Most animal cell culture media can be buffered using bicarbonate and high pressure CO2 in a closed system. However, in an open system, the pH of the culture media increases continuously due to the marked difference in CO2 pressure between the culture media and the atmosphere. Therefore, it is important to measure the exact pH of the culture media in an intact closed system. In this study, a pH measurement method was developed using visible light. The pH was calculated from light absorbance by the cells and by the culture media. This method was successfully applied to both suspension and anchorage-dependent cell cultures.