Chemically modified poly(gamma-methyl-L-glutamate) membranes having several lengths of diamine segments generated oscillation of membrane potential under a concentration gradient of several salts, although nonmodified membranes showed constant potential. The oscillation of membrane potential is considered to be caused by the fluctuation of the fixed charge density originating from the thermal fluctuation of partially ionized amine segments in the modified membranes. The amplitude of the oscillation was greater in the modified membranes having longer diamine segments. It was also influenced by the salts used, and the highest amplitude of the oscillation was observed when LiCl was used as the salt. Fast Fourier transport analysis revealed that the oscillation of membrane potential is not chaotic but has specific frequencies. It might be possible to recognize a particular salt present in the solution from the information on the amplitude and the power spectrum of fast Fourier transport in the oscillation of membrane potential.