The development of in situ chemical sensor plays a key role in understanding of chemical and biological processes in the ocean. There are few chemical sensors for deep sea use and their accuracy and resolution are not satisfying for the purpose of chemical oceanography. ISFET (Ion Sensitive Field Effect Transistor) is a semiconductor that can measure ion concentrations in aqueous media. Since the ion selectivity of ISFET depends on the membrane attached onto its surface, H (super +) , P (sub CO2) , Ca (super +) , K (super +) and Na (super +) can be measured when proper ion sensitive membranes are used. Furthermore, the silicon chip technology will make it possible to develop the integrated chemical analysis system using ISFET sensor and micro actuator. In this article, we introduce an in situ pH sensor newly developed using ISFET and present its possible application to oceanography.