A mathematical model of silica deposition in a porous medium is presented. It consists of equations of fluid flow, silica concentration and silica deposition. In the model, the effects of aluminum on the silica deposition is taken into account since the element is likely to play an important role on the silica scale formation. Then, we apply the model for the experimental results which were carried out using the Otake hot water (R. Itoi et al. , 1984). As an inflow concentration of silica, a value of 1. 6 multiplied by 10** minus **6 g/cm**3 is given for the calculation in spite of the dissolved silica concentration about 520 multiplied by 10** minus **6 g/cm**3. A reasonable match has been obtained for calculated and measured flowrates, silica deposition and permeability distributions.