PURPOSE: To improve the yield of CO by combining a nitrogen cleaning column and a separating column and processing an H2/CO raw material gaseous mixture at the time of partly condensing and separating this mixture.

CONSTITUTION: For example, the H2/CO synthetic gaseous mixture contg. a small amt. of N2 is introduced by a conduit 1 into a heat exchanger E1 where the mixture is cooled and partly condensed. The cooled mixture is introduced into the bottom of the nitrogen cleaning column T1. The mixture is scrubbed by the liquefied scrubbing nitrogen cooled in the heat exchanger E1 and supplied to the head part of the column T1 from a conduit 2. As a result, the H2-rich component free from CO is generated in the head part of the column T1 and is discharged by a conduit 3. On the other hand, the COrich component is collected in the bottom of the column T1 and is discharged by a conduit 5 following which this component is expanded into a separating column T2. The H2-rich component generated in the head part of the column T2 is discharged by a conduit 6 and the product in the bottom of the column T2 is discharged by a conduit 7 and is so expanded as to impart chill. After this component is heated in the heat exchanger E1, the component is made to a CO condensate free from H2. The gas in the conduit 3 may be fed to an NH3 synthesizing apparatus by adding N2 thereto from the conduit 4 at need.