To provide a self-priming pump capable of stably discharging fluid to be transferred even if the fluid is brought into a gas-liquid mixed state.

A liner ring 50 which rubbing-contacts with an impeller 29 when the impeller 29 moves to the front side during its rotation is provided at a circular plate 26b at a position on the rear side of the innermost step thereof. In a cover member 37 of the impeller 29 provided at a position facing a front partition wall 26, the center 37a of a circular plate has an opening having a size substantially the same as an opening of a cylindrical section 26a of the front partition wall 26. The edge 37a of the opening of the circular plate is formed in a ring-like shape so as to project to the front partition wall 26 side. A backflow prevention projection 37a is inserted inside the liner ring 50 of the front partition wall 26 with a predetermined gap formed between the projection 37a and the liner ring 50. A mouth ring 51 which rubbing-contacts with the liner ring 50 when the impeller 29 moves to the front side is provided at the cover member 37 at a position facing the liner ring 50.