PURPOSE: To enhance the permselectivity of a gas separation element for a specified gas by heating the element with the Joule heat produced by energizing at least one electrode itself before a voltage is impressed between the electrodes.

CONSTITUTION: An inner electrode 7 and/or an outer electrode 8 are energized through the lead wires 10 and 10 and/or the lead wires 11 and 11 to produce Joule heat in the electrodes 7 and 8. The gas separation element 6 and the gas around the element 6 are heated to about 800°C by the Joule heat to impart conductivity to the element 6. A voltage is then impressed between the electrodes 7 and 8, gaseous oxygen is recovered from the inside of a porous support 3, and gaseous nitrogen is recovered from a discharge port 13. Since the recovered gases are kept at high temp., the gases are subjected to heat exchange, and heat energy is appropriately recovered.