Detection of change in microenvironmental pH may be essential for analyzing biological processes, if transitory biodegradable scaffold materials are used for tissue engineering in bone regeneration. In this study, we created a pH indicator immobilized-gel-sheet for pH measurement. By using gel-sheet, we can investigate the local microenvironment of biodegradable materials, such as octacalcium phosphate (OCP), under the various protein adsorptions. The gel-sheet was made of a photocrosslinkable prepolymer solution (TUPR-5), which was easily immobilized by the method that the pH indicator is entrapped in a porous polymeric substrate. Bromothymol Blue (BTB) immobilized-gel-sheet was mounted in a biochip made of polydimethylsiloxane (PDMS) with a flow channel. The pH value was calculated by detecting the color changes, and presented as the pH distribution via a custom-made application. The pH sensing was carried out based on the surrounding color information of an OCP granule if incubated in culture medium. The gel-sheet detected the pH values of the condition around an OCP granule, showing the decrease from pH 7.8 to 8.8. As a whole, the proposed pH indicator is capable of providing pH level by high-spatial resolution.