To provide a self-priming pump in which self-priming time is shortened than that in conventional self-priming pumps to improve efficiency of the pump.

The self-priming pump includes a casing divided into two parts of a first casing and a second casing. In the second casing, an impeller chamber provided with an impeller to agitate water and air, a tank chamber for steam separation, and a running water channel for water to run from the impeller to the tank chamber are formed so as to be partitioned by partition walls. A return groove is formed below the axis to return the water separated from steam after communication between the impeller chamber and the tank chamber, and a return passage communicating with the return groove is formed from the tank chamber.