PURPOSE: To conduct the suction of priming water at an early stage and in abundance, by opposing a return current hole which is at a partition plate partitioning an impeller case and a pump cover and which leads from a water sink to an impeller chamber, to a cut-out formed at the outer perimeter of each impeller side plate.

CONSTITUTION: A pump main body is constituted with a drive chamber case 1 housing the drive machine of an impeller 8, an impeller case 2 forming an impeller chamber 6 that houses the impeller 8 and an air-water separation chamber, and a pump cover 3 forming a suction chamber, a water sink 14 and a discharge chamber. At this constitution, cut-outs 26 are formed at the impeller side plates 21 of respective water delivery passages 23 between respective blades 22 of the impeller 8. And a return current hole 27 which is at a partition plate 5 partitioning the impeller case 2 and the pump cover 3 and which leads to the impeller chamber 6 from the water sink 14, is opposed to the outer perimeter of each impeller side plate 21, at the cut-out part. As a result, the expected purpose is attained by operating alternately positive pressure and negative pressure against the return current hole 27.