To promote the wetting of a corrugated fin type material by a method wherein overflow weirs are arranged in such a manner that a liquid may be supplied to both sides of a second heat-exchanging passage, and the corrugated fin type material is extended so as to be higher than the second heat-exchanging passage so that the liquid may surely flow into respective corrugations.

Horizontally oriented corrugated fin type materials 32 are arranged in overflow weirs 26. The overflow weirs 26 is separated from a second heat-exchanging passage by a dividing bar 34 on which a corrugated fin type material 25 is mounted. In order to surely wet all the corrugations, e.g. a corrugation 36, the corrugated fin type material 25 is formed so as to exceed the top part of the second heat-exchanging passage 22, in other words, so as to extend above a plate 18. In order to supply the liquid to the corrugation 36 of the corrugated fin material 25, collecting pools 38 are provided in such a manner that the overflow weirs 26 may be arranged between the collecting pools 38.