The seeding method using silica gel has been suggested as a way to inhibit silica scale formation. In order to improve the silica removal performance, the effect of coaddition of aluminum ion on the silica removal was experimentally examined. When only aluminum ion solution is added in the supersaturated silica solution, the decrease in silica concentration increases with time. This is because aluminum ion produces aluminosilicate deposits. The decrease in silica concentration becomes larger with increasing aluminum ion concentration or with increasing initial pH. When initial pH is 9, the decrease in silica concentration in the coaddition is Larger than three times that in the silica gel addition. The coaddition of aluminum ion is found to improve the performance of silica removal when the initial pH is higher than 8.