The distribution of the lateral earthquake response on the mid-story isolated system is strongly influenced by the position of isolated story and the damper. In some cases, the seismic response on the upper structure of isolated story is amplified greatly in comparison with the base isolation. In this study, by sweeping the modal response with free-free mode-shape vectors, it is clarified that the response amplification on the upper structure of isolated story is caused by modal coupling between modes of vibration on upper and lower structure. Based on the analytical study, the prediction method of the lateral earthquake response distribution on the mid-story isolated system is proposed.