The surface structure of an artificial membrane composed of a Millipore filter and a synthetic lipid analogue, dioleylphosphate (DOPH), immersed in KCl solution changes when CaCl2 concentration reaches a critical value. Using a photomultiplier mounted on the eyepiece of a stereoscopic microscope the structure change can be measured quantitatively as an intensity variation of reflected light of the membrane surface. Calcium ions decreased finally the intensity to more or less 50 % as compared with the original hydrophilic state whose solution was only KCl. However, the degree of structure change was dependent on following conditions: adsorbed DOPH densities, solution temperatures and pH values. On the other hand, using the light reflectance curves obtained from the structural observations, an unknown CaCl2 concentration in a KCl solution can be detected. Since the reflectance curve shifts widely with various densities of DOPH adsorbed in the Millipore filter, it is possible to detect the CaCl2 concentration of wide range. The detection range was from 0.1 to 50 mM CaCl2.