There is provided a unit capable of simultaneously producing nitrogen of ultra high purity and oxygen of ultra high purity from air as a feed material. Feed air is introduced into the bottom 15 of a first rectification column 6. Liquid nitrogen of ultra high purity is recovered from between the upper rectifying part 12 and middle rectifying part 13, and liquid air free of high boiling point components is recovered from between the middle rectifying part 13 and lower rectifying part 14. Oxygen-rich liquid air collected in the bottom 15 is reduced in pressure by an expansion valve 31, and then introduced into a nitrogen condenser 8 as a refrigerant. After a portion of said liquid air is reduced in pressure by an expansion valve 33, it is introduced into the second rectification column 7, where low boiling point components are separated from the top part 21 and liquid oxygen of ultra high purity is recovered from the bottom 23. The remaining portion of said liquid air is reduced in pressure by an expansion valve 32, and then introduced into the nitrogen condenser as a part of the refrigerant. Accordingly, the quantity of a reflux liquid flowing through the lower rectifying part 14 is regulated, and the quantity of said liquid air introduced into the second rectification column 7 is regulated.