PURPOSE: To improve purification efficiency by passing impure Ar having a specific oxygen content through rectification and an excess reducing agent through a catalyst bed comprising at least one kind of platium group metal particles supported on a carrier having a high specific surface almost at room temperature.

CONSTITUTION: Air for processing is introduced in an intermediate-pressure rectifier column 5 constituting a double rectifier column 1 from its bottom part, three kind of remaining components LR, LP1, and LP2 are injected into a low-pressure rectifier column 6 to extract a product N2 from the column top, and waste gas W is discharged from an LP1 injection point. A main remaining component is introduced into the bottom part of an impure Ar production rectifier column 2 having 80-110 logical rectifier plates through a duct 9. Impure Ar having an oxygen content not higher than 1000 vpm is discharged from a condenser 11 at the column top, sent to a heat exchanger 13 and a compressor 14 by a dust 12 and heated and pressurized, and mixed with an H2 stream, so that the mixture is introduced into one bottom part of a bottle 17. A mixed gas containing Ar, N2, and water is produced by the platium group metal catalyst bed 18, dried by a water adsorption bed 19 and then cooled, and sent to a denitrification rectifier column 4, where liquid Ar of high purity is obtained from the column bottom and a mixed gas of N2 and H2 is obtained from the column top.