PURPOSE: To heighten recovery efficiency of light oil and improve qualities, such as flash point, of (heavy) gas-oil, by providing a crude oil atomspheric distillation unit with a high-performance side-cut column.

CONSTITUTION: A main distillation column 1 is provided with a side-cut column 101 having a number of actual plates of 8W40 and provided with a reflux system and a reboiler. The side-cut column 101 is operated at a column overhead pressure of 30W400mmHg abs. A crude heavy gas-oil stream withdrawn from an intermediate plate of the main distillation column 1 is fed through a line 18c to the side- cut column, and gas-oil components as a byproduct withdrawn from the column 1 through a line 113 and a pump 109 is divided into two streams, one of which is recovered as a gas-oil product from a line 21b and the other is recycled through a line 114 to the column 101. A kerosene fraction from the overhead of the column 101 is divided, and one stream is recycled through a line 112 to the column 101, and the other is recovered as a byproduct (kerosene fraction) from a line 115. On the other hand, crude heavy gas-oil from the main distillation column 1 is fed as such to the side-cut column 101 and, after stripping the light fraction therefrom, is withdrawn from the bottom of the column 101 as a heavy gas-oil.