The authors have developed a novel chemical imaging sensor, which enables us to observe two-dimensional pH-distribution in an electrolyte solution. A sample solution is in contact with the insulator (Si3N4/SiO2) on the Si substrate, which forms the electrolyteinsulator-semiconductor (EIS) structure. When modulated laser beam illuminates the backside of the Si substrate, ac photocurrent flows through the EIS structure. Since the photocurrent depends on pH value on the illuminated region, the two-dimensional pH distribution can be microscopically obtained by scanning the focused laser beam. The thickness of the Si substrate as well as the spot diameter of the laser beam restricts the spatial resolution of this sensor, This can be theoretically explained by the carrier diffusion model. The bonded Si on insulator (bonded SOI) wafer is employed for the thinner Si substrate. The chemical anisotropic etching is used in order to remove the Si bulk of SOI wafer. Using the sensor with the active Si thickness of 20 μm, the spatial resolution as good as 5 μm was attained. It is also estimated that the pH resolution is good as 0.01 in pH.