The concept of a georeactor in geothermal environment has been proposed for direct utilization of geothermal energy in the ground. Chemical processes in the georeactor must accommodate a range of operating conditions, because the georeactor utilizes natural resources that are sometimes difficult to control. Combination of hydrogen generation by solar energy from hydrogen sulfide, and reduction of waste sulfur and its compounds using geothermal energy is environmental energy supply system. The self-oxidation and reduction of sulfur is the considered to be suitable for the thermal condition around 200 oC. In previous studies, the self-oxidation and reduction of sulfur proceeded in neutral or alkaline solution. The amount of formed hydrogen sulfide in strong alkaline solution was larger than neutral solution. But it was difficult to use the strong alkaline solution for self-oxidation and reduction of sulfur to georeactor. Therefore we applied the seawater for sulfur reduction because the seawater is alkalescent solution and it can be available at a low cost. In order to study a self-oxidation and reduction of sulfur in seawater, we conducted the reaction between sulfur and seawater in hydrothermal conditions using a batch-type autoclave. The maximum conversion rate of elemental sulfur to hydrogen sulfide with the reactions in seawater is almost 50 % and the highest concentration of formed hydrogen sulfide is 0.026 mol/l in the experiments.