PURPOSE: To obtain stabilized steam conditions for a heating medium and a working medium that are not influenced by steam and hot water heated geothermally, by providing a gas-water separator of which internal pressure will be kept at atmospheric pressure, and an open type evaporator.

CONSTITUTION: A mixture of steam and hot water conducted from a geothermal well 1 is flushed into the atmospheric gas-water separator 11 having a valve 10 for opening to air, and is separated into the hot water and the steam having a temperature of approximately 100°C and a pressure kept nearly to atmospheric pressure. That steam is led via a sluice valve to the open type evaporator 12. The steam kept at about 100°C is allowed to enter an inner tank 17 of the evaporator 12, and after the heat exchange it leaves the inner tank 17 from the upper part. The excess steam and the uncondensed gas are discharged from an outlet 13 open to air so that the steam pressure and temperature are kept approximately constant.