An adsorbent for separating nitrogen from a mixed gas of oxygen and nitrogen is MSC wherein an oxygen and nitrogen separation ratio α and a ratio (t95/t50) of a time t50 required for adsorbing 50% of the oxygen equilibrium adsorption amount and a time t95 required for adsorbing 95% of the oxygen equilibrium adsorption amount satisfy the inequality
(t95/t50)<0.4×(α−24)α≧35.
The amount of adsorbent used with respect to a predetermined amount of nitrogen generated can be reduced since a method is carried out which uses this adsorbent to produce nitrogen by separating nitrogen from the mixed gas of oxygen and nitrogen by means of a PSA method. By this method, it is possible to reduce the cost of the apparatus and reduce and miniaturize the scale of the apparatus, and the power consumption amount can be reduced.