PURPOSE: To forcedly suck out air bubbles staying inside an impeller by cutting a nozzle groove at the end edge of a junction between the outer peripheral wall of a diffuser and a casing cover at a specified position with specified size and angle.

CONSTITUTION: In a diffuser (a), in which the end edge of connection to a casing cover is adopled as the upper face, and a delivery opening 2 is arranged vertically on the left end, a nozzle groove 4 of specified width and depth is cut out at an angle of forming the tangents to the tip of an impeller in the junction end edge 1 in the neighborhood of a 315° datum line 3 on a rectangular coordinates in which the axis of ordinate is taken in the vertical direction, and the center of entrance is taken as the origin, and a circulating water for priming is strongly jetted by means of running impellers toward the tip of the impeller as an air-water mixed fluid having operating energy from the nozzle groove 4. At this time, air bubbles staying inside the impeller are forcedly sucked out to quickly separate water, and then exhausted to produce negative pressure at the inlet of the impeller. Quick sucking of water can thus be performed by strong suction.