This paper describes the development of a dissolved methane sensor that will be used for the methane leakage monitoring in the methane hydrate production in Japan in the future. As for the requirements for the sensor, continuous, long-time use in a great water depth, real-time measurement, and outstanding response to the change of dissolved methane concentration are given. A commercially produced dissolved methane sensor (known as the METS sensor) from Franatech GmbH (Germany) provides the basis to satisfy such requirements. Thus, the authors have optimized the METS sensor for the purpose of utilization in the Research Consortium for Methane Hydrate Resources in Japan (also known as the MH21 Research Consortium).