The Sand Compaction Pile Method (SCP) is a major method to improve soft clay ground. Natural sand is generally used as a filling material of SCP. However, iron and steel slag has been recently applied as a new material because natural sand is difficult to be obtained in Japan. And, it is one of the important problems to clarify seismic behavior in applying such a new material. In this study, the cyclic loading tests using hollow cylindrical torsional test apparatus and dynamic centrifuge model tests were conducted to investigate the seismic behavior of improved ground using iron and steel slag. Furthermore, the deformation behavior of these materials is examined through simulation of the results of these tests by numerical analyses.