To detect methane gas leaks early in the production of methane hydrate, a dissolved methane sensor to sense changes in the dissolved methane concentration in seawater with a high response is necessary. It was suggested that the standard METS sensor that Franatech GmbH of Germany has made available in the market would meet the requirement of such a sensor with its enhanced response as a result of improvements in the mechanical structure. In this study that standard METS sensor was tested and evaluated in various aspects. Based on this, an improved METS sensor was developed to be used for detecting methane gas leaks in the production of methane hydrate. The paper discusses the results of various improvements in the METS sensor, and outlines a prototype of the improved METS sensor that was made to reflect the results.