A new hypothesis clarifies the phenomenon of liquefaction of saturated sandy soil during large earthquakes. It is based on the multi-reflection theory and dynamic response calculation using FEM. The results using the hypothesis applied to soil at Hachinohe Port during the 1994 Far--Off Sanriku Earthquake （M=7.5） in calculating, showed good correspondence with the observed phenomena associated with this earthquake. The method was applied to other areas that had liquefied in the past and confirmed within acceptable limits the mechanism of liquefaction. The Fukui Earthquake （June, 1948,M=7.1） occurred at depth of 15 km at the center of the Fukui basin and caused widespread damage and 5 300 casualties. The earthquake induced largescale liquefaction everywhere and covered rice field over a wide area with sand and gravel. However, regardless of very strong acceleration several parts of the basin did not liquefy. Using the hypothesis, we analyzed differences in surface layer under the same conditions. As a result, we found that the liquefaction depends on the size of strain caused by the rigidity of the surface layer and that the hypothesis accurately corresponds to what happens in practice.