PURPOSE: To provide an improved method for starting up a fractionator smoothly and quickly.

CONSTITUTION: A fractionator, which distills a liq. mixture contg. components having different boiling points into fractions, is started up by delivering an intermediate fraction between the overhead and the bottom fractions of the fractionator to at least one of side reflux systems installed in the fractionator to allow the fraction to stay there, introducing the liq. mixture at an elevated temp. into the fractionator simultaneously with the start-up of the side reflux system where the intermediate fraction is staying, and thereby causing a forced internal reflux in the fractionator before a spontaneous internal reflux occurs.