An impeller comprising: an inner diameter portion (20, 320) of which one side in an axial direction of an axis of a rotation shaft of a rotary machine is configured to be fitted at the outside of the rotation shaft by thermal deformation; a disk portion fitted at the outside of the inner diameter portion by thermal deformation at the other side in the axial direction of the inner diameter portion; a blade portion protruding from a surface of the disk portion facing toward the one side in the axial direction; and a cover portion formed in a single-piece together with the blade portion and the disk portion and covering the blade portion from the one side in the axial direction; wherein the inner diameter portion (20, 320) includes: a thin portion formed at the one side in the direction of the axis, and configured to be fitted to the rotation shaft by shrink fitting, a thick portion (22, 322) formed at the other side in the direction of the axis, and having a thickness thicker than the thin portion, and an expanding diameter portion formed between the thin portion and the thick portion, and having a diameter of which expands gradually toward the other side in the direction of the axis, wherein the inner diameters of the expanding diameter portion and the thick portion are formed so, when the impeller is fitted on the rotation shaft, as to be the same as an outer diameter of the rotation shaft or larger than the outer diameter of the rotation shaft; and wherein the disk portion includes: a main body portion provided with the blade portion, and a fixing portion disposed at an inner side from the main body portion in a radial direction of the rotation shaft and fitted at the outside of an outer peripheral surface (22a, 322a) of the thick portion by shrink fitting, wherein the fixing portion is formed so as to protrude toward the other side in the axial direction from a surface of the main body portion. an inner diameter portion (20, 320) of which one side in an axial direction of an axis of a rotation shaft of a rotary machine is configured to be fitted at the outside of the rotation shaft by thermal deformation; a disk portion fitted at the outside of the inner diameter portion by thermal deformation at the other side in the axial direction of the inner diameter portion; a blade portion protruding from a surface of the disk portion facing toward the one side in the axial direction; and a cover portion formed in a single-piece together with the blade portion and the disk portion and covering the blade portion from the one side in the axial direction; wherein the inner diameter portion (20, 320) includes: a thin portion formed at the one side in the direction of the axis, and configured to be fitted to the rotation shaft by shrink fitting, a thick portion (22, 322) formed at the other side in the direction of the axis, and having a thickness thicker than the thin portion, and an expanding diameter portion formed between the thin portion and the thick portion, and having a diameter of which expands gradually toward the other side in the direction of the axis, a thin portion formed at the one side in the direction of the axis, and configured to be fitted to the rotation shaft by shrink fitting, a thick portion (22, 322) formed at the other side in the direction of the axis, and having a thickness thicker than the thin portion, and an expanding diameter portion formed between the thin portion and the thick portion, and having a diameter of which expands gradually toward the other side in the direction of the axis, wherein the inner diameters of the expanding diameter portion and the thick portion are formed so, when the impeller is fitted on the rotation shaft, as to be the same as an outer diameter of the rotation shaft or larger than the outer diameter of the rotation shaft; and wherein the disk portion includes: a main body portion provided with the blade portion, and a fixing portion disposed at an inner side from the main body portion in a radial direction of the rotation shaft and fitted at the outside of an outer peripheral surface (22a, 322a) of the thick portion by shrink fitting, a main body portion provided with the blade portion, and a fixing portion disposed at an inner side from the main body portion in a radial direction of the rotation shaft and fitted at the outside of an outer peripheral surface (22a, 322a) of the thick portion by shrink fitting, wherein the fixing portion is formed so as to protrude toward the other side in the axial direction from a surface of the main body portion.