Robots tend to be unstable at contacting environment, since robots receive high order disturbance like a impact force. In order to continue stable motion, robots should not only track to command value, but reduce the impact force. In many conventional methods, impedance control is applied to these kinds of problems. However, there is a problem that robustness of position control may be low with the impedance control. It is because that the characteristics of position control and force control are derived from same parameters. Hence, designers troubled over tuning of these parameters. In this paper, control system transformation method is shown. Consequently, designers are able to change the system behavior as they like. Selected control system for contact motion is that, physical model is second order system, and controller is set as sliding mode control. Second order system is compliant for the external force, and sliding mode controller ensures high robustness of position control and force control, and the stability is also assured. In the result, robots are able to suppress the impact force, and to continue stable motion. The effectiveness of the proposed method is confirmed by experiment.