PURPOSE: To enable application of dia-cut system for varying the pump characteristic through reduction of diameter of impeller onto a self-priming volute pump by arranging a deflector pin between the impeller and a deflector through micro gap at least against the impeller.

CONSTITUTION: Upon operation of self-priming volute pump under the condition where the water level in casing 1 is at the position (L), the flow speed will increase in the gap between the imepller 3' and a deflector 2 to deposite the air in the water while to separate the water and the air in a separation chamber 4 and through continuous function to return the water through recirculation hole 5 into the pump, the air in a suction tube 6 is discharged to the delivery side 7. When reducing the diameter of impeller 3' through dia-cut, a cylindrical deflector pin 8 is placed in a gap between a deflector having reduced diameter to make narrow the gap. Gaps C', E are provided respectively between said pin 8 and the impeller 3' and the deflector 2.