An impeller comprising: a disc rotatable around an axis, the disc having a main surface extending outward in a radial direction toward one side in an axial direction; a plurality of blades disposed on the main surface at an interval in a circumferential direction, and defining a flow path extending to an outlet on one side from an inlet on the other side in the axial direction; and a cover disposed to face the main surface to cover the plurality of blades, wherein at least one of an inlet side region including an end portion on the inlet side and an outlet side region including an end portion on the outlet side has a small fillet portion curved in an arc shape when viewed in an extending direction of the blade at a connection portion between the blade and the main surface and a connection portion between the blade and the cover, an intermediate region formed between the inlet side region and the outlet side region has a large fillet portion curved in an arc shape when viewed in the extending direction of the blade at the connection portion between the blade and the main surface and the connection portion between the blade and the cover, a radius of curvature of the large fillet portion is larger than a radius of curvature of the small fillet portion, wherein the impeller further includes a transition region (A2, A4) formed in at least one of a portion between the inlet side region and the intermediate region and a portion between the outlet side region and the intermediate region, and having the radius of curvature which gradually increases from the small fillet portion toward the large fillet portion, the blades are inclined with respect to the main surface and the cover, the large fillet portions are each formed in a portion between the disc main surface and the pressure side surface of the blade, a portion between the disc main surface and the suction side surface, a portion between the inner peripheral surface of the cover and the pressure side surface of the blade, and a portion between the inner peripheral surface of the cover and the suction side surface, the respective large fillet portions have mutually the same radius of curvature within the same cross section, the small fillet portions are each formed in a portion between the disc main surface and a pressure side surface of the blade, a portion between the disc main surface and a suction side surface, a portion between the inner peripheral surface of the cover and the pressure side surface of the blade, and a portion between the inner peripheral surface of the cover and the suction side surface, and the respective small fillet portions have mutually the same radius of curvature within the same cross section. a plurality of blades disposed on the main surface at an interval in a circumferential direction, and defining a flow path extending to an outlet on one side from an inlet on the other side in the axial direction; and a cover disposed to face the main surface to cover the plurality of blades, wherein at least one of an inlet side region including an end portion on the inlet side and an outlet side region including an end portion on the outlet side has a small fillet portion curved in an arc shape when viewed in an extending direction of the blade at a connection portion between the blade and the main surface and a connection portion between the blade and the cover, an intermediate region formed between the inlet side region and the outlet side region has a large fillet portion curved in an arc shape when viewed in the extending direction of the blade at the connection portion between the blade and the main surface and the connection portion between the blade and the cover, a radius of curvature of the large fillet portion is larger than a radius of curvature of the small fillet portion, wherein the impeller further includes a transition region (A2, A4) formed in at least one of a portion between the inlet side region and the intermediate region and a portion between the outlet side region and the intermediate region, and having the radius of curvature which gradually increases from the small fillet portion toward the large fillet portion, the blades are inclined with respect to the main surface and the cover, the large fillet portions are each formed in a portion between the disc main surface and the pressure side surface of the blade, a portion between the disc main surface and the suction side surface, a portion between the inner peripheral surface of the cover and the pressure side surface of the blade, and a portion between the inner peripheral surface of the cover and the suction side surface, the respective large fillet portions have mutually the same radius of curvature within the same cross section, the small fillet portions are each formed in a portion between the disc main surface and a pressure side surface of the blade, a portion between the disc main surface and a suction side surface, a portion between the inner peripheral surface of the cover and the pressure side surface of the blade, and a portion between the inner peripheral surface of the cover and the suction side surface, and the respective small fillet portions have mutually the same radius of curvature within the same cross section.