PURPOSE: To perform proper heat exchange with compact structure between a high pressure column and a low pressure column by forming a high pressure column fin vertically elongated into a fluid passage of the high pressure column, making the low pressure column be a plate column equipped with a low pressure fin on each plate and placing the low pressure column fin and the high pressure column fin in parallel.

CONSTITUTION: In a high pressure column, the part located in a low pressure column is made a high pressure column finned part 28 in which vapor on the high pressure column side rises. On the other hand, the low pressure column is made a plate column and a low pressure column finned part 26 is provided for each plate adjacent to the high pressure column finned part and simultaneously liquid on a column plate 38 is made flow in the low pressure column finned part 26. Thus a zone of height where heat exchange between the high pressure column and the low pressure column is positively performed is only a zone (submerged zone) where liquid flows on each column plate 38 of the low pressure column. Thereby the local condensation in the high pressure column 1 0 is prevented from being excessively accelerated, causing proper ratios of vapor to liquid to be secured, preventing separation efficiency from being lowered, and exchange assuring heat.