PURPOSE: To provide high purity of product nitrogen gas by a method wherein a part of liquid nitrogen after refining and separating substances of low boiling point is fed out of a refining plate lower by several stages than a refining plate at the top part of a complex refining tower while remaining oxygen is low in its volume and gas having substances of low boiling point separated through gas-liquid separation is discharged and removed.

CONSTITUTION: Raw material air (B) cooled down to near its liquifying point after passing through an air filter, an air compressor, a cooling decarbonization drying device and a heat exchanger is fed into the bottom part of a medium refining tower 5 acting as a lower refining tower of a complex refining tower and refined there. Then, the refined and separated gaseous nitrogen is condensed and liquified at a condenser 15 arranged at the bottom part of a low pressure refining tower 6 acting as an upper refining tower, the condensed liquified nitrogen is fed again into the top part of the medium pressure refining tower 5 as a circulating flow. Then, a part of the circulation flow is fed out of the refining plate lower by several stages than the refining plate at the top part of the medium pressure refining tower 5, over-cooled at an over-cooler 9 and after its free expansion, the part of the super high purity liquid nitrogen gas-liquid separated through the gas-liquid separator 10 is gasified through gasification means 11 and 4 and then fed out as product nitrogen gas.