A method and a device with which a bulky decorative piece made of a thermoplastic synthetic resin can be produced. A method for producing a three-dimensional decorative piece made of a thermoplastic synthetic resin includes a step of preparing a decorative piece material including an upper layer that has a thermoplastic synthetic resin film and a lower layer that has a thermoplastic synthetic resin sheet, a step of disposing the decorative piece material on a flat-plate die serving as an electrode, and a softening step of softening and welding the decorative piece material by bringing a flat-plate die serving as another electrode closer to the flat-plate die serving as the electrode, and performing high-frequency dielectric heating while pressing the decorative piece material.