A number of geothermal brine column flow tests were conducted at the Sumikawa geothermal power plant to evaluate inhibitors for the prevention of silica scaling. The silica concentration of the brine was 1150 mg/L and nine inhibitors were examined at concentrations up to 25 mg/L. The most effective inhibitors and concentration were found to be polyacrylic and phosphinocarboxylic acid salts and 1 mg/L, respectively that higher concentrations were not necessarily better in suppressing scaling and generally the inhibiters were better than acidification. The projected cost associated with this inhibition method is close to that of the standard pH adjustment method.