To improve cooling effect of a humid gas by utilizing condensate liquid produced in cooling the humid gas, and improving heat transfer efficiency between the humid gas and a cooling medium flowing in a heat transfer tube.

In a heat exchanger for cooling the humid gas by allowing a cooling medium r to flow into groups of heat transfer tubes 9a, 9b disposed in a flow channel of the humid gas a, heat exchanging effectiveness is increased by spraying or dripping the condensate liquid obtained by collecting liquid droplets c1 by dew condensation of the humid gas a, to the other parts of the groups of heat transfer tubes 9a, 9b.