For adjusting the capacity of a low-temperature rectification unit having a high pressure column and a low pressure column in which a fluid is introduced into the high pressure column (1) and in which liquid from the bottom of the high pressure column is directed into the low pressure column (2), the amount of liquid that is removed from the bottom of high pressure and fed to the low pressure column (2) is controlled via flow adjustment (4), wherein a nominal value for flow adjustment (4) is set to a desired amount of flow, and the liquid level of the liquid at the bottom of column (1) that operates at higher pressure is fixed without a set nominal value corresponding to the amount of liquid that is removed.