Utilizing the difference in the size of the immiscibility regions with alkali ions, phase separation was induced at the surface of 70SiO(2)center dot 15B(2)O(3)center dot 15Na(2)O (mol %) glass by ion exchange in which Na+ ions were replaced by Li+ ions in molten LiNO3 salt at 400 degrees C. The heat treatment after the ion exchange made the phase separation proceed and formed phases which can be dissolved by acid treatment. Through the ion-exchange-induced phase separation, we prepared monolithic glasses, in which only the surface region is porous and the rest is homogeneous and transparent.