PURPOSE: To maintain heat exchanger duty in a condenser constant in spite of changes in nitrogen purity in argon gas by installing a variable resistance between an upper tower of a duplex fractionating column, at least a part of which is constituted by packing, and a coarse argon tower.

CONSTITUTION: During rated operation or when nitrogen purity in argon gas is not varied, an automatic valve (resistance) 14 provided on a feed line 8 between an upper tower 1 and a coarse argon tower 4 is throttled to make pressure at a top of the coarse argon tower 4 minimum. Also, in the case where nitrogen purity in argon gas is increased, the automatic valve 14 is opened to raise pressure at the top of the coarse argon tower 4 to increase temperature at the top of the tower, thus making a temperature difference between a coarse argon gas and a liquid fluid constant. An opening degree of the automatic valve 14 is determined by an arithmetic unit 16 which calculates pressure so that heat exchanger duty in a condenser 5 is made even by that nitrogen purity in argon gas which is detected by a nitrogen analyzer 13.