The effect of the backfilled soil on the dynamic response property of the foundation is investigated. For this purpose the new analytical method is also presented. The outer domain of a soil is analyzed by the indirect boundary integral equation method in the thin layered soil model which has been developed by authors. The inner domain which contains an irregular soft soil is discretized using the axi-symmetric isoparametric finite elements. Combining equations for the total domain are obtained by applying the principle of virtual displacements to the stiffness equation of the outer domain. The three types of the foundation are chosen as analytical models. The first is rested on the elastic base through a surface stratum. The second and third are floating in the homogeneous half space soil and the upper layer of the two layered soil respectively. The results obtained by numerical analyses are as follows. 1) The rigidly backfilled soil reduces the maximum response of the superstructure within the natural period of 0.3 or 0.4 second. Conversely the softly backfilled soil is usefull for the structure with the natural period of longer than it. 2) The maximum response of the foundation bonded to the elastic base becomes lower in a soft backfill rather than a rigid one.