PURPOSE: To improve a rate of recovery of CO and further improve a purity of hydrogen gas by a method wherein hydrogen gas separated under a cooled state of raw gas and a part of liquefied part having as its major constituent CO are extracted and expanded and the raw gas is cooled with circulating gas and liquid nitrogen.

CONSTITUTION: Raw gas enters heat exchangers 1 and 2 through conduits 15 and 16 and is cooled by liquid nitrogen and return gas low temperature supplied by a conduit 29. A part of hydrogen gas separated by a separator of low temperature 3 is extracted from the conduits 19 and 21 and expanded by a valve 12 up to about atmospheric pressure. Further, a part of liquefied part having as its major constituent CO is extracted from conduits 20 and 23, expanded by a valve 11 up to about atmospheric pressure and merged with hydrogen gas. The merged circulation gas is rich in hydrogen and expanded up to about atmospheric pressure, so that it becomes a low temperature gas of about -200°C. This gas is supplied to a heat exchanger and then it can be cooled down to a lower temperature than that of the gas having only liquid nitrogen.