The studies on a gravel pile method used for the prevention of liquefaction of saturated sand deposit during an earthquake have been carried out. To evaluate the pore pressure developed in the sand deposit 20m in depth, in which the gravel pile was installed, we solved the consolidation equation, using the finite difference method. The radii of the gravel piles 20m in length were taken 0.075, 0.15, 0.25 and 0.40m. The EL Centro earthquake record (NS-Comp., May, 1940) and five others were used as the base motion in these analyses. As a result, the optimum radius and the optimum distance, of the gravel pile, used in a design was determined in the relationships between the cost of construction and the limited value of pore pressure buildup.