To provide a leak preventing device for a volute pump capable of greatly improving pumping efficiency by minimizing leak of pressurized water in a volute chamber into a suction passage, and of relatively easing processing precision.

In a part forming a gap 11 between an inner circumference of a casing 5a of a volute pump and an impeller 1, an annular seal ring 13 with a cut off part made of material superior in sliding property is inserted in the impeller 1 slidably in a circumference direction and an axial direction. A contact surface 14 of the seal ring 13 slid in the axial direction is provided at a position in the suction passage 4 side of the seal ring 13 of the inner circumference of the casing 5a. The gap 11 is sealed by the seal ring 13 to prevent pressurized water from leaking into the suction passage 4 side.