|This paper presents a study of a design method for improving the ground around piles using the vacuum consolidation method、 one of various ground improvement methods、 and using the increased ground strength to reflect onto the horizontal resistance of piles. This is a rational design method that can reduce construction costs for site conditions with soft ground、 where horizontal resistance determines pile specifications. In this design method、 the range of influence of the horizontal resistance、 or the range of ground improvement requiring the vacuum consolidation method、 is set from 1/β (the characteristic length of piles: β=4√(KD)/(4EyI)) to the range of passive earth pressure、θ=45+φ/2. The horizontal subgrade reaction of piles is determined by converting the shear strength C of ground、 increased by consolidation、 to the modulus of deformation E. The validity of the design method was empirically verified by applying it to an actual site and conducting horizontal loading tests on actual piles. The seismic behavior of the piles in the improved ground was examined using the conventional earthquake-resistant design method.