PURPOSE: To increase the production of intended components without any trouble by reintroducing a part of the components which are produced additionally and are drawn in an atmospheric distillation tower into a distillation tower, extracting reflux liquid from the part in the upper stage of the distillation tower, and radiating heat therefrom then reintroducing the same into the distillation tower.

CONSTITUTION: When the intermediate fraction, for example, gas oil, in the stage of distilling ulticomponent hydrocarbons is produced additionally, the gas oil discharged through a gas oil discharge pipe 41 is subjected to heat exchanging with the waste gas discharged through a gas discharge pipe 42 from a heating furnace 27 by a heat exchanger 43, and is heated to above its b.p., then the gas oil is reintroduced into the part of an atmospheric distillation tower 28 where the temp. is equal approximately to the heating temp. Further the reflux liquid in the tower 28 is discharged through a reflux liquid pipe 44 and is subjected to heat exchanging with crude oil in a heat exchanger 45 then the liquid is introduced again into the tower 28. As a result, the reflux liquid in the gas oil fractional distillation part in the tower 28 is increased. Therefore, the degree of rectification of the gas oil is improved and the production of the gas oil is increased.