In Japan, there is a serious and urgent issue on seismic retrofit for a lot of old Japanese- style two-story wooden houses built by a Japanese traditional framed-construction method. In order to investigate the seismic performance of an old Japanese-style two- story wooden house, 3-D non-linear collapsing process analysis of this wooden house was conducted against a strong earthquake ground motion with the Japan Meteorological Agency seismic intensity of “6 upper” level. The effect of post fixing condition under the floor of wooden house on the seismic response of an old Japanese- style two-story wooden house was numerically investigated in this paper. As a result, it was found that seismic collapsing behavior of the wooden house strongly depends on the post fixing condition under its first floor.