PURPOSE: To obtain a volute pump having high lift and suitable for well pump by communicating the bottom of groove of impeller with the back side of impeller through a hole while providing a labyrinth for reducing detour of pressure water at the water path side to the backface of impeller on the outercircumference of impeller.

CONSTITUTION: An impeller 1 is provided with a vane section where an annular groove 2 having approximately semi-circular cross-section is partitioned by plural number of vanes 18 while a communication hole 3 coupling between the bottom of vane section and the sideface side and an annular projection 16 for providing a labyrinth against the separation chamber wall 21 on the outercircumference of backface are provided. Because of reduction of water flowing to the backface side through labyrinth and pressure balancing effect through communication hole 3, static pressure balance at the backface side and the water path 14 side can be maintained without modifying the lift characteristic. Consequently, loose mounting of impeller onto the shaft will never cause defective operation.