PURPOSE: To improve efficiency of a pump by forming a cooling water passage which communicates with an outlet pressure part of an impeller at a certain stage in the center of a pump shaft therethrough and making a feed water bore which branches from the passage through a bearing for cooling it, thereby reducing a loss of pressure water.

CONSTITUTION: By rotating an impeller 1, water is sucked from a feed water port 12 and boosted. The boosted water passes through a dischare passage 14 and is then released to a discharge port 15. Cooling water is supplied from a cooling water passage 17 in communication with a certain-stage impeller's outlet pressure part which is located at the suction side of the pump, a pressure being boosted to an insufficient value which is however satisfactory for cooling the bearing part. The water is further carried to cool the bearings 10a and 10b and then passes through a return tube 13 before returned to the suction side of the pump. As mentioned above, the pressure water of the impeller 1 which is not boosted completely cools the bearings 10a and 10b and a motor 9. Since the pressure water at the back of the impeller is not repeatedly used, a loss of the pressure water is minimal and efficiency of the pump is thereby improved.