A terahertz chemical microscopy (TCM) has been proposed and developed in order to visualize the chemical potential changes in the water solution. In this paper, ion sensing in solutions, as an application of terahertz chemical microscopy, are introduced, and we observed the variation of amplitude of THz wave. In TCM, we detect the change in the amplitude of radiated THz wave from a THz sensing plate. The sensing plate is that the amplitude of radiated THz wave changes when chemical reactions are undergo on the THz sensing plate. We immobilized ion sensitive membranes, which can take the certain ion, on the surface of SiO2side of THz sensing plate. We dropped the sodium solution of 10-4M and 10-1M. The amplitude of radiated THz wave as the concentration of 10-1M was large in magnitude than the concentration of 10-4M where the ion sensitive membrane was immobilized. We can also visualize the concentration of ions to get image. From these results, we can detect ions in solution using TCM.