Cooling performance of a cooler for pre-cooling humid gas supplied to a gas turbine power generation plant, etc. is improved by increasing heat transfer efficiency between the humid gas and cooling liquid flowing in heat exchanger tubes with utilization of condensed liquid generated due to the cooling of the humid gas. In a heat exchanger for cooling humid gas a flowing surrounding heat exchanger tubes 8 of the tube groups 9a and 9b by flowing cooling liquid r in the tubes, the liquid droplets cl generated by condensation of moisture contained in the humid gas are gathered and sprinkled or fallen in drops on a part of the heat exchanger tube group 9b, which results in increased heat exchange efficiency.