The gas separator fixing structure of the present invention is characterized in that it is provided with a gas separator 10 having a gas separation membrane 30 formed on at least one surface of a tubular support 31 having a through hole 7 in axial direction and comprising porous ceramics, a cap-like metal member 1 and a ring-shaped metal member 4 are fixed to one and the other open end of the gas separator 10 through seal members, respectively, and seal members are gland packings 11 and 12. A gas separating device can be provided in which there hardly occurs breakage of the tubular support constituting the gas separator 10 due to the thermal stress and there hardly occurs reduction of air-tightness between the gas separator 10 and the supporter supporting the gas separator caused by load of heat cycles, and besides which can be used even under the conditions of high temperatures.