NOVELTY - A scale reducing apparatus comprises insoluble chelate unit (1a) which captures polyvalent metal ion of influent containing silica component and/or calcium component, container (1) into which insoluble chelate unit is placed, alkaline substance addition apparatus (50a) which adds alkaline substance to influent after making insoluble chelate unit contact and makes pH of influent greater than or equal to 7, acidic substance addition apparatus (40a) which adds acidic substance to influent before making insoluble chelate unit contact and makes pH of influent less than or equal to 7, and control apparatus (11a).    USE - A scale reducing apparatus for geothermal energy system (claimed).    ADVANTAGE - Production of silica-type and calcium-type scale can be suppressed at low cost.    DETAILED DESCRIPTION - A scale reducing apparatus comprises insoluble chelate unit (1a) which captures polyvalent metal ion of influent containing silica component and/or calcium component, container (1) into which insoluble chelate unit is placed, alkaline substance addition apparatus (50a) which adds alkaline substance to influent after making insoluble chelate unit contact and makes pH of influent greater than or equal to 7, acidic substance addition apparatus (40a) which adds acidic substance to influent before making insoluble chelate unit contact and makes pH of influent less than or equal to 7, and control apparatus (11a) which switches operation of alkaline substance addition apparatus and operation of acidic substance addition apparatus by turns. INDEPENDENT CLAIMS are included for:    (1) geothermal energy system; and    (2) method of suppressing scale which suppresses scale production of influent which contains silica component and/or calcium component.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the state of scale reducing apparatus during alkali addition.    Container (1)    Insoluble chelate unit (1a)    Control apparatus (11a)    Acidic substance addition apparatus (40a)    Alkaline substance addition apparatus (50a)