A compressor-impeller assembly, comprising: a compressor impeller; a shaft inserted through a hub of the compressor impeller, the shaft including a screw portion protruding toward an upstream side of the compressor impeller in an axial direction from the hub of the compressor impeller; and a compressor-impeller fixing nut configured to fix the compressor impeller to the shaft by being screwed with the screw portion, the compressor-impeller fixing nut having a tubular shape, and comprising: a curved-surface shaped portion disposed in a first range from a first end side in an axial direction, the curved-surface shaped portion having an outer diameter (D1, Dc) which increases toward a second end side in the axial direction and including an outer peripheral surface having a curved shape which protrudes outward in a radial direction in a cross section along the axial direction; and a nut portion disposed in a second range closer to the second end side from the first range in the axial direction, the nut portion having an outer peripheral surface with a nut shape, wherein an expression (D1max-D1min)/2<L is satisfied, where L is a length of the first range in the axial direction, and D1max is a maximum value and D1min is a minimum value of the outer diameter of the curved-surface shaped portion in the first range, wherein a recess portion is formed on an upstream end surface of the hub of the compressor impeller with respect to an axial direction, wherein the compressor impeller fixing nut includes: an accommodated portion disposed in a third range from the second end side in the axial direction and accommodated in the recess portion; and a flange portion disposed between the nut portion and the accommodated portion in the axial direction and protruding outward in the radial direction; and wherein an expression Dc>Dr is satisfied, where Dc is an outer diameter of the flange portion and Dr is an inner diameter of the recess portion. a compressor impeller; a shaft inserted through a hub of the compressor impeller, the shaft including a screw portion protruding toward an upstream side of the compressor impeller in an axial direction from the hub of the compressor impeller; and a compressor-impeller fixing nut configured to fix the compressor impeller to the shaft by being screwed with the screw portion, the compressor-impeller fixing nut having a tubular shape, and comprising: a curved-surface shaped portion disposed in a first range from a first end side in an axial direction, the curved-surface shaped portion having an outer diameter (D1, Dc) which increases toward a second end side in the axial direction and including an outer peripheral surface having a curved shape which protrudes outward in a radial direction in a cross section along the axial direction; and a nut portion disposed in a second range closer to the second end side from the first range in the axial direction, the nut portion having an outer peripheral surface with a nut shape, a curved-surface shaped portion disposed in a first range from a first end side in an axial direction, the curved-surface shaped portion having an outer diameter (D1, Dc) which increases toward a second end side in the axial direction and including an outer peripheral surface having a curved shape which protrudes outward in a radial direction in a cross section along the axial direction; and a nut portion disposed in a second range closer to the second end side from the first range in the axial direction, the nut portion having an outer peripheral surface with a nut shape, wherein an expression (D1max-D1min)/2<L is satisfied, where L is a length of the first range in the axial direction, and D1max is a maximum value and D1min is a minimum value of the outer diameter of the curved-surface shaped portion in the first range, wherein a recess portion is formed on an upstream end surface of the hub of the compressor impeller with respect to an axial direction, wherein the compressor impeller fixing nut includes: an accommodated portion disposed in a third range from the second end side in the axial direction and accommodated in the recess portion; and a flange portion disposed between the nut portion and the accommodated portion in the axial direction and protruding outward in the radial direction; and an accommodated portion disposed in a third range from the second end side in the axial direction and accommodated in the recess portion; and a flange portion disposed between the nut portion and the accommodated portion in the axial direction and protruding outward in the radial direction; and wherein an expression Dc>Dr is satisfied, where Dc is an outer diameter of the flange portion and Dr is an inner diameter of the recess portion.