In this analysis, Seto Inland Sea level is defined as the standard sea level of reference datum during zero sea level gradientbetween North Pacific and West Pacific, and zero atmospheric pressure gradient in North West Pacific Ocean.Based on the assumption, the difference between Seto Inland Sea level and Pacific Ocean level depends on the summation oftwo components of sea level fluctuation; such as, local atmospheric pressure gradient and sea water density. It seems reasonable tosuppose that the mean sea level in Seto Inland Sea can be estimated by global sea level fluctuation.It is concluded that the sea level fluctuation in Seto Inland Sea is mainly caused by variation of sea water density during winter, and by sea water exchange during summer. The mean sea level in Seto Inland Sea is lower than that of the Pacific Ocean, especially in Kii Channel and Suo Sea of Seto Inland Sea.