The vibrational characteristics of the improved ground by Deep Mixing Method are analysed by using the acceleation records observed, and the seismic response of
the improved ground computed by Finite Element Method using the equivalent linear method are compared with the records observed. The results are summerized as follows: (1) The improved ground behaves like a rigid body. (2) Acceleration and dynamic displacement of the improved ground are almost equal to those of the sand layer under the improved ground. (3) The maximum acceleration of the top of the improved ground are smaller than that of the surface of soft clay layer far from the improved ground. (4) The maximum accelerations of the improved ground and of the clay layer do not occur at the same time. (5) It was proved that this computer program was very useful for analysing the vibrational characteristics of the improved ground.