We investigated the pore properties of inorganic membranes applied for hydrogen separation industry. Inorganic membranes were derived from polysilazanes. The thermal reactions involved were studied using thermogravimetry(TG) and IR spectroscopy(FTIR) of the solids. To determine the thermal effect of pore properties, polysilazanes were pyrolysed in inert atmosphere. Pore volume and BET surface area showed the maximum value at a pyrolysis temperature of $500^{circ}C$. For amorphous SiCN membrane derived from polysilazanes, selectivity of $H_2/N_2$ was 4.81 at $600^{circ}C$.