A rotary machine comprising: a rotating shaft which extends along a central axis; an impeller having: a disc formed in a disc shape and provided to be rotatable around the central axis; and a plurality of blades provided on a disc surface facing a first side in a central axis direction at intervals in a circumferential direction around the central axis, wherein a rear surface of the disc facing a second side in the central axis direction is formed to have a convex curved surface which is convex toward the second side in the central axis direction in at least part of a center portion of the disc; and a collar which is provided on the rotating shaft and collides with the convex curved surface of the rear surface of the impeller, wherein a concave curved surface is further formed in a radially outer part of the disc and at a radially outer side of the convex curved surface, wherein the concave curved surface is concave toward the second side along the central axis direction, and the disc is totally formed at the first side in the central axis direction with respect to a contact area on which the convex curved surface abuts the collar characterized in that an outer circumferential convex curved surface which is convex toward the second side in the central axis direction and is formed with a curvature different from that of the convex curved surface is continuously formed on an outer side of the convex curved surface and an inner side of the concave curved surface in a radial direction of the disc. a rotating shaft which extends along a central axis; an impeller having: a disc formed in a disc shape and provided to be rotatable around the central axis; and a plurality of blades provided on a disc surface facing a first side in a central axis direction at intervals in a circumferential direction around the central axis, wherein a rear surface of the disc facing a second side in the central axis direction is formed to have a convex curved surface which is convex toward the second side in the central axis direction in at least part of a center portion of the disc; and a collar which is provided on the rotating shaft and collides with the convex curved surface of the rear surface of the impeller, wherein a concave curved surface is further formed in a radially outer part of the disc and at a radially outer side of the convex curved surface, wherein the concave curved surface is concave toward the second side along the central axis direction, and the disc is totally formed at the first side in the central axis direction with respect to a contact area on which the convex curved surface abuts the collar characterized in that an outer circumferential convex curved surface which is convex toward the second side in the central axis direction and is formed with a curvature different from that of the convex curved surface is continuously formed on an outer side of the convex curved surface and an inner side of the concave curved surface in a radial direction of the disc. a disc formed in a disc shape and provided to be rotatable around the central axis; and a plurality of blades provided on a disc surface facing a first side in a central axis direction at intervals in a circumferential direction around the central axis, wherein a rear surface of the disc facing a second side in the central axis direction is formed to have a convex curved surface which is convex toward the second side in the central axis direction in at least part of a center portion of the disc; and a collar which is provided on the rotating shaft and collides with the convex curved surface of the rear surface of the impeller, wherein a concave curved surface is further formed in a radially outer part of the disc and at a radially outer side of the convex curved surface, wherein the concave curved surface is concave toward the second side along the central axis direction, and the disc is totally formed at the first side in the central axis direction with respect to a contact area on which the convex curved surface abuts the collar characterized in that an outer circumferential convex curved surface which is convex toward the second side in the central axis direction and is formed with a curvature different from that of the convex curved surface is continuously formed on an outer side of the convex curved surface and an inner side of the concave curved surface in a radial direction of the disc.