Provided are: a sewing device provided with a sewing mechanism which performs sewing on a work to be sewn using a sewing-machine needle, while holding the work to be sewn in a state of being shaped in a predetermined shape on a plurality of seating portions arranged in a sewing direction; and a sewing method. When the seating portions are withdrawn from the work to be sewn so as to avoid interference between the sewing mechanism and the seating portions, a guiding mechanism enables movement in a first withdrawing direction for avoiding a first undercut portion of the work to be sewn, and movement in a second withdrawing direction for avoiding a second undercut portion of the work to be sewn.