The transport property of ions in highly concentrated electrolyte solutions, which are governed by many body Coulomb interaction, are found to be well described by a model that ions behave as high density strongly coupled plasma irrespective of ion species. The frequency of plasma oscillation, which is the long-range collective motion of plasma, is estimated to be order of 1 THz for concentrated electrolyte solutions, and evidence of the plasma oscillation can be searched by submillimeter spectroscopy. Transmission and reflection spectra of the concentrated electrolyte solutions have been measured in the 10 to 50 cm(-1) range, and the real and imaginary parts of refractive indices for the solutions are obtained. A close inspection of these spectral data suggests that the plasma oscillation is not excited in the highly concentrated electrolyte solutions. This is probably caused by that the long-range collective motion of ions is disturbed by short-range solute-solvent interaction.