PURPOSE: To obtain an improved membrane to produce high purity nitrogen from air by arranging and operating a membrane stage for a recycle stream so that the recycle stream has the same composition with a holding stream to be blended with, by removing mixing loss, and by reducing membrane surface area and power requirements.

CONSTITUTION: A component of low selective permeability (example, N2) is produced with high purity from a feed gas (example, air) containing a low selective permeability component (example, N2) and a high selective permeability component (example, 02). First, a compressed air is passed from a line 4 through a first stage hollow fiber membrane module 5 and a recycled holding stream is introduced into a second stage hollow fiber module 10 by passing through lines 8, 9. A holding material from the second stage is recycled from a line 11 as a high purity N2. A permeated material from the module 10 is taken out from a line 13, recompressed to high pressure by a compressor 14 and passed into a third hollow fiber membrane module 16 after passing a line 15. The holding material from this stage is fed into a module 10 by blending the first holding material.