To provide an impeller further improved in performance.SOLUTION: An impeller 20 includes: a disc 30 rotatable about an axis O and having a main surface 33 extended to a radial outer side as it goes toward an axial one side; a plurality of blades 40 disposed on the main surface and defining a flow channel Fi extended from an inlet 51 at the axial other side to an outlet 52 at one side; and a cover 36 disposed in opposition to the main surface to cover the plurality of blades. A small fillet portion curved into a circular arc shape and having a relatively small radius of curvature is formed on at least one of an inlet-side region A1 including an inlet-side end portion and an outlet-side region A5 including an outlet-side end portion, at a connection portion of the blades and the main surface and a connection portion of the blades and the cover, and a large fillet portion having a relatively large radius of curvature is formed on an intermediate region A3 between the inlet-side region and the outlet-side region.SELECTED DRAWING: Figure 2