To reduce abrasion such as deep recesses to be locally caused in the surface of an impeller by taking flowing particles into a space between the impeller and a side linear with a vortex flow formed in the tip of an auxiliary vane of the impeller.

This impeller includes: a front shroud having opposed faces, an outer peripheral edge portion and a rotation axis; a plurality of pumping vanes on one of the faces of the front shroud and extending away from the rotation axis, each pumping vane having an outer peripheral edge portion; a rear shroud having pumping vanes between the front shroud and the rear shroud; and a plurality of auxiliary vanes (26) on the other face of the shroud, the auxiliary vanes of each having an outer edge portion wherein the dimension (Da) from the rotation axis to the outer peripheral edge portion of the front shroud is greater than the dimension (Db) from the rotation axis to the outer edge portion of the auxiliary vanes and greater than the dimension (Dc) from the rotation axis to the outer peripheral edge of the pump vane.