Fluorescence spectrum measurement of fluorescein was performed for monitoring the pH of water between mica and silica surfaces by varying the surface separation (D) using a surface forces apparatus (SFA) fluorescence spectroscopy. The pH value of water between silica surfaces decreased drastically with a decrease in D to values below 1200 nm, which was longer than that in the case of mica (600 nm). This pH decrease was attributed to concentrated proton in electric double layer of negatively charged surfaces.