This study is to synthesize knowledges about the vibratory characteristics and the earthquake response of a mid-story isolated system. First it is shown that an equivalent two mass system is useful enough for estimating the vibratory characteristics of the mid-story isolated system from the results of eigenvalue analysis. Then a response analysis based on energy balance applies to the equivalent two mass system, and the seismic prediction method which summarized the foregone knowledges is proposed. Futhermore, it is clarified that the mid-story isolated system holds promise of adecuate aseismic performance by using the seismic prediction method proposed.