The purpose of this study is to decompose methane hydrate into hydrogen gas by the in-liquid plasma method under conditions similar to those on the seabed. In previous studies, an experiment using the radio frequency (RF) in-liquid plasma jet method generated stable plasma at low temperature and high pressure, which has also had satisfactory results in decomposition of methane hydrate under severe conditions. In the present study, by performing the experiment under higher pressures, there is an improvement in the mechanism for the electrode to withstand increased pressure changes and to generate more stable plasma. At the present, this electrode can generate stable plasma under pressures ranging from 0.1MPa to 5MPa. In addition, as part of the ultimate goal, mixed gases of argon and CO2 were used to generate a plasma jet that is generated stably at higher pressure levels. This would enable the conversion of methane hydrate into CO2 hydrate under specific conditions such as those found at extremely low temperatures and high pressures. This suggests that this method could be used to solidify CO2 or other carbon components on the ocean floor