dynamic property of the human operator are analyzed for different values of the robot impedance. From the experimental results, it is shown that the human operator tries to keep the dynamic property of the overall system as constant as possible by adjusting his or her own impedance property.