The present invention relates to a method for measuring dissolved methane in seawater, the method comprising the steps of: a) injecting a sample into a vacuum container into which a cadmium chloride solution has been injected, followed by refrigeration; b) vibrating the container, followed by temperature equilibration with the periphery; c) passing the sample in the container through a methane separator to separate dissolved methane and residues; and d) analyzing the separated dissolved methane using a mass spectrometer. The measuring method according to the present invention has advantages of maximally raising the recovery rate of dissolved methane to increase the ratio of analysis and being able to analyze even a small amount of samples. Furthermore, in the measuring method according to the present invention, in addition to the dissolved methane measurement, sulfur isotope analysis can be also simultaneously conducted using the sample from which the dissolved methane has been extracted.