Following the authors' previous numerical and experimental analyses of a reverse osmosis desalination system utilizing the static pressure head in the case of a seawater channel laminated on one side with a reverse osmosis membrane, numerical analysis is performed for a system in which the seawater channel is laminated with the membrane on both sides. A dimensionless expression is formulated from the numerical results to predict the transmitted fresh water flow rate from the submerged depth, the physical properties of seawater, the permeability of the membrane, and the width and the axial length of the seawater channel. It is also estimated that the present desalination system may reduce the specific pourer consumption in the order of 50% in comparison with those of reverse osmosis desalination systems on land.