Batchwise concentration and partial demineralization of skim milk were performed by nanofiltration up to 1/3.22 times the original volume, and the major salt flux data were analyzed using the extended Nernst-Planck equation. The following observed rejections of major salt elements during nanofiltration were obtained: Na and K, 55-65%; Cl,11.5% at the start of nanofiltration, -85.2% at the end of nanofiltration; Ca and Mg, >98%; P, >97%. The reduction rates of Na, K and Cl were 56.6, 57.7 and 79.4% respectively. The subsequent analysis was based on two assumptions: 1) the membrane completely rejects solutes other than Na, K and Cl; 2) the membrane carries a fixed charge due impermeable ions such as proteines in the retentate. Rejections of Na, K and Cl calculated from the analysis were good agreement with the experimental data.