This paper proposes a methodology to assess the seismic performance of existing pile group foundation, which includes the approaches of investigation, setting of earthquake motion, response analysis and seismic-performance evaluation. This methodology was developed based on the lesson of the Hyogoken-Nanbu Earthquake on January 17, 1995 that caused devastating damage. In order to grasp the suitability of the approach, a pile foundation which experienced the earthquake was taken as the object for assessment, and comparison study was conducted between the analysis and reconnaissance. As a result, the adequacy of the methodology was confirmed to prove good agreement between the two results from the viewpoint of engineering practice.