PURPOSE: To enable cooling water more efficiently and make a water-cooling device compacter.

CONSTITUTION: Water to be cooled us passed through hollow fibers 2 which are placed in contact with air; water is cooled as it is passed through such a hollow fiber 2 by a method wherein the water oozing out through a membrane of the hollow fiber and to the surface is vaporized. A water-cooling device comprises a cooling tower 1 in which hollow fibers 2 are provided, a water- feeding means 9 which feeds water 3 to be cooled to the cooling tower 1 and passes the water through the hollow fibers 2, and an air-feeding means 18 which feeds air to the cooling tower 1 and puts the air in contact with the hollow fibers 2.