Solar thermal, wind power, and geothermal heat have become popular as the new energy with the small impact of greenhouse gas emissions, and Methane hydrate is one of them. IPCC （2007） showed that CH4 has high global warming potential which is 23 times as large impact as CO2 in 2007. It means that changing CO2 from CH4 plays the part of decreasing greenhouse gas emissions. In addition that methane is combustible material, and would become the best fuel energy. However, methane hydrate dissolves sea water with pressure and/or water temperature changing in the seafloor because of unstable solid. Therefore, it is important to measure volume of methane gas for recognition of methane hydrate in the seafloor. However, the studies of measuring volume of gas in the seafloor are hardly found. Therefore, we investigate possibility of using the Time Domain Reflectometry （TDR） method as measuring volumetric water content in seafloor sediments because a gas phase in a core sample of the seafloor sediment may be obtained as Va＝V－（Vw－Vs） where V is the total volume of a core sample （m3）, Va is a gas phase of a core sample （m3）, Vw is a liquid phase of a core sample （m3）, and Vs is a solid phase of a core sample （m3）. We recognize TDR method is useful of measuring volumetric water content in seafloor sediments. However, we found each site needs each calibration under the sediments used for volumetric water content measurements by TDR method.
