Coastal areas along the Pacific Ocean suffered extensive damage owing to the 2011 tsunami due to the Tohoku earthquake. Hence, reinforcement methods against tsunamis need to be developed for existing caisson type breakwaters. This study proposes installing a row of piles behind the caisson using backfilling, with rubble in the space between, as reinforcement against tsunamis. To determine the cross-section and embedment length of the pile used in this structure, understanding the load condition acting on the pile is necessary. Model experiments were conducted based on bending moment distribution to estimate the external force acting as distributed load on the pile’s offshore side. The subgrade reaction acting on the pile from the onshore side was assumed by correcting the coefficient of the subgrade reaction and its depth direction in the Winkler–Spring type subgrade reaction model. There are insufficient points in the external force distribution estimated in this method owing to insufficient evaluation of the subgrade reaction. The impact of the subgrade reaction on a pile’s horizontal resistance characteristics depends on its deformation mode. Hence, it is necessary to evaluate the subgrade reaction in the pile deformation mode under distributed load along the perpendicular pile axis direction. This study conducted loading experiments on the piles using different ground conditions and loading methods to examine the difference in horizontal resistance of the piles. The deformation mode of the piles was found to cause changes in the coefficient value of the subgrade reaction and its changing behavior along the depth direction.