The porous silica membrane was prepared from Si(${OC}_2H_5)_4-H_2O$ system by sol-gel method. To investigate the characteristics of gels and porous silica membrane, we examined gels and porous silica membrane using TG-DTA, X-ray diffractometer, IR spectrophotometer, BET, SEM and TEM. The optimum mole ratio of Si(OC$_2$H$_{5}$)$_4$ : $H_2O$ $C_2$H$_{5}$OH for porous silica membrane was 1 : 4.5 : 4. The porous silica membrane was obtained by heat treatment of the gel above 700 $^{circ}C$. The specific surface area of sintered gel was 3.8 $m^2$/g to 902.3 $m^2$/g at 100 $^{circ}C$ to 1100 $^{circ}C$ The pore size of sintered gel was in the range 20 $AA$~ 50$AA$. The particle size of sintered gel was 15 nm to 30 nm at 30$0^{circ}C$ to 700$^{circ}C$. The performance of the porous silica membrane was investigated for the separation of $H_2$/$N_2$ gas mixture. Gas separation through porous silica membrane depends upon Knudsen flow and surface flow. The veal separation factor($alpha$) of $H_2$/$N_2$ was 5.17 at 155.15 cmHg and $25^{circ}C$. The real separation factor($alpha$), head separation factor($eta$), and tail separation factor( $ar{B}$) increased as the pressure of permeation cell Increased.sed.