This research aims to analyze the characteristics of the tsunami that had stricken Osaka Bay Area in the initial Jomon period. The bathymetry of Osaka Bay and topography of Osaka Plain of 11,000 years before present are reproduced rigorously from boring data and acoustic profiling data for tsunami numerical simulation. Tsunami numerical simulations assuming ordinary level and maximum level of Nankai Trough earthquake tsunami are carried out with the water levels of every 1,000 years of initial Jomon period. It is found that tsunamis in early initial Jomon period were relatively low, and by contrast, tsunamis in late initial Jomon period were high. The area where high tsunami struck in the initial Jomon period were found from the simulation results.