In a process of preventing liquefaction of the structure foundation ground, the ground where the structure weight is supposed to produce distribution of relatively large stress in the event of earthquakes is reinforced with a Iron-Lime stabilized soil that is highly resistant to water and is able to connect hard and soft portions in a well balanced manner, thereby forming a buffer layer against impacts to improve safety of the pile foundation and prevent liquefaction of the foundation ground. To this end, the depth at which the load stress due to a structure to be constructed is equal to the bearing capacity of the existing ground is determined, the equivalent depth converted from the aforesaid depth on an assumption that the modulus of deformation is 21,000 kgf/cm.sup.2 is determined, and a refilled portion of the structure underground corresponding to the equivalent depth is formed as a stabilized layer comprising natural soil iron oxide and lime.