produced in another step and it is for example one having a whole pressure of about 25 bar in which the mole ratio of nitrogen gas exceeds u2153 on the other hand it is required that a product gas has a pressure meeting with an ammonia synthesis process a mixed gas of about 25 bar is therefore raised in pressure in turn to about 50 bar by a first booster c 11 and to about 100 bar by a second booster c 12 as the boosters a centrifugal compressor is properly used and one capable of carrying out two stage compression at a compression ratio of 2 will be preferable in addition a water cooled cooler for temperature maintenance use is provided just after each of the boosters but it is omitted in the drawings in this embodiment it is intended to use a booster for double purpose as a booster for raising the pressure of a product gas and a booster for generating a pressure difference for gas separation thus two stage gas separation will be carried out in accordance with the following steps namely a gas partially led out of the outlet side line of said second booster c 12 is fed to a first gas separation section s 1 via a line l 1 and separated to a first.