A method for inhibiting scale including inorganic cations, and an economically operable geothermal power generating device which can inhibit deposition of scale. The geothermal power generating device includes: an inorganic cation concentration measuring device for measuring the concentration of bivalent or more inorganic cations in geothermal water collected from a production well; a flowmeter for measuring the flow rate of the geothermal water collected from the production well; a heat removal unit for lowering the temperature of the geothermal water; a thermometer for measuring the temperature of the geothermal water after removing heat; a pH measuring device for measuring the pH of the geothermal water after removing heat; a calculation processing unit for calculating the additive amount of a scale inhibitor; and a control unit for adding the scale inhibitor to the geothermal water by the amount calculated by the calculation processing unit.