Dielectric spectroscopy (DS) was applied to a nanofiltration (NF) membrane to detect its double-layer structure and ion permeation. Dielectric measurements were carried out oil the systems composed of the NF membrane NTR7450 and dilute solutions of eight electrolytes, LiCl, NaCl, KCl, NH4Cl, MgCl2, CaCl2, BaCl2, and CuCl2. Two relaxations were observed in the frequency range from 40 Hz to 4 MHz for each system. On the basis of characteristics of the dielectric spectra and the Maxwell-Wagner interfacial polarization theory, the low-frequency relaxation was attributed to inhomogeneity of the membrane structure itself, whereas the high-frequency relaxation was attributed to interfacial polarization between the membrane and the solution. A multiphase dielectric model previously developed by one of the authors and co-workers was adopted to present systems to analyze the dielectric spectra, and electric parameters, i.e., capacitance and conductance, of the two layers composing the membrane were obtained. The electric properties estimated for the two layers were different and changed with the environment in a different manner. Further analyses suggest that the two layers had a different separation mechanism due to their difference in materials. looseness. and fixed charge content. The fixed charge density of one layer was estimated, and the ion permeation difficulties in both layers was compared. This research revealed that DS was by far an effective method to obtain detailed electric parameters about the inner multilayer structure of the NF membrane and to elucidate separation mechanisms of each layer.