Effective design method for response spectra of a single degree of freedom (SDOF) structure taking into consideration liquefaction of ground layer and ground improvement by the compacting method is presented. This method consists of the 2D effective stress analysis of surface ground layers, the simulating analysis of the sand compaction pile method and the seismic response analysis of a SDOF structure. The renewal natural period of the ground due to the ground improvement and the natural period of a SDOF structure are used as two key parameters for evaluating response spectra. Results of numerical computation show that the natural periods of the ground and a SDOF structure are useful for aseismic design of a SDOF structure and the ground improvement works to reduce the responses of a SDOF structure with the long natural period.