This paper discusses a methodology of semiactive control strategy using a variable-friction damper. In this variable-friction damper only the slip-force level is controlled in response to the magnitude of ground motion in such a way that the damper should exhibit a hysteresis to dissipate the energy in any seismic event. The employed control algorithm is rather simple, yet quite effective. The effectiveness of such a variable-friction damper is demonstrated through the computer simulation applying the damper system to a twenty-story building. Then, the control effectiveness is estimated in terms of the equivalent damping factor by utilizing system identification technique.