Abstract: The results of a numerical experiment on water interchange among 21 sections of Setonaikai, or the Seto Inland Sea, are reported in this paper. The numerical model in this experiment is based on the following assumptioms; The first assumption is that the spatial distribution of chlorinity in Setonaikai is given as constant by the annual mean of chlorinity and water mass. Thus, the solution of simultaneous equations with two unknown quantities of inflow and outflow gives the inflow and outflow ratios as shown in Fig. 6. Next, the staying periods of fresh water are obtained by solving a matrix with corn-non en is of inflow and outflow ratios. They are shown in Table 1.