Geothermal energy, which is eco-friendly and completely domestic natural energy, takes the various forms from the high temperature volcanic thermal energy at the deeper part of the underground to the normal temperature geo-heat energy at the shallower part. We comprehend the geothermal energy as a system (the geothermal system) that consists of heat source, fluid and underground structure. And we conduct the theoretical and applied researches for exploration, development and utilization of the ecological and sustainable geothermal energy. The researches contribute not only to the promotion of natural energy utilization and the solution of environmental problems but also to the disaster prevention such as the monitoring of volcanic activity.The figures shown below are a part of our themes.