It is very important for tsunami disaster prevention in the coastal area of Shikoku Island to clarify a mechanism of tsunami propagation in the Seto Inland Sea. Especially, a resonance of tsunami is a dominant factor in such a narrow inland sea. In this study, numerical simulation of tsunami propagation is executed to estimate tsunami height and characteristic period in the Seto Inland Sea. According to the numerical analysis, a tsunami height does not decline during about 10 hours and a characteristic period of Harima-nada is about 60 minutes. The tsunami continued to be resonated for more than 20 hours in the Seto Inland Sea.