The purpose of this study is to consider the application of strength generated following ground improvement as horizontal resistance of piles constructed in improved ground and to systematize it as a rational design method. In the proposed design method, the range of influence of horizontal resistance of piles in improved ground, or the necessary range of ground improvement, was established based on engineering assessment and horizontal subgrade reaction of piles was determined from shear strength increased by ground improvement. The validity of the design method was empirically verified by on-site horizontal loading tests of piles and finite element analysis that was conducted to supplement its parameters. Also, because the improved ground would become stronger than the unimproved original ground, the effect of the hardness of surrounding ground on piles during earthquakes was verified using a centrifugal excitation experiment and the dynamic finite element method.