PURPOSE: To remove silica and hydrogen sulfide contained in geothermal water, by constituting a geothermal power plant to comprise an alkali injecting means for injecting an alkali, a means for absorbing hydrogen sulfide by way of chemical reaction, and a silica separating means for removing silica contained in geothermal water.

CONSTITUTION: Hydrogen sulfide contained in steam is absorbed by injecting an alkali 2 into steam-containing geothermal water drawn up from a production well and mixing them sufficiently in a mixer 3. Then, the geothermal water is introduced into a multi-stage flash type heat exchanger 4. Hot water 1 is removed of incondensible gas containing hydrogen sulfide and carbon dioxide gas in a first-stage evaporation chamber 5, and then carried into the next evaporation chamber 5. Thus, the temperature of hot water 1 is lowered gradually through successive exchange of heat in the evaporation chambers 5. In a hydrogen sulfide absorbing means 13, hydrogen sulfide contained in the hot water 1 is absorbed by way of chemical reaction, and at the same time, pH of the hot water 1 is lowered. Then, silica sticks as scale in a silica separating means 17, and this sticking of silica is promoted along with lowering of pH and the temperature of hot water 1.