A water pump including a pump housing having a central axial through-hole; a pump shaft inserted in the through-hole; a pulley fixed on the pump shaft at one end thereof; an impeller fixed on the pump shaft at the other end thereof; a bearing fitted with the through-hole for rotatably supporting the pump shaft in the vicinity of the pulley; a water seal provided between the impeller and the bearing for sealing water in a pump chamber where the impeller is installed, wherein an intermediate chamber is defined among the pump housing, the bearing and the water seal. A first ventilation passage is formed in a wall of the pump housing so as to induce an atomspheric air into the intermediate chamber. A second ventilation passage is formed in the wall of the pump housing so as to discharge a water vapor in the intermediate chamber to an outside of the pump housing. A vacuum generating portion is formed by rotation of the pulley in the vicinity of an outlet of the second ventilation passage opening to a space defined between the pulley and the pump housing, whereby when the pulley is rotated, the water vapor in the intermediate chamber is positively ventilated through the second ventilation passage by vacuum to be generated in the vacuum generating portion.
