We applied magnesiothermic reduction for porous silica glass, which was prepared from sodium borosilicate glass. The powdered porous silica glass was reduced to silicon and magnesium silicide at temperatures higher than 600 degrees C. The silicon was isolated by the hydrochloric acid treatment and the subsequent hydrofluoric acid treatment for the reduction products. The SEM observation revealed that the prepared silicon had a porous structure. The pore size distribution measured with a porosimeter was similar to that of the stating porous silica glass in the region less than several tens nanometers.