According to the present invention, part of carbon dioxide heated by a heater is supplied to a dry gas seal portion of a compressor so that a device for pressurizing and heating a seal gas is not additionally required, configuration can be simplified and cost can be reduced. In addition, a seal gas flow path includes a low-temperature seal gas flow path and a high-temperature seal gas flow path so that, in an initial driving mode, carbon dioxide heated by a high-temperature portion of the heater is used as a seal gas and in a turbine-driving mode, carbon dioxide heated by a low-temperature portion of the heater is used as a seal gas and thus a more efficient operation can be performed.