A method of generating nitrogen comprising: compressing air taken from outside by using an air compression means; introducing the compressed air into an adsorption column (6, 7) of air pretreatment apparatuses including a pair of adsorption columns (6, 7) and adsorptively removing moisture and carbon dioxide from the compressed air; cooling the compressed air down to a low temperature in a main heat exchanger and then introducing the compressed air into a high-pressure column; cryogenically separating the compressed air introduced into the high-pressure column by using a difference in boiling point between components in the compressed air; storing liquid air in a bottom portion of the high-pressure column and taking nitrogen in gaseous form for use as a reflux liquid from an upper portion of the high-pressure column; introducing the liquid air stored in the bottom portion of the high-pressure column through a liquid air extraction passage into a low-pressure column; cryogenically separating the liquid air introduced into the low-pressure column by using a difference in boiling point between components in the liquid air; storing oxygen-enriched liquid air in a bottom portion of the low-pressure column and taking nitrogen in gaseous form from an upper portion of the low-pressure column to introduce the nitrogen as a product gas into a product gas extraction passage; wherein the high purity nitrogen gas is heated by passing the high purity nitrogen gas through a super-cooler and the main heat exchanger; wherein after a nitrogen compressor 24a in a nitrogen compression apparatus compresses the high purity nitrogen gas to a predetermined pressure, the compressed high purity nitrogen gas is introduced into the product gas extraction passage and is supplied as a product nitrogen gas (GN 2) to a customer site; introducing the nitrogen in gaseous form extracted for use as the reflux liquid from the upper portion of the high-pressure column into a condenser to liquefy the nitrogen; refluxing part of the liquid nitrogen as the reflux liquid to the high-pressure column and feeding the remainder of the liquid nitrogen to the upper portion of the low-pressure column; oxygen-enriched air in gaseous form is taken from an exhaust gas outlet provided below the rectification part of the low-pressure column and above the condenser; and the oxygen-enriched air is introduced into the main heat exchanger to cool the compressed air; characterized in that the method further comprises: introducing liquid nitrogen or liquid oxygen from outside a system into the low-pressure column as a coolant for supplementing heat loss in the heat exchanger (8, 19) or heat entering from outside, wherein the liquid air in a bottom portion of the high-pressure column taken through the liquid air extraction passage is introduced into a portion of a rectification part of the low-pressure column in which the number of theoretical plates from a column bottom side is set within the range of one to ten; the oxygen-enriched air, after being introduced into the main heat exchanger to cool the compressed air, is then introduced into the heat exchanger of the air compression means and the adsorption column (6, 7); and after being used as a cold source for the air compression means and as a regenerating gas for the adsorption column (6, 7), the oxygen-enriched air is released to the atmosphere; wherein liquid nitrogen within a backup liquid nitrogen tank is evaporated by backup liquid nitrogen evaporators in a backup system line to feed the nitrogen gas into the product gas extraction pipe, thereby avoiding the interruption of the supply of nitrogen gas when there is an apparatus failure or there arises a shortage of the product nitrogen gas (GN 2).