The phase separation of SiO₂-B₂O₃-Al₂O₃-ZrO₂-RO-R₂O glasses and the composition of resulting porous glasses were compared with those of conventional Vycor type porous glasses, Phase separation progressed by the nucleation growth mechanism, pore diameter increased with 1/3 power of time. The rate of phase separation in a ZrO₂ containing glasses was smaller than that of Vycor type by factors of 1/10-1/100. The rate determining step was considered to be the viscous flow of network structural units. Amounts of Al₂O₃ and ZrO₂ remaining in the SiO₂ skeleton increased with increasing temperature and time. The composition of Vycor type porous glasses remained almost constant regardless of the heating conditions.