In this study, we proposed a method, which promotes the entrainment of seawater by equipping the river mouth with the weir, and the effect was examined based on experimental and theoretical analysis. These results could be summarized as follows : (1) The energy, in which the river water entrains bay-seawater, consists of kinetic energy and potential energy of the river water following into the bay. The former is evaluated by densimetoric Froude number of the river water, and the latter is evaluated by the water-level difference of bay and river. (2) The effect of the water-level difference of bay and river on entraining the seawater in the bay is more effective as the kinetic energy of river water is smaller.
