Various types of devices have been developed to reduce seismic response of the mechanical systems. Base isolation system is one of them. In such devices, friction characteristic is used. The authors have developed a base isolation system with friction for relatively small mechanical system. In ordinary case, response of mechanical system decreases with the increase of friction coefficient. However, in the experiment to examine the effectiveness of base isolation system, it is found that acceleration response takes minimum value at some friction coefficient. In this paper, experimental results are examined using a simplified model. As earthquake excitations, some types of random vibration are used. Responses of the mechanical system are obtained from moment equations using the equivalent linearization method. It is demonstrated that acceleration response takes minimum value at some friction coefficient in some conditions.