To complement the authors＇ previous numerical analysis for a RO seawater desalination system utilizing the static pressure due to the depth of the sea, a numerical analysis is performed for a RO brackish water desalination system utilizing the static pressure head of brackish water in a vertical pit, by taking into consideration the solute permeation through membrane. Numerical results are obtained for the effects of the length of channel, the submerged depth of device, the pure water permeability of membrane, the solute permeability of membrane, and the Schmidt number of brackish water on the axial variation of velocity, concentration, and pressure in laminar natural convection flow of brackish water, caused by the concentration difference in the moderately narrow annular channel formed between a tube-type membrane and a cylindrical rod. The effects on the axial variation of the velocity of transmitting fresh water and the concentration of transmitted fresh water are also examined. From the results, it is shown quantitatively that the application of large submerged depth of device and the adoption of high pure water permeability of membrane bring about a increase in the flow rate of fresh water with low solute concentration.