Strong ground motion which exceeds 1G was observed at KiK-net Mashiki (KMMH16) site in Mashiki Town, Kumamoto Prefecture, during the 2016 Kumamoto Earthquake. Since such large amplitude ground motion was observed, we discussed effects of soil-structure interaction on damage of a building located near the observation point. For large input ground motion, the interaction phenomenon becomes more complicated because of nonlinearity of soil deposit, soil near foundations, and structures.<br> In this paper, our objective is to explain soil-structure interaction of Mashiki Town Office, whose damages of superstructure and piles have been reported. Since strong motion records were obtained at KMMH16 site and first floor of the town office building, we got rare data to evaluate the nonlinear soil-structure interaction of buildings supported by piles during large earthquakes.<br><br> In order to achieve this objective, numerical analyses were conducted. At the beginning, nonlinearity of soil deposit of KMMH16 site was discussed based on one-dimensional nonlinear dynamic analysis. Secondly, the effect of gap between piles and clay soil on horizontal soil resistance of piles was discussed based on horizontal loading analysis for pile group which supports footing of the town office. Finally, using soil-pile-building system model of Mashiki Town Office, the effect of the gap on response at first floor and damage of piles was discussed.<br><br> Results obtained from this study are summarized as follow:<br> 1) In the analysis at KMMH16 site, we assumed to decrease shear wave velocity in order to consider nonlinearity of deep soil for large input ground motions. As a result, calculated ground motion at the surface was good agreement with observed one.<br> 2) We calculated ground motions at Mashiki Town Office (MTO) site in order to obtain input ground motions for the interaction analysis. At this site, seismic motion amplification at soft volcanic clay layer was larger than that of KMMH16 site. While calculated shear strain of KMMH16 site ground was less than 1%, that of MTO site ground exceeded 2%.<br> 3) For cyclic loading on piles in the clay layer, the hysteresis of subgrade reaction-displacement curve had slip behavior because of the gap between piles and soil.<br> 4) In the analysis of the pile-building system of Mashiki Town Office, calculated acceleration response of the first floor was good agreement with observed one. From the result of the analysis considering slip behavior, it is suggested that the gap between piles and soil and continuous strong motions influenced response at the first floor and damage of piles because of decrease of horizontal soil resistance.