PURPOSE: To fractionate a gas mixture into a most volatile component by partly consensing a fluid in first and fifth circulation passages, reheating a non- condensed gas fraction in the second circulation passage, subjecting the foregoing fraction to secondary cooling in a third circulation passage, and providing required cooling thereto with a condensate evaporated in a fourth circulation passage.

CONSTITUTION: Gas to be fractionated is carried to a circulation passage C5 through lines 1, 3, and flows out therefrom as a gas phase/first condensate phase mixture through a line 2. Both phases are separated from each other in a drum B1, and the gas phase being supplied to a circulation passage C1 through a line 16, where it is cooled to form a second condensate which condensate is returned to B1 through a line 17. Non-compressed gas flows out from a head of C1 and is carried to a circulation passage C2 through lines 5, 7. In a re-heated state it flows out through a line 14 from a bottom of the circulation passage C2. Thus, a fraction with lightest batch load or fractionated gas is obtained.