In a study on prevention of silica scale formation in geothermal applications, two silica removal methods a ere examined using actual geothermal brine in the Sumikawa and Onuma geothermal areas: one was the addition of silica gel seeds. The other was the addition of aluminum ions. The results for silica gel seed show that the material precipitates on the seeds withdrawn from Sumikawa and model geothermal brine is only silica, On the other hand, the seeds withdrawn from Onuma brine contain not only silica but a small amount of aluminum. In the case of aluminum ion addition, the deposits withdrawn from Sumikawa geothermal brine contain not only silicon and aluminum, but also sodium, potassium and calcium. The composition of the deposits is similar to that of common silica scale. Also, the distribution ratios of these elements between the deposits and the brine are consistent with their common distribution ratios between silica scale and brine, In each case of silica gel seed and aluminum ion addition, the silica removal performance in the actual geothermal brines is found to agree with that in the model ones. Therefore results obtained in model geothermal brine can be applied to actual geothermal brine.