To solve a problem in a self-priming pump, that when water is returned from the water tank part of a priming casing and a reflux chamber to the inside of the impeller chamber of an impeller casing through a communicating hole under a water supply condition, a clearance between a diaphragm and the blade of an impeller is small, therefore interference of the communicating hole with the impeller is increased so as to increase noises.

In this self-priming pump, an impeller 12 is stored in an impeller casing 3, a priming casing 4 is arranged through a diaphragm 6 having a communicating hole 8, and the diaphragm side edge 12b of the peripheral side end part of a blade 12a in the impeller 12 is tapered, thereby the end part of the blade 12a of the impeller 12 is prevented from being interfered with the communicating hole 8.