To provide an energy supply system reusing an existing cavern, without affecting a sight scene resource and an environment, while using efficiently a geothermal source at a large scale.

A geothermal conversion means 6 is provided in one adit 4d in adit groups 4a-4f, in the existing cavern having a vertical shaft 2 excavated downwards and formed from a ground surface 3, and the plurality of adit groups 4a-4f communicated with the vertical shaft 2 and extended horizontal-directionally, and an electric power generating means 7 generates electric power by heat energy acquired by the geothermal conversion means 6.