An equation of rejection curve was derived for a membrane system where the membrane charge strongly affected the partition of ions into the membrane surface. The equation predicted that the rejection varied linearly with the inverse of the volume flow. The equation was applied to the experimental data. As the result, it was shown that the equation was fit with experimental data very well. In addition, it was also shown that the ion flux due to the ion migration and the ion flux due to the solvent drag are obtained separately from the analysis of the rejection curve with the equation.