For a hydroelectric power station utilizing the osmotic pressure generated between pure water and sea water, a performance prediction method was proposed in considering the concentration polarization on the flow parallel to the membrane surface. The analysis was one-dimensional and was based on equations of concentration polarization, water permeate flux, salt permeate flux, mass conservations both of water and salt, and energy conservation. From prescribed equations, the salt concentrations at the flow core and in the neighborhood of membrane surface, permeate flux, core flow velocity and also core pressure were numerically analyzed. From the analysis, the hydroelectric power performance was calculated and compared with one in case of non-concentration polarization. The result showed that the polarization lowered the performance. Based on the present analysis, the ability of electric power extracted from main rivers in Japan was estimated. It showed that the total electric power equal to the several nuclear power stations of 1 000 MW class was possible.