PURPOSE: To prevent white smoke from being produced from an exhaust port of a closed cooling tower in the rainy season and in the winter season by producing high temperature dried air with a simplified structure and mixing the dried air with wet air.

CONSTITUTION: A plurality of closed heat exchangers 11 are disposed below a plurality of spreading water pipes B vertically hierarchically, in which circulation cooling water flows. Many vertical rectifier plates 10 are arranged at straight pipe portions of these closed heat exchangers 11, separated axially of the same. An air-only passage 12 is formed between the rectifier plates 10, 10 disposed alternately at one interval for guiding only air from an outside air intake port to an exhaust port side, and a gas/liquid contact passage 13 is formed between the remaining rectifier plates 10, 10, and further the spreading water pipes B are disposed on the upper end of each gas/liquid contact passage 13. A baffle plate 14 is provided at the upper ends of the rectifier plates 10 forming opposite side walls of the air-only passage 12, which plate 14 is opened and closed with respect to the gas/liquid contact passage.