A method of manufacturing produces an assembled product where a ring-shaped seal is attached to a channel in an inner circumferential surface of a ring-shaped component. The method includes: a holding step that moves an engaged portion of the seal inwardly in a radial direction using a deformation member, while restricting movement of the seal in a direction parallel to an inner circumferential wall of the channel, to cause deformation so that the outer diameter of part of the seal aside from the engaged portion becomes smaller than the channel inner circumferential diameter; and an attachment step that attaches the seal to the channel by releasing the seal to restore the seal from the deformation. The attachment step includes releasing the seal by releasing engagement by the deformation member and moving the seal body inside the channel while pressing out the seal body from inside.