½éÉÜ£º Many existing infrastructures constructed prior to 1980 in Japan were designed for a particular seismic load which is smaller than the Level 2 ground motion caused by the maximum considered earthquake (MCE). Those existing structures must be retrofitted to comply with the seismic requirement for the level 2 ground motion to be newly designated after 1995 Hyogoken-Nanbu earthquake. For such pile-supported structures, a new seismic retrofitting approach is proposed by introducing additional reinforing piles. The effectiveness for this approach will be discussed based on the seismic performance-based design method