An absorber having a shell, wet tubes and cooling fluid supply and collection portions. Partition walls divide the shell into at least three zones. The wet tubes are disposed parallelly in the shell, passing through the partition walls. The opposite open ends of the wet tube locate in the uppermost and lowermost zones of the shell, respectively. The cooling fluid supply portion communicates with the open end of the wet tube to supply a cooling fluid thereto. The cooling fluid collection portion communicates with the other open end of the wet tube and discharge it outside. A gap is formed between the wet tube and the partition walls facing the intermediate zone. The cooling fluid continuously flows in the wet tubes, while supplying the liquid working fluid to the uppermost zone so as to flow down from the gap. The liquid working fluid flows through the gap to the zone locating below the intermediate zone and is discharged outside. The gaseous working fluid is supplied to the intermediate zone so as to come into contact with the liquid working fluid flowing on the outer peripheral surfaces of the wet tubes.