The decrease in permeability of a porous column (50 cm long and 5 cm in diameter) in a cylinder due to the deposition of silica scale has been examined. The temperature of the Otake water used for the experiment was 90-92 degree C and contained 522 ppm of silica. Permeability was calculated using the potential measured by 11 manometers which are set at various levels in the cylinder. The results indicate that the most of the silica scale was deposited within the first 10 cm of the porous medium and thereafter rapidly decreased in amount. In inverse proportion to the amount of silica deposits, the permeability was reduced to 0. 5-2 % in the uppermost part of the porous medium whereas it retained 80-90 % of the initial value in the places distant from the upper surface.