The nonlinear soil response is examined by using KiK-net vertical array records from a large (the 2003 Tokachi-oki earthquake; M8) and small (M-6) events. First we search stations which show nonlinear soil response by comparing the S-wave spectral ratio of the Tokachi-oki earthquake with those of three small events; the S-wave spectral ratio is taken from the surface spectrum divided by the borehole one. We find three stations from 106 KiK-net stations in Hokkaido that show the nonlinear soil response. These stations' spectral ratios show amplitude decay at high frequencies and peak shift to lower frequency for the spectral ratio from the Tokachi-oki earthquake. The TKCH07 station at Toyokoro in Tokachi-district, Hokkaido, is chosen to evaluate the nonlinear soil response. This site has very soft ground different from another KiK-net stations; S-wave velocity (Vs) is less than 200m/s down to a depth of 30m. This station observed peak ground acceleration (PGA) of 403.9cm/sec² during the Tokachi-oki earthquake. Next we construct the soil model (Vs and Qs (quality factor for S-wave) structures) at TKCH07 based on the S-wave spectral ratios from 22 small earthquakes by using the genetic algorithms (GA) inversion method. Finally we evaluate the nonlinear soil response at TKCH07 by comparing the surface accelerogram observed during the Tokachi-oki earthquake with those calculated using the borehole accelerogram and the constructed soil model; the linear response analysis, equivalent linear response analyses and nonlinear response analysis are used in the calculations. The frequency dependent equivalent linear response analysis is newly proposed; this method well explains the observed surface accelerogram.