The water pressure variations acting on the surface of a highly saturated sand bed change the pore water pressure distribution in the bed with time. The occurrence of excess pore water pressure produces a decrease in the strength of the sand bed. Under certain conditions the effective stress may become equal to zero; that is, liquefaction will occur. In this study the characteristics of the settlement of a concrete block placed on such a liquefied sand bed are investigated experimentally and theoretically. Experimental results show that the main factors influencing the characteristics of the settlement are the porosity of the sand bed and the attached area, among the many factors related to the settlement phenomenon. We obtained.the changes of pore water pressure and effective stress distributions with a progress of the settlement by numerical analyses, and clarified the dynamical mechanism of the settlement to some extent.