PURPOSE: To prevent occurrence of cavitation by eliminating gaseous body by allowing a part of discharged fluid body to be jetted to an inlet, by connecting one end of a small-diameter tubing in the neighborhood of outlet of a pump, leading the other end of the tubing in the neighborhood of the pump outlet and providing an opening.

CONSTITUTION: In a middle section 5 of a passage through which a fluid having been discharged from an impeller 1 is led to a following conduit 4 through a casing 3, a small-diameter tubing 7 is connected so that an open end 6 on the one side contacts the fluid, and an open end 8 on the other side of the small- diameter tubing 7 is opened in the neighborhood of an inlet 9 of the pump. When the impeller 1 rotates, the fluid covering the pump's inlet 9 is sucked into the impeller 1. In this process, even if gaseous body is to exit by being mixed up into the fluid in the impeller 1, by allowing a part of the fluid from the pump outlet to be jetted to the pump inlet through the small-diameter tubing 7, the gaseous body is eliminated for resumption of the normal operation.