Since silica undergoes polymerization, precipitation, and dissolution depending on the change in pH or temperature, the chemical behavior of silica would be much complicated when cement for the construction of geological disposal system greatly changes the pH (8 to 13) of groundwater. To clarify the dynamic behavior of silica in such an alkaline solution, the concentrations of silica in both soluble and colloidal form in the supersaturated solution in the presence of solid phase have been traced over a 40-day period. In the experiment, the concentration of silica in a soluble form was determined by the silicomolybdenum-yellow method, and the concentration of silica in soluble plus colloidal forms was determined by adjusting the pH of the solution to 13, where all the silica changes into a soluble form (mainly monomeric). In order to examine the dynamic behavior of colloidal silica with solid phase of silica, this study has used natural quartz and pure commercial amorphous silica, both in a size fraction of 74–149 μm, whose specific surface-area (BET, N2 gas) were respectively 1.0 m2/g and 400 m2/g. The Na2SiO3 solution (250 ml, pH>10, 298 K) was poured into a polyethylene vessel containing quartz or amorphous silica (0.1 g or 0.5 g), HNO3 and a buffer solution. The pH of the solution was set to 8. The silica initially in a soluble form at pH>10 (6.8×10-3 M or 1.2×10-2 M) became supersaturated and either deposited on the solid surface or changed into the colloidal form. The ratio of silica in those form depended both on the initial concentration of soluble-silica and the surface area of the solid. The concentration of colloidal-silica gradually decreased, where the logarithm of its concentration decreased linearly against time after the concentration of soluble-silica decreased to a metastable concentration slightly higher than the solubility of soluble-silica.