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The hydrothermal plume spouting out from the sea bottom is one of the most interesting phenomenon on the deep-sea science. This study aims to develop the in situ pH sensor that can work at the deep-sea hydrothermal vents for the chemically observation of the hydrothermal plume. We applied Ion Sensitive Field Effect Transistor (ISFET) as a pH electrode, and Chloride Ion Selective Electrode (Cl-ISE) as a reference electrode to expect the good response, the good accuracy, the strong shock resistance, and the stable long-term characteristics. We firstly carried out the fundamental experiments on land to obtain the basic characteristics of ISFET, and then carried out the sea test at the site of active hydrothermal vents in the Okinawa Trough, Japan to confirm its usefulness. A ISFET pH sensor could be expected to become a very helpful tool for the survey of the plume source and the chemical oceanography by combining the other ion sensitive membrane.